(repost from two years ago)
R. Mordechai Willig calculates the preferred minimum amount of matzah to eat, i.e. the size of a ke-zayis (link – audio). This is, by necessity, based on approximations and averages. Here is how I understand his steps:
- According to measurements in Israel, the average size of a contemporary egg is 50 ccs. (The majority of classical authorities do NOT double this size.)
- Measurements have shown that an egg’s volume decreases by 10% when the shell is removed, which leaves us with 45 ccs.
- The strict view is that a ke-zayis is half of an egg’s volume, which is 22.5 ccs.
- Measuring the required volume of a matzah should be done on the matzah as it is, not by crushing it into crumbs. The weight of 22.5 ccs of water is 22.5 grams.
- Experiments show that the equivalent volume of matzah has half the weight of water. This means that a ke-zayis of matzah weighs approximately 11.25 grams.
- 11.25 grams is about 0.4 ounces.
- There are on average 7.5 handmade matzos per pound (16 ounces) and 15 machine-made matzos per pound, which means that an average handmade matzah is 2.13 ounces and an average machine-made matzah is 1.07 ounces.
- Therefore, a ke-zayis is less than 1/5th of a handmade matzah and less than 2/5th of a machine-made matzah.
R. Willig emphasizes that matzah at the seder is the only biblical obligation to eat a specific food and that, according to R. Soloveitchik, the Rambam is of the view that you fulfill a voluntary mitzvah for eating more than a ke-zayis of matzah. The minimum here should not be mistaken for the maximum.
For those of us who have the custom of eating horseradish as marror, this might help. For a few years, I would not recite the blessing over marror because I did not think I would eat a ke-zayis. Even though now I can eat plenty of horseradish, I think these measurements might be useful sometime in the future.
(See also this post: link)