Computer Science › Math for Programmers · also in Product Building Blocks

Rounding and Money

Never store money as a float; use integers or decimals.

Also known as: storing money, floating point money, currency rounding

Never store money as a floating-point number. Floats can’t represent most decimal fractions exactly, so tiny errors creep in, and they add up.

0.1 + 0.2                  # 0.30000000000000004
sum([0.10] * 10)           # 0.9999999999999999

A report that’s off by a cent is a bug that auditors, customers and regulators notice (floating point).

Two safe approaches

1. Integers in the smallest unit (cents, or the currency’s minor unit):

price_cents = 1999            # $19.99
total_cents = price_cents * 3 # 5997, exact
f"${total_cents / 100:.2f}"   # format only for display

2. An exact decimal type:

from decimal import Decimal
Decimal("19.99") * 3          # Decimal('59.97')

Create decimals from strings, not floats (Decimal(19.99) copies the float’s error). Databases have NUMERIC / DECIMAL, and Java has BigDecimal.

Rounding

Decide how and when to round, and be consistent.

• Round only at defined points (per line or per invoice), not after every step.
• Pick a rule: half-up is common in invoices, while “banker’s rounding” (half to even) is the default in some libraries. Check what yours does.
• Splitting an amount (a $100 bill across three people) leaves a remainder cent. Decide who gets it, so the parts add up to the whole.
• Percentages and tax produce fractions of cents. Round per your tax rules.

More pitfalls

• Currencies differ: some have no minor unit (JPY), some have three decimals (KWD). Store the currency with the amount (multi-currency handling).
• JSON numbers become floats in JavaScript. Send money as integer cents or a string.
• Never compare floats for equality, and don’t use floats in database money columns.
• Keep an audit trail, and use balanced entries for movements of money (ledger).