Beverage Volume - Revi'is

Q. What is the volume of a revi’is?

A. Since a revi’is is equal to 1½ eggs (see Mishna Berura 271:68), the real question is, what is the volume of a k’beitza (halachic egg)? This is a matter of great dispute. Rav Chaim Naeh and many others have measured the average size of eggs and have calculated that a beitza is about 2 fluid ounces. According to this calculation, a revi’is is about 3 oz. However, the Noda B’Yehuda (Tzlach - Pesachim 109) questions the approach of calculating a revi’is by measuring our eggs, because Chazal also say that a revi’is is equal to 10.8 cubic finger widths. Measuring in this manner yields almost twice the volume. The Noda B’Yehuda posits that our eggs might be smaller than the eggs of Chazal, and the more accurate way to measure is with fingers. The Aruch Hashulchan (OC 210:1) dismisses this concern, and rules that one need only calculate based on the measurement of the egg. The Mishnah Berurah (Beiur Halacha 271:13, s.v. Shel) offers proofs against the Noda B’Yehuda, but concedes that there is no simple answer to the apparent contradiction. He therefore offers the following recommendation: When we are dealing with a Torah obligation, such as Kiddush Friday night, we should use the larger measurement of the Noda B’Yehuda (which is based on a revi’is equaling approximately 5 oz). However, when we are measuring for mitzvos d’rabbanan (Rabbinic obligations), such as the daytime kiddush of Shabbos as well as kiddush on Yom Tov, one may use the smaller measurement of a revi’is (approx. 3 oz), or, preferably, a compromise amount equal to two eggs (approx. 4 oz). The four cups of wine are Rabbinic. However, when Pesach falls on Shabbos, the first cup is a Torah obligation because it serves as kiddush for Shabbos, while the remaining three cups are Rabbinic mitzvos.

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