money-decimal-arithmetic

Off-by-a-penny errors occur because binary floating-point numbers lose precision during monetary arithmetic. You fix this by refactoring calculations to use integer minor units or arbitrary-precision decimals, ensuring exact reconciliation invariants.

Largest-remainder allocation is a technique that distributes remaining pennies across line items so the split sums exactly to the original total. It prevents precision loss and leaking pennies when dividing charges across multiple entries.

Handle currency-aware rounding by applying ISO 4217 exponent rules per currency, treating JPY with a 0 exponent differently than USD with 2 or BHD with 3. This ensures correct minor unit calculations across global currencies.

Prevent float precision loss by replacing floating-point numbers with integer minor units or arbitrary-precision decimals for all tax and discount calculations. This ensures exact pricing logic and eliminates rounding inconsistencies.

You need either arbitrary-precision decimal types or strict integer minor units to avoid float drift in accounting software. Binary floating-point numbers cannot accurately represent decimal fractions, causing reconciliation failures.

The best way to ensure exact allocation is implementing a largest-remainder strategy that distributes leftover pennies across line items. This guarantees the split totals sum exactly to the original charge without leaking pennies.