Shiur of K'zayis

How large is a Kezayit?

1. All sizes are measured in volume and not weight.
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.

How large is a reviyit?

1. A Reviyit is a Kebaytzah and a half. 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) </ref>

Bottom line

1. 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).
2. The minhag of the world and many other authorities is to follow Rav Chaim Noeh’s opinion.

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.
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.