Handling multiple currencies correctly is crucial for financial applications. Poor implementation leads to rounding errors, incorrect totals, and unhappy users. These best practices help you avoid common pitfalls.
Store Amounts in Minor Units
Store monetary values as integers in the smallest currency unit (cents, pence). $10.50 becomes 1050 cents. This avoids floating-point precision issues. Only convert to decimal for display. Most payment processors require amounts in minor units anyway.
Cache Exchange Rates Appropriately
For display purposes, caching rates for 1-4 hours is usually acceptable. For transactions, use fresher rates (minutes to real-time). Always show users the rate being applied and when it was fetched. Consider rate guarantees for checkout flows.
Handle Rounding Consistently
Different currencies have different decimal places (USD: 2, JPY: 0, BHD: 3). Always round to the correct precision for each currency. Use bankers rounding (round half to even) for financial calculations to avoid systematic bias.
Display Currencies Correctly
Use proper currency symbols and formatting for each locale. $1,234.56 in US becomes 1.234,56 € in Germany. The Intl.NumberFormat API handles this automatically. Always show the currency code for ambiguous symbols ($ is used by many countries).