Leviticus 27:25

27:25 Every conversion value must be calculated by the standard of the sanctuary shekel; twenty gerahs to the shekel.

Exodus 30:13

30:13 Everyone who crosses over to those who are numbered is to pay this: a half shekel according to the shekel of the sanctuary (a shekel weighs twenty gerahs). The half shekel is to be an offering to the Lord.