The Measurement of an Olive

I have had a question for a long time about the measurement of an olive: how did they arrive at the conclusion that an olive is 25-30 cubic centimeters? Why don't they say, as in the Mishnah, that it is a medium olive, which is about 7 cubic centimeters? After all, the Gaonim wrote: "What is the size of a large dried fig and a medium dried fig, as well as a large olive, a small olive, and a medium olive. These are the measurements, and how will there be a measurement for a measurement. And if you say by weight, our teachers did not specify weight, and the Almighty was not precise with us in weight, and each one, doing according to his understanding, fulfills his obligation. And there is no need to learn the measurement from another, as it is taught: Rabbi Yehuda says: bring the largest of the large and the smallest of the small, put it in the water and divide the water. Rabbi Yossi said: who will inform me which is the largest of the large and which is the smallest of the small, but everything is according to the understanding of the observer." (Teshuvot HaGeonim (Herzog) 268) and the Rashba (Mishmeret HaBayit 4:1) 'fifteen eggs which are much more than sixty olives', - meaning that the olive is much less than a quarter of an egg. And the Ritva (Shabbat 76b, addition at the end of the tractate, Mosad HaRav Kook edition) 'the dried fig - it contains several olives', and since, according to him, a dried fig is a third of an egg, therefore in an egg at least 6 eggs. And the Netziv of Volozhin (Meromei Sadeh Pesachim 39) in a letter to his son: 'How much you, my dear son, are strict about the measurement of an olive for bitter herbs... And it is clear that the measurement of an olive explained in the Shulchan Aruch is very small, and why should we be strict'. And Yad Yehuda (Yoreh Deah 41:2) wrote: 'And I am very surprised at what he saw to come to a discussion about the measurement of an olive, for in the Gemara nothing is found except an olive everywhere, and they did not give a general measurement on eggs, and olives were very common in their places, and it was known what the measurement of an olive is, and on this they wrote the measurement for all prohibitions and commandments, only the Tosafot are precise... Therefore, it would have been better to inquire from people coming from the Holy Land and other places where olives are common, and he would know well how to estimate the measurement of an olive. So why don't they say that an olive is simply like an olive?

Hello,

The discussion is lengthy, and you can certainly find a lot of material on this topic in books. Therefore, I will write only very briefly:

Indeed, the opinion of the Gaonim [Otzar HaGeonim by Rabbi Sherira Gaon, and several other responses in the Gaonim) is that the measurement of an olive and an egg is according to the reality of that place and time. 

However, on the other hand, there is a source in the Gemara where the sages did not leave things only under the name 'egg' or 'olive'. Rather, it is stated in the Gemara (Pesachim 109): Rav Chisda said, a quarter of the Torah is fingers on fingers, in the height of a finger and a half finger and a fifth of a finger.

The Noda BiYehuda (Tzlach Pesachim 116) estimated in his generation that a finger is 2.4 cm wide, and according to this, the volume is 150 cubic centimeters. And since it is explained in the Gemara (Menachot 87) that a quarter is an egg and a half, it follows that an egg is 100 cubic centimeters.

This measurement does not match the average egg of our time, whose volume is 50 - 55 cubic centimeters. Therefore, the Noda BiYehuda was compelled to say that the eggs have halved in size from what they were in the times of the sages. 

Opposing him were the sages of his generation (Teshuva MeAhava Yoreh Deah 354, and others), who estimated that the finger is only about 2 cm wide, and according to this, they estimated the quarter at 86 cubic centimeters, and consequently, the measurement of the egg is about 55 cubic centimeters.