How much must one eat to recite a brocha acharona? How much bread must one eat to fulfill one’s obligation of seudas Shabbos?

Although Chazal chose to describe measurements in terms of commonly used items or foods such as a k’zayis (olive) and a k’beitzah (egg), the size of a standard size egg 1800 years ago may have been larger than today’s egg. Similarly, there are many varieties of olives, and we are uncertain as to which one is used for the k’zayis measure. Therefore, shiurim must be defined in contemporary terms.^{1} The following is based on the psak of Rabbi Moshe Heinemann shlit”a.

I. K’zayis Measurement^{2} – 1.27 fl. oz. (38 ml) – If one eats a k’zayis^{3} of bread, he must recite birchas hamazon.^{4} Similarly, if one eats a k’zayis of any other food a brocha acharona must be recited.^{5}

Our testing indicates that this is the approximate size of a golf ball (40 ml) or a roll of quarters (37 ml). Approximately seven Manischewitz Tam Tams comprise a k’zayis. Therefore, if one consumes this amount, an al hamichya blessing is required. A complete graham cracker (both halves) has a volume of approximately 27 ml. Therefore, if one eats only one graham cracker, a brocha acharona is not recited. An additional one half must be eaten in order to say the brocha acharona. A brocha rishona is required even for any amount less than a k’zayis from which one is neheneh (derives benefit).

When eating bread or cake, one should calculate in his mind whether it has a volume of 38 ml (e.g., whether it has the volume of a golf ball). It is impossible to state how many slices of bread equal a k’zayis, as bakeries and bread companies bake bread in many different sizes.

On the first two nights of Pesach, one should eat two k’zayisim of matzah at Motzi Matzah.^{6} This is equivalent to approximately one half of a standard size machine matzah, or 1/3 of an average hand matzah. If eating this amount is too difficult, one may eat one k’zayis – i.e., 1/4 of a machine matzah or 1/6 of an average size hand matzah. A k’zayis is also the amount of marror one must eat to fulfill that mitzvah.^{7}

II. K’beitzah Measurement – Two k’zayisim, 2.53 fl. oz. (75 ml) – The k’beitzah^{8} measurement is relevant to numerous halachos throughout the year. It is equivalent to the size of approximately two golf balls. When washing before eating bread, one recites the brocha of Al Nitilas Yadayim only if one intends to eat a k’beitzah of bread.^{9}

It is preferable for one to eat more than a k’beitzah of bread at each of the three Shabbos meals and two Yom Tov meals.^{10} Similarly, on the first two nights of Sukkos ideally one should eat this amount of bread in the sukkah.^{11} During the rest of Sukkos, if one eats more than a k’beitzah of bread or cake, a brocha of “Layshev B’sukkah” is recited.

As with all halacha, for a definitive psak one must consult his rav.

*(This is an excerpt from a previous Kashrus Kurrents article, The Guide to Halachic Food Measurements, which can be found at www.star-k.org.)*

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- Note: It is important to emphasize that shiurim are measured in terms of volume, not weight. For example, a k’zayis of cake will volumetrically equal a k’zayis of potato kugel, yet the weight of the potato kugel will exceed the weight of the cake.
- Tosfos (Yuma 80a s.v. Hachi) says the amount one can fill his entire mouth (both cheeks) is greater than a revi’is; hence, a m’lo lugmav (one cheekful) is often referred to as “rov revi’is.” This fact is the basis of calculating the volume of a k’zayis. If a revi’is is less than two cheekfuls, and two cheekfuls equal two eggs (available nowadays – see Biur Halacha 271:13), and one egg nowadays has a volume of 1.9 fl. oz., this means a revi’is must be less than 3.8 fl. oz. A revi’is equals 1.5 beitzim (at the time of Chazal). This means 1.5 beitzim are less than 3.8 fl. oz., one beitzah is less than 2.53 fl. oz., and one k’zayis (half of a k’beitzah) is less than 1.27 fl. oz. (38 ml). Therefore, if one eats food with a volume of 1.27 fl. oz. (38 ml), he has certainly eaten a k’zayis and may recite a bracha acharona.
- Literally, “like [the size of] an olive.”
- Shulchan Aruch, Orach Chaim (henceforth, all references to Shulchan Aruch refer to Orach Chaim) 184:6.
- An after-brocha is only recited if one eats a k’zayis in the amount of time known as k’dei achilas pras. This means, to recite Birchas hamazon or a bracha acharona one has to have eaten a k’zayis ideally within a two minute span (b’dieved within a four minute span). The same applies to fulfilling various mitzvos (e.g., achilas matzah).
- Shulchan Aruch 475:1
- A k’zayis is also a commonly used shiur with regard to “onshin.” In the days of the Sanhedrin, if after being warned one ate a k’zayis of non-kosher food or milk and meat that were cooked together, he was chayav malkos. Eating less than a k’zayis (known as chatzi shiur) is also prohibited min haTorah (but there is no onesh in Beis Din).
- Literally, “like [the size of] an egg” – also pronounced k’baya.
- Shulchan Aruch 158:2. One washes without reciting “al netilas yadayim” (or is yotzai with someone else’s brocha) when eating any amount, even if it is less than a k’zayis (see Mishna Berura 158:10).
- Mishna Berurah 291:2. B’dieved, a k’zayis is enough.
- M.B. 639:22. B’dieved, a k’zayis is enough (Shulchan Aruch 639:3). If it is raining, one should eat a k’zayis without reciting “Layshev B’sukkah” (M.B. 639:35).