Money precision

Package: CURRENCY_PACKAGE. Registered by createEngine(); for a slimmer engine, register it explicitly (see choosing packages).

Money is exact. A price is a decimal, not a binary fraction, so amounts in the same currency add, subtract, multiply and divide without the rounding error a floating-point number carries. This is what keeps a column of prices adding up to the cent instead of drifting by a fraction of a penny.

$0.10 + $0.20 // $0.30
$19.99 * 3 // $59.97
$100 - $99.99 // $0.01
$10 / 3 // $3.33
$0.70 * 1.10 // $0.77

A half-cent rounds away from zero, the way a till rounds it, rather than the way toFixed rounds the nearest double sitting just below it.

$1.005 // $1.01
$2.675 // $2.68
$0.10 + 15% // $0.12

Exactness holds wherever a currency is involved, a currency against a plain number included, and that includes adding a percentage: $0.10 + 15% is $0.115, which the half-cent rule rounds up. A bare decimal on its own is an ordinary floating-point number, and a conversion between two currencies goes through a live rate, which is not exact.