# Shiur of K'zayis

## How large is a Kezayit?

1. According to Ashkenazim, all sizes are measured in volume and not weight. [1]However, according to Sephardim the actual halacha is that measures are in volume but the minhag is to follow the measures in weight. [2]
2. There’s a dispute in the rishonim whether the kezayit (olive size) is a half or third of a KeBeytzah (egg size), and the halacha follows the opinion that a Kezayit is half a KeBeytzah. [3]
3. Rav Chaim Noeh’s opinion is that the Kezayit is 27 grams (.92 oz) and preferable to have 28.8 grams (.96oz). The Chazon Ish’s opinion is that the Kezayit is 33.3 grams (1.3 oz). [4]
4. The minhag of the world and many other authorities is to follow Rav Chaim Noeh’s opinion. [5]

## How large is a Reviyit?

1. A Reviyit is a Kebaytzah and a half. [6] Therefore, according to Rav Chaim Noeh, the Reviyit is 86 grams (3 oz) and according to the Chazon Ish the Reviyit is 150 grams (5.3 oz). [7]

## Practical applications

1. If one ate between a third and a half a KeBaytzah one should not make a Bracha achrona because of safek Brachot. Therefore, one should have less than a third and not make a Bracha achrona or more than a half and make a Bracha achrona. [8]
2. On מצות Deoritta such as eating Matzah one must be strict to hold a Kezayit is a half a Kebaytzah. However, regarding מצות Derabbanan it’s preferable to hold that a kezayit is a half a kebaytzah, however, one may rely on the opinion that a kezayit is a third a kebaytzah. [9]

## References

1. Mishna Brurah 456:3, Vezot HaBracha (pg 6)
2. Yalkut Yosef (Klalei Brachot pg 195-202) discusses this at length
3. S”A 486 rules like Tosfot and not the Rambam that the Kezayit is half a Kebaytzah and not a third.
4. Vezot HaBracha (pg 6)
5. Vezot HaBracha (pg 6, Birur 1, pg 221) in name of Rav Elyashiv and Rav Shlomo Zalman that one may follow Rav Chaim Noeh’s opinion and there’s no need to be strict for the Chazon Ish’s opinion. Vezot HaBracha also mentions that such is the minhag of the world.
6. Mishna Brurah 486:1
7. Vezot HaBracha (pg 6)
8. Mishna Brurah 486:1
9. Mishna Brurah 486:1