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March 28, 2011

Evolution of the Olive

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The Halachic History of the Expanding Kezayis

Rabbi Natan Slifkin

Copyright © 2010 by Natan Slifkin

Version 1.1

This essay was written as part of the course requirements for a Master’s

Degree in Jewish Studies at the Lander Institute (Jerusalem).

This document may be freely distributed as long as it is distributed

complete and intact.


The Evolution of the Olive


R. Moshe Sofer (Chasam Sofer, 1762-1839) notes that today, when there is no Temple,

there is only one Biblically-ordained mitzvah involving eating: that of eating matzah on the

first night of Pesach.1 Accordingly, he stresses that one must be careful to be punctilious in

the fulfillment of this mitzvah. Aside from the mitzvah requiring a certain type of food, there

is also a requirement of a sufficient minimum quantity to qualify as “eating.” This quantity is

defined in the Midrash:

There is no “eating” with less than a kezayis (equivalent to an olive). (Toras Kohanim, Acharei

12:2; Emor 4:16)

How much is this quantity? R. Chaim of Volozhin (1749-1821) is widely revered as the

father of the yeshivah world. Less known and certainly less popular in the yeshivah world is

his view as to the size of the matzah that one is obligated to eat on Pesach. R. Chaim was of

the view that this kezayis is actually the size of an olive – around three or four cubic

centimeters.2 This results in a piece of matzah about half the size of a credit card.

Yet this is in sharp contrast to common custom today. The widespread policy is to

quantify a kezayis as 28.8 cubic centimeters. The Mishnah Berurah states that one should eat

a volume equal to an egg, which is about 55cc. And there are boxes of machine matzot which

state on the packaging that one whole matzah equals a kezayis! The greatest irony is that, in

the effort to perform the mitzvah as scrupulously as possible, some might engage in achilah

gasah (gorging oneself), which surely could not be the intent of the mitzvah and which might

prevent a person from fulfilling his obligation.

Recently, there have been efforts by some individuals to prove that the kezayis should be

scaled down, but they have met with little success and much opposition. In this study, while

proving that the kezayis is the size of a regular olive, the focus will be on exploring how it

happened that so many authorities ruled it to be far bigger, and why it is difficult to

overcome this view.

Logically, in order to reach the conclusion that a kezayis is much larger than olives are

today, two separate positions must both be taken: First, that olives of ancient times were

much larger, and second, that we are obligated to follow the size of ancient olives rather than

1 Responsa Chasam Sofer, Choshen Mishpat 196.

2 R. Yisrael Yaakov Kanievsky, Kehillas Yaakov, Pesachim 38. See too A. Z. Katzenallenbogen, Shaarei

Rachamim (Vilna 1871) p. 19, #165 note 3.


the olives of today. Neither one of these positions on their own is sufficient to require a

larger measurement; they must both be adopted. Let us begin by evaluating both of these

positions in turn.

Were Olives Bigger in Ancient Times?

Were olives of the Biblical or Talmudic era larger than those of today? From the

standpoint of archeology, there is clear evidence the olives of ancient times were not any

bigger than those of today. Many olive pits from ancient times have been discovered,

including a huge number in the remains of the settlement at Masada and in caves in the

Judean Desert dating from the Bar-Kochba revolt. These pits were mostly from the Nabali

strain of olives, but also included the local Suri and Melisi varieties, as well as the large Shami

and Tohaffi olives that were imported as luxuries from other countries. All these pits are not

significantly different in size from the pits of those olive strains today.3 One could claim that

the flesh-to-pit ratio used to be greater, but this is unlikely, and should not be accepted

without good reason.

Furthermore, there are dozens of olive trees alive today of the Suri variety, in Israel and

elsewhere, which are around two thousand years old, and seven in Israel that are over three

thousand years old. These trees even still produce fruit, which are no different in size from

the fruit produced by young olive trees.4 One could claim that they used to produce larger

fruit, but there would have to be strong grounds not to presume that they always produced

the same size fruit.

All the empirical evidence, then, indicates that in Talmudic and even Biblical times, olives

were no larger than those found today. The Mishnah specifies which of the various strains is

intended when the olive is given as a halachic measurement:

The kezayis of which they spoke is neither a large one nor a small one, but rather a mediumsized

one, which is the egori. (Mishnah Keilim 17:8)

The large olive mentioned in the Mishnah would correlate with the Shami, which

measures around 12-13cc, and the small olive would correlate with the Melisi, which

measures around 0.5-1cc. The medium-sized olive would be the prevalent Suri and/or the

slightly larger Nabali strains. The Suri ranges from 2.5-3.5cc, while the Nabali ranges from

4-6cc.5 The kezayis of the Talmud, which would be the same as the kezayis of today, would

range from 2.5-6cc with an average of around 4cc.

3 Mordechai Kislev, “Kezayis – The Fruit of the Olive as a Measure of Volume” (Hebrew), Techumin 10 pp.

427-437; “Everything is According to the Opinion of the Observer – A New Evaluation of the Measurement of

a Kezayis,” (Hebrew) BDD vol. 16 pp. 77-90.

4 M. Kislev, Y. Tabak & O. Simhoni, Identifying the Names of Fruits in Ancient Rabbinic Literature, (Hebrew)

Leshonenu, vol. 69, p.279.

5 Kislev, ibid.


The Geonim: Following the Observer

Already, then, we see that there appears to be no reason to ever assume that an olive was

any larger than olives today. But what if, for whatever reason, someone were to believe that

perhaps olives of ancient times were larger – would they be obligated to replicate that

quantity? The Geonim rule that this is not the case. Around 130 years ago, three responsa on

this topic from the Geonic period were discovered. The first is from Rav Sherira Gaon

(Babylonia, c.900-c.1000):

You asked me to explain if there is a weight given for the fig, olive, date and other

measurements, in the weight of Arabic coins, and you explained that Rav Hilai Gaon clarified

that the weight of an egg is 16 2/3 silver pieces. [You wondered,] if the others do not have an

ascribed weight, why is the egg given one?

It is known that these other measurements are not given any equivalent weight in silver, not in

the Mishnah nor the Talmud. If [the Sages] had wished to give a measurement in terms of the

weight in dinarim, they would have done so originally. Rather, they give the measurements in

terms of grains and fruit, which are always available, and one is not to say that they have


…We practice according to the Mishnah: Everything goes according to the observer… And

likewise with regard to the olive and date, it is explained in this Mishnah that it is not referring

to a large one, or a small one, but rather an average one – and it is also according to the view of

the observer. The reason why some rabbis gave their view as the size of an egg, and did not do

the same with an olive, date or fig, is that there are many things that are dependent upon the

size of an egg – the kab, the sa’ah, the efoh, the omer; all are evaluated in terms of eggs, and

therefore they estimated it according to their views, but these other measurements are left to

the opinion of the observer… (Cited in Sefer Ha-Eshkol vol. II, Hilchos Challah 13 p. 52)

The intent may be that since the kab, sa’ah etc. are multiples of eggs (a kab is 24 eggs, a

sa’ah is 144 eggs), it is difficult to visualize this in terms of eggs, and it is easier to visualize it

in terms of silver.6 However, with measurements given as a kezayis, there is no reason or basis

for giving an alternate measurement.

Rav Sherira Gaon’s son, Rav Hai Gaon (Babylonia 939-1038), writes as follows:

…And therefore the Torah gave measurements in terms of eggs and fruits – for divrei sofrim

were given at Sinai… – because eggs and fruit are found in every place. For it is known and

revealed before the One Who spoke and brought the universe into existence, that Israel is

destined to be scattered amongst the nations, and that the weights and measures that were in

the days of Moses and that which were added to in the Land of Israel would not be preserved,

and that the measurements change in different times and places… Therefore the Sages related

the quantities to fruit and eggs, which always exist and never change. They made the quantity

of an egg depend upon the view of the observer. (Ibid. pp. 56-57)

A final responsum, from an unknown Geonic author, states:

6 R. Chaim Beinish, Midot VeShiurei Torah, pp. 522-523.


And that which you wrote regarding the size of a large fig and a medium fig, and likewise a

large olive and a small and medium olive – surely these are shiurim, and how can there be a

shiur for a shiur? And should you say that it is [a matter of ascribing] a weight – our rabbis did

not specify a weight, and the Holy One was not particular with us regarding the weight. Every

person, in acting according to his own assessment, has fulfilled his obligation, and there is no

need to learn the quantity from another… (Teshuvos HaGeonim 268, Harkavey ed.)

In all these responsa, we see that the kezayis is intended to be assessed, very simply, by each

person looking at an olive. Even if one were to believe that the olives of the Talmudic era

were larger than those of today, there would be no need to attempt to replicate that

quantity.7 With the Geonim, we see a presumption that the size of olives does not change,

and that in any case each person is supposed to follow his own assessment of an olive. This

was seen to be the underlying rationale of the Torah prescribing quantities in terms of

familiar fruit rather than by some independent system of measurement.

The Rishonim of Sefarad

Let us now turn to the era of the Rishonim, and we shall begin with the Rishonim of

Spain and comparable regions. Rambam (Spain/Egypt 1135-1204) makes no statement

regarding the size of a kezayis. But an inference regarding its maximum size can be drawn

from his statement that a dried fig is 1/3 of an egg.8 Since the Talmud notes that a kezayis is

smaller than a dried fig,9 this would result in a kezayis being less than 1/3 of an egg.10

7 There is a statement in the Talmud which might seem to show that we are supposed to replicate the

measurements of the Talmud rather than to use the measurements of our own era: “Rabbi Elazar said: One

who eats chelev nowadays must record for himself the quantity, in case a future Beis Din will increase the

measurements (for which one is liable)” (Talmud, Yoma 80a). A similar ruling is found in the Yerushalmi:

“Rabbi Hoshea said: One who eats a forbidden food in our day must record the quantity, in case a later Beis

Din will arise and change the quantity (for which one is liable), and he will know how much he ate”

(Yerushalmi, Pe’ah 2a). This sounds like there is an absolute measurement of a kezayis, valid for all times and

places. Each Beis Din does its best to assess what this measurement is, but because it is possible that they are

mistaken, one must record the amount eaten in case a future Beis Din assesses matters more correctly.

Accordingly, it seems that the objective is to figure out the quantity used in the Talmud, not to follow the size

of olives in one’s own era! However, further analysis shows that this could not be the intent of the Talmud.

How is the person going to be recording the amount that he ate? There was no possibility of a person recording

it in terms of cubic centimeters or some other such absolute unchanging standard; and if such a standard had

existed, surely the Sages would have used it for their measurements! Instead, the intent of the Talmud is that he

is recording whether, for example, he ate the volume of a big olive, a medium olive, or a small olive. The

concern is not that the size of olives will change, but rather that the quantity for which one is liable will change

– is one liable for a big olive, a medium olive or a small olive. See Sdei Chemed, Ma’areches HaAlef, 34, s.v.


8 Mishneh Torah, Hilchos Eruvin 1:9.

9 Talmud, Shabbos 91a.

10 There is a potential difficulty with this inference, in reconciling it with an inference from two statements in

the Talmud. As we shall see later, the Talmud in one place states that a person can swallow food up to the size

of two olives, while elsewhere it states that a person can swallow food up to the size of an egg. These passages


It is important to note that our inference of Rambam’s view regarding olives does not tell

us anything as to the absolute size of a kezayis, only that it must be less than 1/3 of an egg

(which is, of course, true of a regular olive). However, this inference was later apparently

later interpreted to mean that Rambam was of the view that a kezayis is actually equal to

slightly less than 1/3 of an egg, and then to mean that he was of the view that a kezayis is equal

to about 1/3 of an egg.11 One can propose how this happened; since it was not known how

much less than 1/3 of an egg it was,12 nor is it convenient to quantify “less than 1/3 of an

egg,” the upper limit was taken as the bottom line and simplified to 1/3 of an egg. The

problem with this result is that when the process itself is forgotten, it is assumed that

Rambam’s position opposes the idea that a kezayis is much smaller than 1/3 of an egg,13

whereas the truth is that he does not oppose it at all.

Furthermore, from the fact that Rambam does not specify the size of a kezayis – whereas

he does specify the size of other quantities14 – one can presumably infer that his position was

that a kezayis is the size of an ordinary olive, and/or that it is up to each person to assess it on

their own, rather than to attempt to calculate the size of a Talmudic olive.15

indicate that an olive is half the size of an egg. How can this be reconciled with our inference that Rambam’s

position is that an olive is less than 1/3 of an egg? The Vilna Gaon (1720-1797) claims that the inference in the

Talmud that an olive is half the size of an egg is referring to a person swallowing an egg without its shell, but an

egg with its shell is three times the size of an olive (Biyur HaGra to Orach Chaim 486:1). This does not

necessarily mean that a shell changes an egg from being twice the size of an olive to being more than three times

its size (which is clearly not the case!). Rather, the point is that a hard-boiled egg without the shell is sufficiently

pliable that a person can swallow a whole one, just as a person can swallow two olives. But the olive is less than

a third the size of an egg with its shell. R. Chizkiya ben David DiSilva (1659-1698, author of Pri Chadash)

writes that the Talmud’s statement that the throat cannot hold more than two olives is imprecisely written, and

actually refers to an olive and a date (Pri Chadash, Orach Chaim 486:1). R. Yaakov Orenstein (author of

Yeshuas Yaakov) states that Rambam simply considers the Talmud’s statement to be disputed by the other

statement about the throat being able to hold an egg, and Rambam does not follow that view (Yeshuas Yaakov,

Orach Chaim 301). According to both these approaches, Rambam is indeed of the view that a kezayis is less

than 1/3 of an egg. R. Avraham Gombiner (c.1633-c.1683, author of Magen Avraham) suggests that according

to Rambam, when the Talmud rated an olive as being smaller than a dried fig, it was only referring to a small

olive, but an average olive is larger than a dried fig. It seems that R. Gombiner interpreted Rambam’s view as

being than an average olive is equal to half an egg. However, it does not seem that anyone else adopted this

understanding of Rambam.

11 See Mishnah Berurah 486:1 and R. Pinchas Bodner, The Halachos of K’zayis p. 25 note 27. R. Yaakov Yisrael

Kanievsky, in Shiurin Shel Torah 11 p. 70 notes that Rambam should not be misinterpreted in this regard.

12 R. Yosef Kotkovski argues that an olive must be significantly smaller than a dried fig; see Darkei HaChaim

(Petrikow 1884), Hilchos Borei Minei Mezonos 4, Chelki b’Chaim 3. However, R. Chaim Na’eh, Shiruin Shel

Torah p. 190 n. 24 disagrees.

13 See Rabbi Moshe Petrover, “The Size of the Kezayis for Eating Matzah: A Clarification of the View of the

Chazon Ish” (Hebrew), Moriah 5754, p. 106.

14 Commentary to the Mishnah, Eduyos 1:2, Keilim 2:2; Mishneh Torah, Eruvin 1:12.

15 R. Hadar Margolin, “A Clarification of the View of the Chazon Ish,” (Hebrew) Moriah 3-4 (5753) p. 100.


For a long time, it was thought that Rambam is the only authority from Sepharad whose

view regarding the size of an olive can be assessed. Recently, however, two more sources came

to light. Rashba (R. Shlomo ben Aderes, Spain, 1235-1310), in discussing a different topic,

mentions that fifteen eggs are “much” more than sixty olives; hence, an olive is much less

than ¼ the size of an egg.16 Ritva (R. Yom Tov ben Avraham Asevilli, Spain, 1250-1330), in

a newly published manuscript, states that a dried fig is the volume of “several” olives.17 Since

he is also of the view that a dried fig is 1/3 the size of an egg, this means that an olive is

around 1/9 the size of an egg.

It is important to note that none of these authorities set out to specify the size of an olive,

or grappled with statements concerning its size (as we shall see to have been the case with the

Rishonim of Ashkenaz). Our knowledge of their position regarding the size of an olive, or

the upper limit of the size of an olive, is inferred from statements of theirs made in a

different context. The clear implication is that they took it for granted that a kezayis is the

size of an ordinary olive.

By the same token, the fact that most authorities of this period did not make any

statement relating to the size of an olive does not mean that we have no idea as to what their

view was. For someone for whom a kezayis is obviously an olive, there is no need to make any

comment about it. One can assume that the reason why they did not comment on the size of

a kezayis is that it was obvious to them that a kezayis is kezayis.

The Rishonim of Ashkenaz

It is in Ashkenaz that we find the olive beginning to evolve with the statements of the

Rishonim themselves (as opposed to with later mistaken inferences regarding the Rishonim).

The Rishonim of Ashkenaz translated the size of an olive into a proportion of an egg, but

they gave different quantities. This was based on differing resolutions of various passages in

the Talmud. In one place, the Talmud states that a person can swallow food up to the size of

two olives:

The Sages evaluated that the throat cannot hold more than two olives. (Talmud, Krisus 14a)

Elsewhere, the Talmud states that a person can swallow food up to the size of an egg:

The Sages evaluated that the throat cannot hold more than a chicken’s egg. (Talmud, Yoma


These passages indicate that an olive is half the size of an egg. However, in a third place, a

different conclusion emerges. The Talmud (Eruvin 82b) discusses the amount of food

required for an eruv. Two of the views cited express their opinion in terms of kabin, which in

turn can be expressed in quantities of eggs (since 1 kav is 24 eggs):

16 Rashba, Mishmeres HaBayis 4:1.

17 Ritva to Shabbos 76b; printed at the back of the Mossad HaRav Kook edition.


 Rabbi Shimon: Two meals are 2/9 of a kav, which is 5 1/3 eggs.

 Rabbi Yochanan ben Beruka: Two meals are ¼ of a kav, which is 6 eggs.

Elsewhere, the Talmud states that two meals are equal to 18 dried figs.18 Now, as we saw

earlier, an olive is known to be smaller than a dried fig.19 This results in the following


 Rabbi Shimon: Two meals = 5 1/3 eggs = 18 dried figs; thus 1 olive is less than 3/10

of an egg

 Rabbi Yochanan ben Beruka: Two meals = 6 eggs = 18 dried figs; thus 1 olive is less

than 1/3 of an egg

How are all these sources to be reconciled?

Ri (R. Yitzchak ben Shmuel the Elder of Dampierre, 12th century) concludes from the

passages concerning swallowing that an olive is half the size of an egg.20 As for the passage in

Eruvin, he states that we do not follow the views of either Rabbi Shimon or Rabbi Yochanan

ben Beruka; thus that discussion has no inferences for the size of an olive. Ri’s view that an

olive is half the size of an egg was adopted by R. Mordechai b. Hillel (Germany, 1240-

1298),21 R. Alexander Zusslein HaKohen (France/Germany, d. 1348)22 and R. Yaakov Weil

(Germany, 15th century).23

Rabbeinu Tam (Yaakov ben Meir Tam, France, c. 1100–c. 1171), on the other hand,

states that we rule in accordance with Rabbi Shimon, an olive must be less than 3/10 of an

egg (although he does not propose how much less). Regarding the statements concerning

swallowing which indicate that an olive is half the size of an egg, Rabbeinu Tam suggests that

the foods are in different states, which affects the volume that can be swallowed. An egg is

much easier to swallow than an equivalent volume of olives. Because olives are hard and

contain pits, only two can be swallowed at a time, even though they are much smaller than a

single whole egg, which can be swallowed in one gulp.

In a variant of this approach, Tosafos Yeshanim reconciles it by suggesting that they are

descriptions of different types of swallowing. When the Talmud spoke of a person being able

to swallow two olives, it was referring to what a person can swallow in the course of ordinary

18 Talmud, Eruvin 80b.

19 Talmud, Shabbos 91a.

20 Tosafos to Yoma 80a s.v. Veshiaru. (The same inference is apparently made by Sefer HaChinnuch, mitzvah

313.) R. Chaim Na’eh argues that Ri must mean that an olive is slightly less than half the size of an egg; see

Shiurei Torah, p. 192.

21 Mordechai, End of Pesachim, Seder Leil Pesach.

22 Sefer Ha-Agudah, Eruvin 82b.

23 Mahari Weil 193.


eating. However, when it spoke of a person being able to swallow a whole egg, it was

referring to the maximum that a person can force themselves to swallow.

Rabbeinu Tam’s approach does not draw any conclusions as to the absolute size of an

olive, only that it must be less than 3/10 of an egg (which is, of course, true). However, this

view was later apparently interpreted to mean that it equals slightly less than 3/10 of an egg.

(The ratio of 3/10 was later slightly expanded to 1/3, for reasons that are unclear; perhaps as

it is a simpler quantity to assess.)

Reasons for the Ashkenazi Expansion

Why did the Ashkenazi authorities relate the size of an olive to the size of an egg, especially

since, according to Ri, this results in the error of considering an olive to be half the size of an

egg? Why did they not follow the position of the Sephardic authorities, that a kezayis is the

size of an olive?

The answer is that the Ashkenazic authorities never saw an olive. Olives do not grow that

far north; they only grow in the Mediterranean region. In medieval Europe, transporting

commodities was expensive, and was only done with foodstuffs for which there was high

demand. Many food items were simply unknown in some regions. In an early 15th century

Bavarian translation of an Arabic pharmacopoeia, the German translator has to explain to his

readers what various foodstuffs (such as sesame seeds and pistachio nuts) actually are.24 In

northern Europe, unlike with the Mediterranean region, olives were not part of the menu

and they were virtually unheard of.25 Only olive oil was imported, and even that was very

expensive and only used by the wealthy. In the oldest German cookbook entered in a

parchment codex in Wuerzburg around 1350, no olives are mentioned, and oil (which may

not even be olive oil) appears only once.26

Thus, the reason why the Rishonim of Ashkenaz greatly increased the size of a kezayis is

simply due to the fact that they were unfamiliar with olives. They were forced to attempt to

calculate the olive’s size based on deductions from various statements in the Talmud. R.

Eliezer b. Yoel HaLevi (Germany, c.1140-c.1225) explicitly acknowledges that they knew

that they were missing direct observation, and that they therefore decided to err on the side

of caution:

And wherever a kezayis is required, the food should be measured generously, since we are not

familiar with the measurement of an olive, and so that the blessing should not be in vain.

(Ravyah, Berachos 107)

24 Prof. Melitta Weiss Adamson of the University of Western Ontario, personal communication.

25 Melitta Weiss Adamson, Food in Medieval Times, pp. 29-30; John Ayto, The Glutton’s Glossary: A Dictionary

of Food and Drink Terms, p. 198.

26 Melitta Weiss Adamson, personal communication.


Another revealing statement comes from one of the Rishonim from the generation of the

Rosh. He was addressing a question that arises from Hillel having eaten a kezayis each of

matzah, maror and charoses simultaneously, which was problematic for those who believed

that two olives was the maximum that the throat can hold. This rabbi pointed out that based

on what he had seen on his travels to Israel, there is no difficulty in this:

To me there is no difficulty, for I saw olives in Israel and Jerusalem, and even six were not as

large as an egg. (Piskei Rabboseinu SheBeAshkenaz, Moriah2:3)

We thus see that the Rishonim of Ashkenaz themselves acknowledged that, living in

Ashkenaz, they had not seen olives.

This in turn answers the following question. Even given the view of Ri that the Talmud

dictates that an olive is half the size of an egg, why assume that this means that olives back

then were larger? Why not explain instead that eggs back then were smaller? This question is

especially potent since the fundamental measurement for the mitzvos of eating is in terms of

olives, not in term of eggs.27 So why pick a different unit of measurement as the barometer?

The answer is that for Ri, and many others after him, there was no first-hand experience

with olives. On the other hand, they were familiar with eggs. Since the olive was the food

whose size they didn’t know and were trying to determine, it was natural to assume that the

egg of the Talmud was the same as their egg, and the olive was half that size. There was no

reason to assume otherwise. Today, however, when we know that olives are not and were not

that big, and we also know that eggs were formerly smaller (as we shall later discuss), there is

no reason why, even if we are reconciling the Talmudic statements about olives and eggs, this

should lead us to the conclusion that olives must have been bigger.

There is a further point to consider in evaluating the adoption of the view of the Rishonim

of Ashkenaz. Ri and Rabbeinu Tam did not deliberate over the size of a kezayis in the context

of issuing a practical ruling, but rather as part of an attempt to resolve a conflict in the

Talmud. It is far from clear that they were of the view that for one’s own obligation, one

always needs to replicate the size of a Talmudic olive. They may well have adopted the view

of the Geonim, that if one has access to olives, one should follow the size of an olive in one’s

own time and place. Perhaps initially the rulings of the Ashkenazi Rishonim were adopted

because nobody had anything better to go with. And even later when people did have

alternatives, the statements of these authorities had already been accepted as formal rulings

regarding what size a kezayis should be.

27 See Moshe Koppel, “The Sages Evaluated” (Hebrew), Higayon 5 pp. 55-62 for a valuable discussion of the

concept of primary and secondary units of measurement.


The Shulchan Aruch’s Ambiguous Ruling

In the Shulchan Aruch of R. Yosef Karo (Spain/Israel, 1488-1575), the chapter concerning

the size of a kezayis contains only one section, and it is probably the shortest chapter in the

entire work. Its wording is intriguing:

The amount of a kezayis – some say that it is around half of an egg. (Shulchan Aruch, Orach

Chaim 486:1)

This ruling is surprising in that R. Karo does not rule what a kezayis is; he just notes what

“some say” it is. This is widely understood to mean that he is citing this view as a stringency,

but that he himself is of the view that it is smaller.28 But how much smaller? An inference can

be drawn from a ruling elsewhere,29 where in discussing the quantity of two meals for an

eruv, he follows the ruling of Rabbi Yochanan ben Beruka in quantifying this as being the

size of six eggs, and equates this with 18 dried figs. The inference is that a kezayis, which is

smaller than a dried fig, must be less than 1/3 of an egg.

Many therefore state that while the Shulchan Aruch records the stringent view of a kezayis

being ½ an egg, it rules a kezayis to be slightly less than 1/3 of an egg. However, a careful

analysis shows that this is not the case. All we can derive from the discussion regarding eruv is

that the maximum size must be somewhat less than 1/3 of an egg; we still have no inference

as to how much less it is. In theory it is still entirely possible that the view of R. Yosef Karo

was that a kezayis is the size of a regular olive. One might claim that since he quotes the view

of Ri, this indicates that he was working within the Ashkenazi approach, which therefore

makes it unlikely that he himself viewed it as being a much smaller quantity. But on the

other hand, the fact that he does not specify what he considers to be the normative view

(only quoting what “some say” it to be) could indicate that he considered the normative view

to be obvious – namely, that a kezayis is the size of an olive. This is especially likely in light of

the fact that R. Yosef Karo himself, unlike Ri and Rabbeinu Tam, would have been familiar

with olives.

Whichever way one understands R. Karo, it seems that the Shulchan Aruch denotes a

critical point of transition; in recording the explicit view of the Ashkenazi Rishonim and

being silent about the silent Sefardic Rishonim, it thereby strengthens the impression that Ri

was the mainstream view and Rabbeinu Tam, interpreted maximally, was the alternative.

Reasons for the Ashkenazi Adoption

When later authorities who were familiar with normal olives nevertheless followed the

positions of the Ashkenazi Rishonim, they were implicitly adopting the notion that olives of

28 Responsa Vayomer Yitzchak, Orach Chaim 8. See too Ner Mitzvah 17 and Benish, Middos VeShiurei Torah

p.254 note 111 and p.527.

29 Orach Chaim, 368:3; 409:7.


ancient times were larger. Some were explicit about this. For example, in adopting the view

of Ri that an olive is half the size of an egg, Rabbi Shlomo Luria (Poland, 1510-1574) writes

as follows:

It is a received tradition in our hands from the Tosafists that an olive is half the size of an egg.

And even though in our time we see with our eyes that the size of an olive is much smaller

than half of an egg, this is not surprising, for in the days of the Sages the fruit of the Seven

Species were unusual in their size, and they have since changed. (Yam Shel Shlomo, Chullin

3:86, also cited in Taz, Yoreh De’ah 44:12)

R. Yaakov Yisrael Kanievsky (1899-1985) similarly writes that although the strain of olives

found today is the same as that mentioned in the Mishnah, “it has become weak and the

fruit have become smaller”30 (however, as we shall see, he was not of the view that the earlier

larger size is to be replicated).

We have seen that the botanical and archeological evidence shows that olives were always

the same size as they are today. But the belief that they used to be larger was consistent with

a general worldview of the “decline of generations” – that the world used to have a golden

age in which people, animals and plants were superior in every way to those of today. The

most radical application of this concept to the size of the kezayis was that of R. Yechezkel

Landau, which we shall now explore.

The Alleged Egg Shrinkage

R. Yechezkel Landau (Poland/Bohemia, author of Noda B’Yehudah, 1713-1793) created a

famous revolution in the determination of halachic quantities.

Since a mitzvah performed at a designated time is precious, and on this night we are required

to eat a kezayis of matzah and maror and to drink a Torah-determined revi’is of the four cups, I

need to clarify my view concerning the size of a kezayis and a revi’is, which I concluded by way

of proofs is not in accordance with the words of the Shulchan Aruch. For in truth it is clear in

the Shulchan Aruch, chapter 486, that the size of a kezayis is half the size of an egg. However, it

is clear to me by way of measurement that with the eggs that we have in our day, a whole egg

of our day is only half the size of an egg that was used for the Torah quantities… (Tzlach,

Pesachim 120a)

R. Landau proceeds to describe his measurements which resulted in a ratio of thumbs to

eggs that differed from the ratio that results from the Talmud. He continues:

And against our will we see that things have changed in our time; either thumbs have grown,

and they are bigger than the thumbs of the days of the Tannaim, or the eggs have shrunk and

in our day they are smaller than the eggs of the era of the Tannaim. And it is known that the

generations progressively decline, and it is therefore impossible that our thumbs should be

larger than the thumbs in the day of the Sages of the Mishnah. (Ibid.)

30 Shiurin Shel Torah p. 8.


R. Landau therefore concludes that the eggs mentioned in the Talmud were larger, and his

calculations enable him to conclude that they were twice as large:

It is therefore necessarily the case that the eggs of our day are smaller… and since it has

become clear that our eggs are smaller by half, therefore the size of a kezayis, which is

(originally) half an egg, is as the size of a whole egg of today. And thus I evaluate the eating of

matzah and maror… (Ibid.)

Many authorities adopted the view of R. Landau.31 In some cases, they only did so vis-à-vis

measurements dependent on thumbs, such as challah and revi’is, but not vis-à-vis

measurements dependent on eggs, such as a kezayis.32 But others adopted it for eggs (and

calculated the volume of today’s eggs as being 50cc and that of eggs in ancient times as being

100cc) and thus for the kezayis too, such as R. Yisrael Meir Kagan (Poland, 1838-1933) in

the Mishnah Berurah:

…And with regard to the ruling, with a Biblical mitzvah, such as the positive commandment

of eating matzah, one should be stringent and eat at least the volume of half an egg… and

know that what the Shulchan Aruch wrote about a kezayis being half an egg is not a final

statement in our day, for some of the Acharonim proved that the eggs found in our day are

much smaller, as much as half, of the eggs that were in ancient times, with which the Sages

gave their measurements. Accordingly, wherever the required quantity is half an egg, one needs

to measure this as a whole egg of our day…. According to this, in our day a person is obligated

to eat matzah of the size of an egg… (Mishnah Berurah, 486:1)

Note the three steps taken in the Mishnah Berurah which result in this gigantic

measurement of 50-60cc for a kezayis.33 First is that, again, we have a presumption that the

obligation is to consume the presumed size of an olive of ancient times, not an olive of today

– which is in contrast to the view of the Geonim. Second is that he states that one should

follow the stringent view that a kezayis is half an egg – which we have seen to based on the

Ashkenazi Rishonim not being familiar with olives. Third is that he claims that the

Acharonim proved that eggs have shrunk – which we shall now demonstrate to be incorrect.

Evaluating the Alleged Egg Shrinkage

From a rationalist perspective, Rabbi Landau’s claim that the relative sizes of thumbs and

eggs has changed, and that it must be that eggs have shrunk rather than that thumbs have

grown, is problematic on several counts. First, we know that thumbs have indeed grown;

second, we know that eggs have not shrunk. As for his difficulty regarding the apparently

changed ratio, there are other solutions. Let us explore these three points in turn.

31 Vilna Gaon, Maaseh Rav 105; R. Akiva Eiger, Responsa R. Akiva Eiger HaChadashos 39; Beis Ephraim, Rosh

Ephraim, Kuntrus HaTeshuvos 16.

32 Chasam Sofer, Responsa Chasam Sofer Orach Chaim 127, 181; Responsa Gidulei Taharah 1; R. Chaim of

Volozhin, as per Shaarei Rachamim 165 and at the end in Minhagei HaGraCh 51.

33 The range of 50-60cc is due to the different assessments of the volume of a contemporary egg.


R. Landau’s belief that people could not have grown larger was based on his understanding

of the decline of generations. The concept of a “decline of generations” sets traditionalists

squarely against rationalists, depending on how it is defined.34 But even if one accepts the

notion of a general spiritual and/or intellectual decline, R. Landau’s extrapolation to a

physical decline is quite a leap. Furthermore, evidence from archeology shows that between

the Talmudic era and the era of R. Landau, mankind did not become any shorter; in fact,

beginning in the 18th century, people began to grow taller.

With regard to eggs, empirical research shows that eggs in ancient times were far from

twice the size of today, which measure 50-60cc. In fact, eggs in ancient times were actually

smaller than those of today. There are several independent lines of evidence for this.

One argument, concerning the size of eggs in Rambam’s era, is based on his relating the

size of an egg to certain Arab coins. R. Chaim Na’eh (1890-1954) used this technique to

calculate the size of eggs in Rambam’s day as being 57.6cc.35 However, R. Chaim Beinish

states that the coin of Rambam’s era was of a different weight than that known to R. Chaim

Na’eh, and it results in an egg size of 49cc.36 The fact is that the size of Arab coins varied

tremendously in different places and eras, which makes any such calculation questionable,37

but there are many other more reliable forms of evidence.

The Talmud records that R. Yehudah HaNasi measured a vessel called the modia as

containing the volume of 217 eggs.38 We know that the modia was one-third the size of a

standard Roman measuring vessel called the amphora, and we are able to measure extant

amphoras at one cubic Roman foot, which equals 25.79 liters.39 This means that the eggs of

R. Yehuda HaNasi’s era measured 39.6cc.40

34 Menachem Kellner, in Maimonides on the Decline of Generations, argues that Maimonides did not subscribe

to this doctrine as a general pattern.

35 Shiurei Torah (Jerusalem 1947) pp. 111-120.

36 Middos VeShiurei Torah 13:7 and 30:6.

37 See, for example, Stefan Heidemann, “The Merger of Two Currency Zones in Early Islam. The Byzantine

and Sasanian Impact on the Circulation in Former Byzantine Syria and Northern Mesopotamia,” Iran 36

(1998) pp.95-112.

38 Eruvin 83a. The Talmud further states that R. Yehudah HaNasi had a tradition that this vessel held the

volume of 207 eggs of the size that existed at the time of the Revelation at Sinai, and attributed the slight

difference of about 5% to the natural change in egg size over so many years.

39 Lesley Adkins, Roy A. Adkins, Handbook to Life in Ancient Rome (Oxford University Press 1998), p. 314.

40 Greenfield, “Has the Egg Volume Really Decreased in the Thousands of Years since Matan Torah?”

(Hebrew) BDD 16 pp. 91-94 uses the argument from the amphora to produce a volume of 43cc; I am unclear

as to how he arrived at this result.


Professor Yehudah Feliks examined eggs that were preserved whole in the volcanic

destruction of Pompeii two thousand years ago, and states that they were “around the size of

the small Arab eggs of our time,” which he defines as 41.4cc.41

My own research indicates that the eggs of ancient times were considerably smaller than

those of today. Domestic fowl have been selectively bred for larger eggs, which would mean

that eggs used to be smaller. Furthermore, we know that the domestic chicken was

domesticated from the red junglefowl several thousand years ago, and its eggs are very small,

only 32.1cc.42 Assuming a gradual increase to the size of today’s eggs, this would indicate

that two thousand years ago, eggs were around 40cc. In addition, records show that the

chickens used in Roman Italy were able to incubate twice as many eggs at a time than

chickens of today are able to do, which shows that their eggs were much smaller.43

We thus see that, contrary to R. Landau’s assertions, neither thumbs nor eggs are smaller

than those of ancient times. As for R. Landau’s question regarding the apparently changed

ratio of eggs to thumbs, other solutions have been presented.44 In fact, the eggs of ancient

times were slightly smaller than those of today; according to my research, around 40cc. This

may also assist in solving the difficulty in the olive-egg ratio implied by the Talmud’s

statement regarding a person being able to swallow a whole egg or two olives.

Recent and Contemporary Poskim

The Mishnah Berurah, as noted above, rules that for Biblically-ordained mitzvot one

should follow R. Landau’s conclusion that the eggs of ancient times were twice the size of

today’s eggs. But most authorities rejected the notion that one should double the size of the

measurements. R. Elchanan Wasserman (1875-1941) noted that based on the words of R.

Hai Gaon (that we cited early in this study), the size of eggs and olives does not change.45 R.

Chaim Na’eh, a prominent rabbinic authority in Jerusalem, compiled an extensive study of

halachic weights and measures. As noted above, he concluded that R. Landau had erred

about eggs in ancient times being larger than those of today. On the other hand, R. Avraham

Yeshayah Karelitz (the “Chazon Ish,” 1878-1953) wrote that regardless of whether it is

41 Feliks, Kelai Zera’im VeHarkavah, p. 184 note 5.

42 Gardiner Bump, Special Scientific Report 62: Red Junglefowl and Kalij Pheasants (Washington DC: U.S. Fish

& Wildlife 1962), gives the dimensions of the egg as 4.53×3.44cm (compare large chicken eggs at around

5.7×4.4cm). Using the calculation V = (0.6057-0.0018B)LB2 in which L is the egg length, and B is the egg

maximum breadth, the volume is 32.14cc. The calculation is from V.G. Narushin, “Egg Geometry Calculation

Using the Measurements of Length and Breadth,” Poultry Science 84:3 (March 2005) pp. 482-484.

43 George Jennison, Animals for Show and Pleasure in Ancient Rome (Manchester University Press 1937), p. 106,

citing Pliny, Varro and Columella.

44 Avraham Greenfeld, “Middah Kenegged Middah,’ Moriah 10 (5742). This provoked heated responses; see, for

example, R. Kalman Kahana, “Lo Zu HaMiddah,” Moriah 11 (5743) 11-12 pp. 67-76.

45 Kobetz Shiurim 2:46.


factually true that eggs have halved in size, since this was the assessment of R. Landau and

others, and it has become widely accepted, it is as though it has been established by a Beis

Din for all Israel and is binding.46

Putting aside the issue of whether one assumes that eggs used to be twice the size, what

proportion of an egg is a kezayis? R. Avraham Danzig (Chayei Adam, 1748–1820)47 rules that

a kezayis is half an egg, as does R. Yechiel Michel Epstein (Aruch HaShulchan, 1829-1908).48

R. Yisrael Meir Kagan (Mishnah Berurah, 1838-1933) rules that when following the

stringent view (e.g. for Biblically-ordained mitzvos such as eating matzah at the seder) one

should follow the stringent view of Ri cited in the Shulchan Aruch that a kezayis is half an

egg. R. Chaim Na’eh also states that for such mitzvos one should follow the view that a

kezayis is half an egg and specifies this as being 28.8cc.49

Partly because R. Chaim Na’eh was the first to address the topic comprehensively, and

partly due to his stature, many of his conclusions became widely accepted. Thus, the most

widespread view today concerning a kezayis is that it measures 28.8cc. This is based on

following the stringent view of Ri along with the egg calculation of R. Chaim Na’eh. This

view has been advanced by authorities such as R. Shlomo Zalman Auerbach50 (1910-1995)

and R. Yosef Shalom Elyashiv.51 It is also the standard adopted for R. Pinchas Bodner’s

popular work The Halachos of Kezayis.

A less widespread view is that a kezayis is 17cc. This is based on following what is

presumed to be the Shulchan Aruch’s own view, following Rabbeinu Tam and Rambam, that

a kezayis is slightly less than 1/3 of an egg. The aforementioned recent and contemporary

authorities also propose that one may rely on this “leniency” for rabbinically-mandated

requirements, especially when it is difficult to rely on the larger quantity, such as when eating

maror at the seder.

Yet even in recent times there were still those who maintained that a kezayis is the size of

an ordinary olive. As noted at the beginning of this study, R. Chaim of Volozhin is one such

example. When the manuscripts of the Geonim came to light around 130 years ago,

revealing their view that one is to follow the size of the fruit of one’s own era, some adopted

this position. Rabbi Akiva Yosef Schlesinger (1835-1922) writes:

46 Chazon Ish, Kuntrus HaShiurim 39:6. However, as we shall soon see, this was far from R. Karelitz’s last word

on the topic.

47 Chayei Adam 50:12

48 Aruch HaShulchan 202:5 and 486:1.

49 See R. Chaim Na’eh, Shiurei Torah, 3:12 p. 193.

50 Halichos Shlomo vol. II p. 90.

51 Kobetz Teshuvos II:30.


The measurement of an egg is not found in the Torah; rather, it says, “a land of… olives” etc.,

that all its measurements are like olives, and this olive was only rated by the measurement of

an egg for those who did not have olives. But not for us, who see the olive in front of us –

there is no need to push aside the ikkar for the tafel. (Tel Talpios, Shevat 5661 p. 103)

R. Yitzchak Elchanan Spektor (1817-1896) likewise stated that the Geonic view is the

fundamentally correct approach.52 Apparently following the same approach, R. Avraham

Bornstein (1838-1910, author of Avnei Nezer) maintained that the kezayis is the size of an

ordinary olive and did not see any basis for expanding it to half the size of an egg, even as a

stringency.53 R. Yaakov Yisrael Kanievsky (1899-1985) states that we follow the size of

today’s olives regardless of the presumed greater size of olives in ancient times.54

The same was stated by R. Avraham Yeshayah Karelitz (the “Chazon Ish,” 1878-1953),

but there is much confusion and dispute with regard to his ultimate ruling. At one extreme,

in some editions of his chart for the measurements of shiurim, he gives a measurement of

50cc for a kezayis. This view reflects his acceptance of R. Landau’s radical expansion of the

kezayis, based on the assumption that eggs have halved in size. Elsewhere he states that if one

wishes to be stringent, one can follow the view of the Ri that a kezayis is half the volume of

an egg.55 The combination of these positions results in a kezayis measuring 50cc – half the

presumed volume of an egg in ancient times. But at the other extreme, Chazon Ish writes

that the essential concept of kezayis is that one follows the dimensions of olives that one

observes and one does not need to concern oneself with calculating if olives in an earlier era

were larger.56 An intermediate position emerges from an account by R. Chaim Kanievsky

that at the seder of the Chazon Ish, he allocated portions that were 17cc in size.57 Reconciling

all these conflicting statements and accounts is difficult, and varying conclusions have been

drawn.58 Suffice it to say that while it is “common knowledge” that a “Chazon Ish shiur” is

the largest measurement for a kezayis, the truth is far more complex and it seems that he

acknowledged that fundamentally a kezayis is the size of an ordinary olive.

52 Quoted by R. Dov Aryeh Rotter, Tel Talpios, 5661 p. 71.

53 Testimony recorded in Middos VeShiurei Torah, p. 510 note 111.

54 Shiurin Shel Torah 11 p. 71. Puzzlingly, however, he rates the size of a contemporary kezayis as being slightly

less than a third of the size of an egg (17-19cc).

55 Ibid. 39:17.

56 Chazon Ish, Shiurin Shel Torah 11; Letters, 194.

57 Related by R. Hadar Margolin, “A Clarification of the View of the Chazon Ish,” Moriah 3-4 (5753) p. 102.

58 See R. Hadar Margolin, “A Clarification of the View of the Chazon Ish,” Moriah 3-4 (5753) pp. 99-103 and

R. Moshe Petrover, “The Volume of a Kezayis for Eating Matzah – A Clarification of the View of the Chazon

Ish,” Moriah 7-9 (5754) pp. 106-109. See too Menachem Friedman, “The Lost Kiddush Cup: Changes in

Ashkenazi Haredi Culture – A Tradition in Crisis,” in Jack Wertheimer, ed., The Uses of Tradition: Jewish

Continuity in the Modern Era, (New York: JTS/Harvard University Press 1992), pp. 175-186.


The Canonization and Conservatism of Halacha

Given the botanical/archeological evidence and the discovery of the Geonic manuscripts,

both of which independently show that there is no reason to exceed the size of a

contemporary olive, why is it that there are so many who rule otherwise? Of course, in the

Charedi world, it is common to treat scientific evidence with suspicion. But there is another

reason why many adopt the position that a kezayis is much larger than olives are today: the

nature of the halachic process in general. The question of whether halachic practice should

be changed in light of new empirical data or newly discovered manuscripts is complex. There

is a strong case to be made for saying that halachah follows its own protocols and should not

be re-evaluated in light of new data, even if it seems clear that the halachah is in opposition

to objective facts.59 A fundamental value in halachah is creating and preserving stability. If a

halachah has become canonized, then it ought not to be changed.

However, this case has two factors that make it easier to rely on the new data if one wishes

to do so. One is that upon closer inspection, the halachah is not at all canonized in the way

that it is commonly assumed. The primary authorities who are assumed to have ruled that a

kezayis is 1/3 of an egg, Rambam and Rabbeinu Tam, in fact did not say any such thing; it is

only the upper limit of a kezayis that can be inferred from them. The same may well also be

true of the Shulchan Aruch.

The second factor is that this is not a case where the halachah was ruled unequivocally in

one direction. There have always been those, such as Rabbi Chaim of Volozhin, the Avnei

Nezer and others who maintained that the kezayis is the size of an ordinary olive. Even the

Chazon Ish acknowledged that this is the fundamentally correct position. It is thus an

established halachic view, which is merely being given greater weight in light of new

discoveries of manuscripts and new data concerning olives and eggs.

Yet while this justifies someone who wishes to evaluate a kezayis as being the size of a

regular olive, we can still understand why others do not take this approach. Even if a

halachah has not been unequivocally canonized, it can still be sufficiently entrenched that it

becomes problematic to change. Judaism is a traditionalist way of life, and traditionalist

religions are inherently and neccessarily conservative.

Further Growth – Weight Replaces Volume

Another factor that is sometimes involved in the expansion of the kezayis is the change

from measuring volume to measuring weight. There is no doubt that the kezayis is supposed

to be a measure of volume, and there are several independent lines of evidence for this. First

is a Mishnah which appears in a chapter discussing how to evaluate the quantity of a food:

59 See Natan Slifkin, Sacred Monsters, pp. 362-367.


An airy loaf is evaluated as it is. If there is a hollow inside, it is compressed. (Mishnah, Uktzin


The Mishnah tells us that an airy loaf, which is much less dense than other foods, is

nevertheless evaluated as it is, and not compressed into a density comparable to other foods.

Only if there is actually a distinct large single pocket of air is it to be removed from the

equation. This clearly means that it is volume being measured rather than weight. This

position, emerging from this Mishnah, is found in a number of halachic authorities.60

We also have an explicit statement from the Geonim that kezayis is a measure of volume

rather than weight:

And that which you asked regarding the measure of… a kezayis etc., surely these are shiurim

(designated quantities), and how can there be a designated quantity for a designated quantity?

And if you suggest to give a weight, our rabbis did not explain things in terms of their weight,

and the Holy One does not exact with us in weight. (Teshuvos HaGeonim 268)

Finally, the discussions of Rabbeinu Tam, Ri and others regarding the calculation of a

kezayis, which were based on reconciling various statements in the Talmud concerning how

much a person can hold in his throat, only make sense if the discussion is regarding volume.

Yet we do find several later halachic authorities stating a weight measure for an egg, a kezayis,

and so on. R. Yaakov Chaim Sofer (1870-1939) cites several such authorities, and affirms

that this is widespread custom amongst all God-fearing Jews.61 He concludes that since an

egg weighs 18 drahms and a kezayis should be half an egg, a kezayis is 9 drahms. But how are

we to understand these halachic authorities in light of the clear positions in the Mishnah,

Geonim and Rishonim that we cited above?

There are two reasons why we nevertheless find some halachic authorities prescribing

measures of weight rather than volume. One is that it is sometimes more convenient to

prescribe quantities in terms of weight, especially since it is less likely to cause measuring

errors. For this reason, some halachic authorities converted measures of volume to measures

of weight. But this does not mean that they considered the halachic requirement to be

essentially one of weight. In fact, it is pointed out that R. Yaakov Chaim Sofer, as well as all

the halachic authorities that he cites, themselves make it clear elsewhere that halachic

measures are all volume rather than weight.62

The second reason why some convert the measurement to weight is that, as we have seen

in the Mishnah, large air pockets are not to be included in the calculation. However it is

60 Rama, Orach Chaim 486:1; Magen Avraham 486:1; Chida, Machzik Berachah 486:2; Mishnah Berurah 486:3.

See too Shulchan Aruch, Orach Chaim 456:1; Chazon Ish, Orach Chaim 39:17.

61 Kaf HaChaim 168:46, citing Bnei David, and Pesach HaDvir, who in turns cites several others.

62 R. Eliyahu Topik, Responsa Kol Eliyahu, Orach Chaim 30, p. 137, pointing to Kaf HaChaim 456:10, Pesach

DaDvir, Kisei D’Chayay 196a, and others; Chida, Machzik Berachah 486:2. A similar point is made by R.

Chaim Na’eh, in Shiurei Torah 1:1, pp. 71-72.


difficult to draw the line between a large air pocket and a small one. R. Chaim Na’eh

therefore rules that any visible air pocket is not to be included in a volume measurement.63

Since it is difficult to calculate the volume of a food item without any air pockets, a weight

measure was sometimes substituted. It should be stressed, though, that R. Chaim Na’eh

himself explicitly stated that the essential definition is one of volume, and that he was only

converting it to weight due to this uncertainty and subsequent stringency.64

However, R. Chaim Na’eh’s stringency in this regard was widely rejected. Contemporary

halachic authorities are emphatic that the kezayis is to be measured by volume, not weight.65

Nevertheless, in popular discourse, a kezayis is often defined in terms of weight – specifically,

30 grams. It is this that leads to the greatest quantity of matzah designated as a kezayis: the

labeling on certain machine-made matzah stating that one whole matzah equals a kezayis.


At the beginning of this study, it was noted that logically, in order to reach the conclusion

that a kezayis is much larger than olives are today, two separate positions must both be taken:

First, that olives of ancient times were much larger, and second, that we are obligated to

follow the size of ancient olives rather than the olives of today.

The first is refuted by empirical evidence. We have living trees from the Talmudic era,

which produce olives that are exactly the same size as olives from the trees of our own era,

and we have olive pits from ancient times that are similar to those of today. Furthermore,

there is no testimony otherwise in any source in the Talmud or Rishonim (contrary to

popular belief). In fact, there is testimony from some of the Rishonim that olives were the

same size as those of today. Alleged indications from inferences regarding eggs having been

larger are likewise disproved by evidence that eggs of ancient times were actually smaller.

The second position, that we are obligated to follow the size of ancient olives, was assumed

by many authorities, but it is explicit in the Geonim, implicit amongst many Rishonim and

acknowledged by several recent authorities that there is no such obligation.

An olive measures 4-6cc. How did it arise that virtually all halachic authorities are ruling

that a kezayis is at the very least 17cc, and most are ruling that it is in the region of 28cc or

even 50cc and more? We have seen that a combination of seven factors was involved:

 As some of them explicitly admit, the Rishonim of Ashkenaz were working with the

basic disability of not being familiar with olives. In one case this led to mistakenly

63 Shiurei Torah pp. 182-184.

64 Shiurei Torah, 1:1, pp. 71-72. In a subsequent work, Shiurei Tziyon p. 18, he himself expressed reservations

about his innovation.

65 R. Avraham Yeshaya Karelitz, Chazon Ish, Orach Chaim 39:17; R. Tzvi Pesach Frank, Chag Ha-Asif p. 316;

R. BenZion Abba-Shaul, Responsa Ohr LeTziyon p. 124; R. Ovadiah Yosef, Chazon Ovadiah, vol. II p. 518.


interpreting the Talmud to mean that an olive is half the size of an egg, and in

another case, it led to only being able to calculate an upper limit for an olive’s

possible size.

 The Rishonim of Sefarad, who were familiar with olives, never saw a need to discuss

their size. Their silence on the matter led to a fundamentally misleading situation:

from the discussion in the period of the Rishonim, the impression arises that there is

a divide between those who rate it as measuring 1/3 of an egg and those who rate it

as ½ an egg. Thus it was those who were not familiar with olives, and thereby

increased its size, who formed the framework for subsequent halachic discussion.

 The view that an olive must be less than 1/3 of an egg, which was explicit in

Rabbeinu Tam and inferred from Rambam, was simplified/misunderstood to mean

that an olive is equal to slightly less than 1/3 of an egg.

 Difficulties with resolving various questions led to the belief that eggs and/or olives

of ancient times were vastly larger than those of today. Given the lack of scientific

knowledge, the understanding of the decline of generations from a golden age, as

well as the intellectual climate that was pervasive at the time, this was seen as a

reasonable position.

 The manuscripts from the Geonim stating that one need only follow the size of olives

of one’s era were only discovered and published relatively recently, as was also the

case with the statements of Rashba and Ritva that olives are very small.

 The substitution of measuring by weight rather than volume, initially instituted for

convenience, led some to believe that matzah ought to be measured this way. Since

matzah is very lightweight, this resulted in a huge increase in volume.

 Finally, the process of halachic tradition, with its canonization and conservatism,

meant that even when the above factors came to light, it was too late for the rulings

to be adjusted.

Those who have attempted to prove that a kezayis is the size of a regular olive have

encountered strong opposition. Understanding how the alternate views arose is the key to

both understanding the cause of this opposition and to overcoming it.


Rabbi Natan Slifkin teaches at Yeshivat Lev HaTorah and is the author of numerous books

and articles. His work can be found at and



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ג-ד (תשס”ז).

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—. “שיעור הכזית והרביעית ביחס לממצאים”, מוריה (שנה כא חוב’ ג-ד טבת תשנ”ז) עמ’ פ-צב.

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עמ’ רכג-רכה.

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