1. How much is a Kezayis?[1]

Regarding saying an after blessings such as Meiyn Shalosh, Borei Nefashos and Birchas Hamazon, after consuming a Kezayis of food/bread, one is not to say an after blessing until he eats 28.8 grams of the food within Achilas Peras. Likewise, initially one is to beware not to enter himself into a doubt of a Safek Bracha, and he is thus not to consume more than 17.3 grams but less than 28.8 grams. Below is a full analysis on the subject and how we reached our conclusion.

1. Background and theoretical size in Talmudic terminologies:[2]

The type of olive referred to in the term Kezayis:[3] The Kezayis which is mentioned in all Halachic literature refers to a medium and average size olive, which is not large or small. This sized olive is called an Agguri olive in Hebrew, as its oil is gathered within it.

Its size in comparison to eggs:[4] The exact measurement of the above-mentioned olive is traditionally ratioed versus the size of an egg. However, it is disputed amongst the Poskim as to the exact ratio that the above-mentioned olive has versus the measurement of an egg.[5] Some Poskim[6] rule that it has the ratio of approximately one half of an egg [without its peel[7]]. Other Poskim[8] rule that it has the ratio of slightly less than one third of an egg [with its peel[9]]. Practically, by Rabbinical commands one may be lenient, while by Biblical commands one is to be stringent, as will now be explained.[10] [Nonetheless, some Poskim[11] rule that even by Rabbinical command one is initially to be stringent. This especially applies by commands, over which a blessing is recited.[12] One who is sick and cannot eat the larger Kezayis amount may even initially be lenient and eat the lesser Kezayis amount.[13]]

The law by an after blessing:[14] Regarding saying an after blessings such as Meiyn Shalosh, Borei Nefashos and Birchas Hamazon, after consuming a Kezayis of food/bread, one is to be stringent to not say an after blessing until he eats the measurement equal to half of an egg.[15] Likewise, initially one is to beware not to enter himself into a doubt of a Safek Bracha, and he is thus not to consume the above questionable amount[16], and is either to eat approximately a measurement equal to half of an egg, or much less than one third of an egg, by all foods that one is not required to eat a Kezayis of them.

The law by other cases of Kezayis: See our corresponding Sefer the laws and customs of the Pesach Seder for the application of this subject to the Shiur for Matzah, Maror, and other foods of the Seder night.

1. How much is a Kezayis-Translating the Kebeitza and Kezayis into grams/volume:

Above it was determined that there exist two theoretical sizes of a Kezayis, one being 1/3 of an egg and the second being ½ of an egg, and that by Biblical commands we use the stringent measurement while by Rabbinical commands one may use the lenient measurement. The difficulty in practically determining the exact amount of gram or volume measurements that this translates into is twofold: 1) It requires one to determine the volume/weight of an egg, for which the 1/3 or ½ Kezayis ratio can then be derived from. Indeed, the practical volume/weight of an egg is debated amongst the Poskim, as will be explained. 2) Another issue present is regarding the form of measurement, whether we measure versus the weight of an egg/Kezayis or versus its volume size. If we measure versus the volume size, then even a food which is lighter than an egg/olive will still contain an equal volume size in determining its Kezayis measurement. Likewise, even a food which is heavier than an egg or olive, will still contain an equal volume in determining its Kezayis measurement. However, if we follow the weight, then a food which is lighter than an egg/olive will require a greater volume. Likewise, a food which is heavier than an egg or olive, will require a smaller volume. Thus, the determination of whether we follow the weight or volume of the gg/olive is crucial for determining the exact amount of a Kezayis. We will now address both of these issues, beginning with the second issue of volume versus weight.

Following volume not weight:[17] All the Torah measurements are measured in volume and not weight. [Nevertheless, due to the difficulty in determining the volume of an item, therefore for simplicity purposes, we practically measure in weight.[18] Nonetheless, this weight must be commensurate to its Halachic Kezayis volume. Thus, a food which is lighter than water [of which its weight and volume is equal], and thus floats on it, will require a lesser weight in its Kezayis then a Kezayis of water. For example, if a Kezayis equals 26cc, which is equal to 26 grams of water, then a food which floats on water will need less than 26 grams for its Kezayis, and within less than 26 grams of weight it already contains a volume of 26cc. On the other hand, a food which weighs more than water, and hence sinks in it, will require a greater weight in its Kezayis, and thus will need more than 26 grams for it to attain its 26cc.[19]]  

What is the volume/weight of a Kebeitza: There exists a major debate amongst the Poskim[20] in regards to the volume size of the Kebeitza, which in turn has created a consequential debate regarding the volume size of a Kezayis. Some Poskim[21] rule that the Kebeitza egg referred to by the Talmud was of a larger size than we have available today, and hence its volume is 100cc. This approach is formally known as Shiur Chazon Ish. Other Poskim[22], however, rule that no change has occurred within the egg sizes from the times of the Talmud, and hence the volume of an egg [without its peel[23]] equals 54cc. This approach is formally known as Shiur Avraham Chaim Naah. Practically, the widespread custom of world Jewry, Ashkenazim and Sephardim, is like the latter approach, and hence a Kebeitza is measured as 54cc.[24] This is equal to 54 grams of water, and hence for simplicity purposes, rather than measuring the volume of the food to make sure it contains 54cc one can simply weigh it to see that it contains 54 grams [unless the food weighs more than water in which case its Kebeitza weight would be more than 54 grams, while if it weighs less than water, then its Kebeitza weight would be less than 54 grams, as explained above]. Nonetheless, there are some Jews of Lithuanian Jewry who are stringent like the former approach and measure the Kebeitza as 100cc/grams. This especially applies by a Biblical Mitzvah.[25] We will now discuss the practical volume/weight of a Kezayis.

What is the volume and weight of a Kezayis: Above it was determined that the volume of a Kebeitza is 52cc, which is equal to 52 grams of water, and that the size of a Kezayis is measured as ½ of a Kebeitza regarding all Biblical Mitzvos, and as 1/3 of a Kibeitza regarding all Rabbinical Mitzvos. Thus, practically, the size of a Rabbinical Kezayis is 17.3cc, which is equal to 17.3 grams of water, while the size of a Biblical Kezayis is 26cc grams [lit. 25.6cc], which is equal to 26 grams of water.[26] Thus, for simplicity purposes, rather than measuring the volume of the food to make sure it contains 17 or 26cc one can simply weigh it to see that it contains 17 or 26 grams [unless the food weighs more than water in which case its Kezayis weight would be more than 17/26 grams, while if it weighs less than water, then its Kezayis weight would be less than 17/26 grams, as explained above], and so is the custom.[27]

Eating a slightly larger volume/weight than the technical Kezayis/Kebeitza size:[28] In all cases that one is required to eat a minimum measurement of Kezayis or Kebeitza one is to eat slightly more than the exact Shiur, as the food that gets stuck in the teeth and is not swallowed does not count towards it.

1. How much is a Kezayis-The practical size in grams/volume:

The law by an after blessing: Regarding saying an after blessings such as Meiyn Shalosh, Borei Nefashos and Birchas Hamazon, after consuming a Kezayis of food/bread, one is to be stringent to not say an after blessing until he eats 28.8 grams of the food. Likewise, initially one is to beware not to enter himself into a doubt of a Safek Bracha, and he is thus not to consume more than 17.3 grams but less than 28.8 grams.

• If already ate between 17-28 grams:[29] If one ate 17 grams of seven species fruit then if he already ate a Kezayis of Mezonos, then one should include an Al Hapeiros in Meiyn Gimel due to the doubt.

The law by other cases of Kezayis: See our corresponding Sefer the laws and customs of the Pesach Seder for the application of this subject to the Shiur for Matzah, Maror, and other foods of the Seder night.

[1] See Admur 486:1-2; Kaf Hachaim 486:1-3; Ketzos Hashulchan 59 footnote 1; Piskeiy Teshuvos 486:1-3

[2] Admur 486:1-2

[3] Admur 486:1; Rambam Machalos Assuros 14a; Brachos 39a; Mishneh Keilim 17:8

[4] Admur 486:1; See Tzemach Tzedek 69 and 209; Shiureiy Torah p. 129 and 193; Yechaveh Daas 1:16

[5] The source of this dispute is based on a similar dispute in the laws of Eiruvin, with regards to how much is the amount of “two meals” that one needs to place for Eiruv Chatzeiros, as will be explained in the footnotes below

[6] 1^st opinion in Admur ibid and 2^nd opinion in Admur 368:3; Michaber 486:1 in name of Yeish Omrim and 2^nd opinion in Michaber 368:3; Darkei Moshe 475:1; Terumas Hadeshen 139; Drashos Maharil Seder Haggadah p. 11; Maryu 123; Tosafus Eiruvin 80b; Yuma 80a; Chulin 103b; Piskei Tosafus Chulin 272; Mordechai Perek Arvei Pesachim Seder Shel Pesach 38; Rashi Eiruvin 4a; Tur 409; Pesakim Ukesavim 26

The source from Eiruv Chatzeiros: The measurement of two meals for Eiruv Chatzeiros is 18 Gerogeros. [Admur 368:3; Michaber 368:1; Rav Yehuda in name of Shmuel Eiruvin 81b] Now, some Poskim rule that this amount calculates to 8 eggs, [2^nd opinion in Michaber 368:3 and Admur 368:3; Rashi Eiruvin 4a; Tur 409] or slightly more than 18 half eggs [Admur 486:1; Tosafus Eiruvin and Yuma ibid; See Bach 409] This proves that according to these Poskim, a Gerogeres is 1:2 of an egg. Now, being that an olive is a smaller measurement than a Gerogeres [See Shabbos 91a], therefore we can conclude that a Kezayis is less than even 1:3 of an egg. [See Admur ibid]

[7] Shiureiy Torah 3:12; Chazon Ish 39:17

Other opinions: Some Poskim rule that the half Beitza is measured with its peel. [M”B 486]

[8] 2^nd opinion in Admur ibid and 1^st opinion in Admur 368:3; 1^st opinion in Michaber 368:3; Rambam Eiruvin 1:9 as learns Peri Chadash

The source from Eiruv Chatzeiros: The measurement of two meals for Eiruv Chatzeiros is 18 Gerogeros. [Admur 368:3; Michaber 368:1; Rav Yehuda in name of Shmuel Eiruvin 81b] Now, some Poskim rule that this amount calculates to 6 eggs. [Rambam ibid; 1^st opinion in Michaber 368:3 and Admur 368:3] This proves that according to these Poskim, a Gerogeres is 1:3 of an egg. Now, being that an olive is a smaller measurement than a Gerogeres [See Shabbos 91a], therefore we can conclude that a Kezayis is less than even 1:3 of an egg. [Admur ibid]

Other opinions who rule it is even less than a 1:3 of a Kebeitza: Some Poskim rule that the Kezayis is much less than 1:3 of a Beitza. [Rashi and Geonim in Shaareiy Teshuvah, brought in Shiureiy Torah 3 footnote 30] Others rule it is less than ¼ of a Beitza. [Rashba, brought in Shiureiy Torah 3 footnote 29-30]

[9] Shiureiy Torah 3:12

[10] Admur ibid; M”B 486:1; See Kaf Hachaim 486:1; See Shiureiy Torah 2 and 3 footnote 30 that the main opinion is like the Rambam

The reason: As it is ruled in 368 and 409 that the main opinion regarding Eiruv Chatzeiros and Eiruv Techumin follow those who rule that we do not need more than 6 eggs worth of food for the two meals, thus concluding that a Kebeitza is 1:3 of an egg. Now, the reason for this ruling by Eiruvin is because an Eiruv is Rabbinical, and we rule leniently by a Rabbinical matter. However, by Biblical matters one is to be stringent. [Admur ibid]

Other opinions: Some Poskim rule that one is to always be stringent to consider a Kezayis as ½ of an egg, even regarding a Rabbinical Mitzvah. [Implication of Michaber 486:1; Kisei Eliyahu 486:1; P”M 486 A”A; Kaf Hachaim 486:1]

[11] Shiureiy Torah 3:13 footnote 30 that so applies even according to Admur ibid, as proven from Admur 160:13 that we don’t initially enter ourselves into a Rabbinical doubt; Implication of Michaber 486:1; Kisei Eliyahu 486:1; P”M 486 A”A; Kaf Hachaim 486:1

Other opinions: Some rule that one may even initially take 1:3 of a Kebeitza for Rabbinical commands. [Simple implication of Admur ibid; Piskeiy Teshuvos 486:2; Shevach Hamoadim in name of Rav Dworkin; Rav Emanuel Shochat in his introduction to the English Rebbes Hagada; Pamphlet “Pesach Chassidi”]

[12] M”B 486:1; Kaf Hachaim ibid in explanation of Admur ibid; See Shiureiy Torah 3 footnote 30

The reason: As Admur ibid rules that by blessings we follow the rule of Safek Brachos Lihakel and thus we always follow the larger Shiur. Now, since a blessing of Al Achilas Maror is recited over the Maror, one must conclude that Admur’s ruling above is limited to a case of Bedieved, that if he only ate 1:3 of an egg, or is unable to eat more than 1:3 of an egg, that he is Yotzei, however, Lechatchila, he must eat ½ of an egg of Maror in order to be allowed to recite the blessing over it.

[13] M”B 486:1 in name of Chayeh Adam; Piskeiy Teshuvos 486:1

[14] Admur 486:1; P”M 486; M”B 486:1; Kaf Hachaim 486:1; Ketzos Hashulchan 59 footnote 1; Shiureiy Torah 3:12; Shaareiy Rachamim in Minahgei Hagr”a

[15] The reason: As by blessings we follow the rule of “Safek Brachos Lihakel.” [Admur ibid]

[16] One should not eat between 1:3 and a ½ of an eggs size in order not to enter himself into doubt.

[17] Admur 486:2; Rama 486:1; M”A 486:1; Maharil Seder Haggadah p. 111; Sefer Ohel Moed Hilchos Yom Kippur; Mishneh Uktzin 2:8; Rambam Pirush Hamishnayos Eidiyos 1:2; So rule regarding Issur Viheter: Peri Chadash Y.D. 99:6; Peri Toar 99:4; M”B 486:3; Kaf Hachaim Y.D. 98:5 [that one who measures by weight is feeding Treifos to the Jewish people]; Shiurei Torah Siman Alef “It is known and public knowledge from all the Poskim Rishonim and Achronim, that all the measurements of Revius, Kezayis, Kivbeitza, are to be measured in volume and not weight” and in footnote 1 “There are many proofs for this, and I have written all this in length to negate the opinion of the Kaf Hachaim 168:46 that we measure in weight and not volume, as in truth we only measure in weight those foods that had already had their volumes measured and then weighed.”; [However, see Shiureiy Torah p. 184 who writes to measure Matzah by weight; Rav SZ”A says that Rav A”C Naah actually told him to measure by volume, unlike what he wrote above to follow weight. See Seder Hearuch 79 footnote 32]; Piskeiy Teshuvos 486:1; Rav Yaakov Yosef z”l ruled to follow volume and completely negated the following of weight, thus disputing his fathers, and brothers positions.

Other opinions: Some Poskim rule that the all Torah measurements are to be measured in weight and not volume. [Maggid Mishneh Eiruvin 1:12  in name of Teshuvos Hageonim; Pesach Hadvir 130, 190 in name of a number of Poskim, brought in Shiureiy Torah 3:8 footnote 18 and Kaf Hachaim 168:46; Beis David; Mateh Yehuda 168:1; Chida; Vhemdas Yamim; Kaf Hachaim 168:46; 486:1 and 3; See Chazon Ovadia Pesach Hilchos Leil Haseder p. 42 and 73; Rav Yitzchak Yosef Shlita that his father held to always follow weight and that so he asked of him to tell the public in his Shabbos Hagadol Drashos, thus negating his brothers position to follow volume]

[18] Kaf Hachaim 168:46; 486:1 and 3 that so is the widespread custom of Jewry, and all Rabbanim, to measure in weight in order to be Yotzei according to all; Shiureiy Torah 1:1 brings that the Rambam measured a number of items in weight and that from this we can learn regarding other items to measure in weight; Shiureiy Torah 3:8 footnote 18 and page 184 writes to measure the Matzah and bread in weight, and practically gives weight measurements for all his Shiurim, after having compared them with their volume size; Chazon Ovadia Pesach Hilchos Leil Haseder p. 42 and 73; Rav Yitzchak Yosef Shlita that so was the custom of Sephardic Jewry for all generations and that his father held to always follow weight and that so he asked of him to tell the public in his Shabbos Hagadol Drashos, thus negating his brothers position to follow volume

[19] See Shiureiy Torah 3:12-13 and p. 184; Piskeiy Teshuvos 486 footnote 3

[20] See M”B 486:1; Piskeiy Teshuvos 486:1

[21] Opinion brought in M”B 271:68; 486:1 and Biur Halacha 271:13 “Shel Revius” [M”B 486:1 concludes to be stringent by a Biblical Mitzvah] ; Nodah Beyehuda Tzelach Pesachim 116a; Chasam Sofer O.C. 97 and 127; Maaseh Rav 74 and 105; Rav Chaim Volozhin in Shaareiy Rachamim 165; Ashel Avraham Butchach 272; Aruch Hashulchan 168:13 that so is custom

[22] Shiureiy Torah 3:9 it is 53.8cc if the egg is measured without the peel and 3:7 that it is 57.6cc if the egg is measured with the peel; Kaf Hachaim 81:3; 168:46; 486:1; The following Poskim rule that the Kibeitza is 18 Dirham, and each Dirham is equal to between 2.7-3.2 grams, thus totaling approximately 55 grams: Rambam

Other opinions: Some Poskim rule that the Beitza is 44 grams with the peel and 33 grams without the peel. [Mahariv brought in Shiureiy Torah 1:21; 3 footnote 30]

[23] This follows the ruling of the Shiureiy Torah ibid and the Chazon Ish 39:17 that the egg is measured with its peel, however the M”B 486 rules that it is measured with its peel. [See Midos Vishiurei Torah 15:16]

[24] Kaf Hachaim 486:1 that so is custom

[25] M”B 486:1 concludes to be stringent by a Biblical Mitzvah

[26] Shiurei Torah 3:12-13; Kaf Hachaim 168:46; 486:1 in name of Rambam and others that it equals 9 Dirham which is equal to 2.7-3.2 grams, thus totaling approximately 27 grams; Piskeiy Teshuvos 486:1 in name of Midos Vishiureiy Torah writes 27 grams

Shiur Chazon Ish: According to the Chazon Ish, the Biblical Shiur Kezayis is between 45-50cc [depending on whether we measure with the shell of the egg, as stated above] while the Rabbinical Shiur Kezayis is 33cc

Other opinions: Some Poskim rule that the Biblical Kezayis is 15 grams. [Custom of many in Ashkenaz prior to ruling of Tzelach, based on Mahariv, brought in Shiureiy Torah 1:21; 3 footnote 30]

[27] Kaf Hachaim 168:46; 486:1

[28] See Shiureiy Torah 3:12-13 footnote 26 based on Chulin 103 that by all Issurin, the food stuck in the teeth is not included; Minchas Chinuch Mitzvah 6 and 10; Matzos Mitzvah 12:23; Piskeiy Teshuvos 486:1; Heard from Rav Yitzchak Yosef in name of his father Rav Ovadia Yosef

Food stuck on gums: Does join the Shiur Kezayis:Kebeitza. [Chasam Sofer 127; Kesav Sofer 96]

Other opinions: Some Poskim rule that food stuck in between the teeth does join the Kezayis. [See Maharam Shick 250 that the benefit of the mouth joins the Shiur]

[29] Chayeh Adam 50:20; Zechor Leavraham 2; SSH”K 54 footnote 71; Yechaveh Daas 6:14; Mishneh Halachos 4:29; 5:36; Shraga Hameir 3:4; Igros Moshe 2:109 [permits by even if a less than the amount for certain]; Piskeiy Teshuvos ibid footnotes 179