What is a Kezayit?

Kezayit literally means “like an olive,” but is used as a noun, meaning “an olive-bulk.” Essentially, it is a semi-standardized1 measurement of volume that has been passed down through the generations, all the way since Moses on Mount Sinai.2

A kezayit is considered a substantial amount of food. “‘Eating’ is not less than an olive-size,” the Midrash rules.3 This means that mitzvahs related to eating usually require a kezayit to be considered substantial.

Referring to the seven species of fruit with which the Land of Israel is blessed,4 the Talmud teaches: “The Sages interpreted ‘A land of olive oil [and honey],’ as… ‘A land, most of whose measures are olive-bulks.’”5 Indeed, it is one of the most prevalent halachic measurements, applying to laws ranging from vows of abstinence to afterblessings to matzah on Passover.

How Big Is a Kezayit?

In the scheme of halachic measurements, a kezayit is less than the dried-fig-bulk (kegrogeret), the date-bulk (kekotevet), and the egg-bulk (kebeitzah).

The Mishnah defines an olive measurement as, “Not a large olive nor a small olive, but an average olive, like the “egori variety.”6 While this sounds simple, converting this ancient fruit into a modern measurement is the subject of significant halachic debate.

While a minority opinion argues that the measurement should be taken at face value based on the olives we see today (approx. 3–5cc),7 the Code of Jewish Law,8 based on the Talmud, rules that the measurement of a kezayit is linked to that of the egg-bulk (kebeitzah).

Thus, depending on varying interpretations of Talmudic passages, the standard size of the kezayit is defined as either a half9 or a third10 of an egg’s size.

Practically, we follow the stricter, half-egg measurement for commandments originating from the Torah (de'oraita), such as eating matzah. For Rabbinic commandments (derabbanan), we rely on the smaller, third-egg measurement in cases where eating the larger amount is difficult.11

Weight vs. Volume

Is the kezayit measured by weight or volume? Halachically, it is a measure of volume (cubic centimeters). However, for ease of measurement, it is often converted to weight based on the density of water (1 cc=1 gram).12

• For dense foods (meat/fish): The weight needed to reach the required volume is slightly higher.
• For airy foods (matzah/bread): The weight is lower.13

Although we have explained that the kezayit is linked to the size of an egg-bulk, we still need to translate that into contemporary measurements. So, in practical terms, what exactly is the measurement of the olive-bulk kezayit?

There is a well-known debate among halachic authorities regarding the volume of a kebeitza (egg-size), which, as mentioned, directly affects the measurement of a kezayit (olive-size).

Some authorities maintain that the egg referenced in the Talmud was larger than today’s eggs and therefore measure a kebeitza as 100 cc. This view is commonly known as the measurement of the Chazon Ish. Others rule that egg sizes have not changed since Talmudic times and calculate a kebeitza (i.e., an egg without the shell) as approximately 54 cc. This approach is known as the measurement of Rabbi Avraham Chaim Na’eh.

In practice, the widespread custom among both Ashkenazi and Sephardi Jews follows the latter opinion, measuring a kebeitza as approximately 54 cc.14 Since 54 cc of water equals 54 grams, one may measure food by weight rather than volume, unless the food is significantly heavier or lighter than water. (There are, however, some communities that are stringent to follow the larger measurement of 100 cc, especially when fulfilling a biblical obligation.)

Accordingly, a Rabbinic kezayit (a third of an egg) is approximately 17 cc (17 grams of water), while a Biblical kezayit (half an egg) is approximately 26 cc (26 grams of water).

• Strict opinion (half-egg)/ Torah commandments: 25.6 cc (approx. 25.6 g of water)
• Lenient opinion (third-egg)/ Rabbinic commandments: 17.3 cc (approx. 17.3 g of water)

Ultimately, Rabbi Na’eh recommends adding a bit to the measurement (for a total of approx. 28.3 grams) to account for any food that might remain stuck between the teeth, since that food isn’t included in the measurement.

What are the Practical Applications of a Kezayit?

The kezayit measurement is used in many Torah laws. Here are the most common:

• Matzah: The minimum requirement for eating matzah on the first two nights of Passover (or the first night in Israel) is at least a kezayit.15 Being a Torah commandment, the preferred minimum amount of matzah is 28.3 cc, to allow for matzah getting stuck between the teeth. This would total less than an ounce due to matzah’s lower density. Depending on the size and thickness of your matzah, this could be up to half a matzah.
  
  The preferred amount, however, is much more than that. Throughout the Seder, we eat matzah a total of three times: during the Motzi-Matzah section, the Korech sandwich section, and finally the Afikoman at the end of the meal. The preferred amount for Motzi-Matzah is two kezayits (one from the broken piece and one from the full matzah); for Korech, one kezayit; followed by another two kezayits for the Afikoman, totaling five kezayits. That is the premium way of observing the mitzvah.
  
  However, it is important to note that only the first kezayit of Motzi-Matzah must be the larger 25.6 cc size. The rest may be the lower 17.3 cc measurement.16
• Maror: In post-Temple times, this is a Rabbinic mitzvah, so there is leniency if someone has difficulty eating a full, large-sized kezayit (up to 1oz) of bitter herbs.17 In such a case, the minimum would be the lower kezayit size—17.3 cc (17.3 grams or slightly more than 0.6 oz of water).
  
• Sukkah: On the first night (or first two nights outside of Israel) of Sukkot, there is a mitzvah to eat bread in the sukkah. The minimum amount is a kezayit.18 As a Biblical commandment, this should be the larger kezayit size, meaning 25.6 cc or the equivalent of 1 oz of water (28.3cc) to account for food sticking to the teeth.
  
• Afterblessing: Although one should make a blessing before eating any amount of food, one should not recite an afterblessing (bracha acharonah) unless they ate a larger-sized kezayit (i.e., 28.3cc to account for teeth).19
  
• Respecting Food: Food should be treated with a measure of respect and not needlessly destroyed. Therefore, while it is okay to sweep up or disregard crumbs, one should not step on or destroy food that is a kezayit in size.20 (Consideration should also be given to smaller amounts of food.21)
  
• Prohibitions: The bulk of kezayit applications concern food-related prohibitions, such as pork, milk and meat mixtures, insects, or meat that was not ritually slaughtered. In Temple times, one was not held legally liable unless a full kezayit was eaten. However, non-kosher food is prohibited in any amount.
  
  Likewise, if one makes a vow not to eat a certain food or to fast, they are only held liable for violating their vow if they eat a full kezayit of the specified food.22 Still, eating any amount remains prohibited.

To summarize with a chart: