Storing money amounts correctly in programs

In short: Money is never stored as a floating-point number (float/double), but as an integer in the smallest unit (cents) or as an exact decimal type.

In more detail: Binary floating-point numbers cannot represent many decimal fractions exactly. 0.1 + 0.2 gives 0.30000000000000004 in most languages. With amounts such deviations add up.

In Depth

• Whole cents: €19.99 is stored as 1999. Addition and comparison are exact. Divide by 100 only for display.
• Decimal types: DECIMAL(12,2) in SQL, BigDecimal (Java), Decimal (Python, C#). They calculate in base ten.
• Carry the currency: Amount and currency code (EUR, JPY …) belong together. Not all currencies have two decimal places (JPY: 0, KWD: 3).
• Rounding: The rounding rule must be set by the business (commercial, banker’s rounding) and applied only at the end of calculation chains. Percentages and taxes produce fractions of a cent.
• Splitting: When dividing (€100 among 3) a remainder cent is left that must be assigned deliberately.
• Exchange: In JSON transfer amounts as a string or whole cents, not as a number with decimals.

See also: floating-point number