The se'ah or seah (Hebrew: סאה sə’āh), plural se'im, is a unit of dry measure of ancient origin found in the Bible and in Halakha (Jewish law), which equals one third of an ephah, or bath.
[3] In the context of a mikveh, forty se'ahs are the minimum quantity of water needed to render bodily purification after an immersion.
A mikveh must, according to the classical regulations, contain enough water to cover the entire body of an average-sized person.
[4][5] The exact volume referred to by a se'ah is debated, and classical rabbinical literature specifies only that it is enough to fit 144 eggs.
[6] In some Orthodox Jewish circles there is a proclivity to follow the stringent ruling of Avrohom Yeshaya Karelitz, or Eliyahu of Vilna, whose view is that the size of eggs has progressively become smaller over the ages, therefore requiring a larger measure.