Sifrei Mitzvot para sa Exodo 30:13

The silver denomination spoken about in the Torah regarding rape, seduction, one who puts out a bad name and one who kills a slave – that silver denomination is the shekel that is spoken about in every [other] place in the Torah. And its weight is three hundred and twenty barley seeds. And the Sages already added to it and made its weight [to be] like the weight of the coin called, [the] sela, at the time of the Second Temple. And how much is the weight of the sela? Three hundred eighty four medium barley grains.

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.

And there was yet another coin there – the weight of which was two sela – and it was called the darkhon. And all of these coins, that we have said and explained the weight of each one of them, are the ones with which we measure in every place. And we have explained them [now], so that I will not have a need to explain their weight in every place.