Êxodo 30:13 Halacá: Mishneh Torah, Sheqel Dues, Sefer HaChinukh, Kitzur Shulchan Arukh & Mishneh Torah, Positive Mitzvot

And the sela is four dinar; and a dinar is six maah. And the maah is what was called gerah in the days of Moshe, our teacher. And a maah is two fundyon; and a fundyon is two issar. And a ferutah is an eighth of an issar. It comes out that the weight of a maah – which is a gerah – is sixteen barley grains; and the weight of an issar is four barley grains; and the weight of a perutah is half a barley grain.

The commandment of the giving of the half shekel during the year: That each one of Israel from twenty years and up — whether poor or rich — give the half shekel, which is the weight of ten gerah of silver, each year to the hand of the priests, as it is stated (Exodus 30:13), “This is what everyone who passes the count shall give.” And they would place all of it in a compartment in the Temple. And from there they would take them out (Mishnah Shekalim 4) to buy daily offerings, additional offerings and any sacrifice brought for the community and their libations, the salt with which they would salt the sacrifices, the wood for the arrangement, the bread of display and the wage of the one making the bread of display, the omer, the two-breads, the red heifer, the goat sent away and the golden strip [it would carry].

It is customary to donate a coin, equal to half of the standard monetary unit currently in use in the country before Purim, as a reminder of the half-shekel, the Jews used to give in Adar to buy the communal offerings. It is customary to give three half-shekels, because in Parashas Ki Sisa, the word terumah [offering] is mentioned three times. These coins are given in the evening before the reading of the Megillah, and then distributed to the poor. A minor is exempt [from giving the half-shekel]; but if his father once contributed for him, it remains his obligation for life. According to some authorities, a thirteen year old boy must give [a half-shekel], while others hold that he is exempt until he reaches the age of twenty.