The **se'ah** or **seah** (Hebrew: סאה) is a unit of dry measure of ancient origin used in Halakha (Jewish law), which equals one third of an *ephah*, or *bath*. Its size in modern units varies widely according to the criteria used for defining it. Seah is similar to the Arabic Saa' ( صاع ).

In Islamic jurisprudence, A Saa’ ( صاع ) is not a weight measurement. It was and still is a measurement of volume, similar to the size of a large salad bowl. It is made up of four mudds, and a mudd ( مد ) is a smaller container, close to the size of a small salad bowl. To be precise, a Prophetic mudd in modern volume measurements is .75L (or 750mL), which means that a Saa’ is three liters.

According to Herbert G. May, chief editor of two classic Bible-related reference books, the bath may be archaeologically determined to have been about 5.75 gallons (22 liters) from a study of jar remains marked 'bath' and 'royal bath' from Tell Beit Mirsim. Since the *bath* unit has been established to be 22 litres, **1 ***se'ah* would equal 7.33 litres or 7.33dm^{3}.

In the context of a mikveh, a *se'ah* can be about twice as much in order to accommodate even the most stringent rabbinical ruling on immersion. A mikveh must, according to the classical regulations, contain enough water to cover the entire body of an average-sized person; based on a mikveh with the dimensions of 3 cubits deep, 1 cubit wide, and 1 cubit long, the necessary volume of water was *estimated* as being 40 *se'ah* of water. The exact volume referred to by a *seah* is debated, and classical rabbinical literature specifies only that it is enough to fit 144 eggs; most Orthodox Jews use the stringent ruling of the Avrohom Yeshaya Karelitz, according to which **one ***seah* is 14.3 litres, and therefore a mikveh must contain approximately 575 litres. This volume of water could be topped up with water from any source, but if there were less than 40 seahs of water in the mikveh, then the addition of 3 or more pints of water from an unnatural source would render the mikveh unfit for use, regardless of whether water from a natural source was then added to make up 40 seahs from a natural source; a mikveh rendered unfit for use in this way would need to be completely drained away and refilled from scratch.