Charles Ellicott Commentary Exodus 30:13

"This they shall give, every one that passeth over unto them that are numbered, half a shekel after the shekel of the sanctuary; (the shekel is twenty gerahs;) half a shekel for an offering to Jehovah." — Exodus 30:13 (ASV)

Half a shekel. —When shekels came to be coined, they were round pieces of silver, about the circumference of a shilling, but considerably thicker, and worth about two shillings and seven pence in the currency of that time. Their average weight was about 220 grains troy. In Moses’s time coins were unknown, and a half-shekel was a small lump of silver, unstamped, weighing probably about 110 grains. The ransom of a soul was doubtless made this light in order that the payment might not be practically felt as a burden by any.

After the shekel of the sanctuary. —Without a standard stored somewhere, weights and measures will always fluctuate greatly. Even with a standard, they will practically vary considerably. The “shekel of the sanctuary” probably designates a standard weight kept carefully by the priests with the vessels of the sanctuary. All offerings were to be estimated by this shekel (Leviticus 27:25).

A shekel is twenty gerahs. —Rather, the shekel, i.e., the shekel of the sanctuary has this weight. A “gerah” was, literally, a bean, probably the bean of the carob or locust tree (C eratonia siliqua), but became the name of a weight, just as our own “grain” did. It must have equalled about eleven grains troy.