Why money in software must be exact, not approximate

In short: Money is stored as exact decimals and totalled on the server, so any invoice can be added up by hand.

Computers store many decimal numbers approximately. In most places that is fine. In billing it is not.

The problem

Add 0.10 and 0.20 as ordinary floating-point numbers and the result can be 0.30000000000000004. Show that on an invoice, or let it grow across many lines and installments, and the totals stop matching.

What I do instead

• Amounts are stored and calculated as exact decimals, with two places.
• Totals are calculated on the server from the lines. The browser's arithmetic is never trusted.
• Amounts travel to the screen as exact text, so no rounding is introduced on the way.
• Installments must add up to the invoice total exactly, or the invoice cannot be issued.
• A payment cannot be larger than the amount still due.

Why it matters to you

You can add the lines of any invoice by hand and get the same total. If you cannot, that is a bug I want to hear about.