Money Is Not a Floating-Point Number

Mon, 01 May 2017 00:00:00 +0000

A booking-time price arrived as four values:

net amount: 407
tax rate: 25.5%
tax amount: 104
gross amount: 511

The amounts are integers in the currency’s minor unit. The rate is a decimal ratio. The contract checks that net plus tax equals gross and that applying the declared rounding rule to net multiplied by the rate produces the tax amount.

None of those details is decorative. Remove the unit, mix the rate’s two representations, or let two services round at different stages, and the same price can stop balancing.

Money on Brian Faust

Money Is Not a Floating-Point Number