Article: How Much is a Kezayis?

The term kezayis is frequently found in Torah sources. A kezayis is the volume of an average-sized olive. However, there is a difference of opinion amongst early authorities as to the volume of an average-sized olive. Many Rishonim maintain that a kezayis is approximately the equivalent of half an average-sized egg (without its shell). Another opinion is that a kezayis is slightly less than a third of an egg (with its shell).

Regarding a Torah rule, such as eating a kezayis of matzah on the first night of Pesach, we must be stringent and go by the larger shiur; regarding the amount of maror to be eaten, which is a Rabbinic ruling, we can be lenient, if necessary. At times the larger shiur is a leniency: For example, regarding the halachah of destroying any chometz over a kezayis found in our possession on Pesach which is a Biblical law, we must be stringent and deem the smaller shiur kezayis as the kezayis. (According to Rabbinic law, we must destroy even chometz which is under a kezayis.)

Before making a brachah acharonah, an after-blessing—which is required only after eating a kezayis—one should be sure to eat at least the equivalent in volume of half an average egg in order to avoid the possibility of saying a brachah in vain. One should avoid the equivocal zone: either eat less than the smaller shiur kezayis and not make a brachah acharonah, or eat more than the larger shiur kezayis so that a brachah acharonah can be said—in compliance with all opinions.

Sources:

שו״ע אדה״ז סתפ״ו ס״א ובהנסמן שם ובמהדו״ח. בקליפתה וכו׳ – ראה שו״ת נוב״י מהדו״ק או״ח סל״ח. וכן הכריע הגרא״ח ע״ה נאה בשיעו״ת ס״ג סי״ב. וחזר בו ממש״כ בקצוה״ש סנ״ט סק״א.

במרור שקו״ט אם מותר לברך – ראה משנ״ב סתפ״ו סק״א. וחלק עליו בשיעו״ת ס״ג סק״ל. וכ״ה בקש״ע סקי״ט ס״ז. וכתבנו מזה בשעתו בקובץ התמים ניסן תשס״ב.

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