Currency Conversion: Handling Exchange Rates Correctly

International business means handling multiple currencies. Whether you're processing payments, displaying prices, generating invoices, or building financial dashboards, getting currency conversion right matters for accuracy, trust, and sometimes legal compliance. Exchange rates are fundamentally different from most data your application handles. They change constantly during market hours, sometimes dramatically within minutes. An approach that works for slowly-changing data—aggressive caching, daily updates—creates problems with currency data. Understanding how exchange rates work, when they change, and how to handle them in applications helps you build systems that stay accurate while remaining performant. How Exchange Rates Work Exchange rates represent the price of one currency in terms of another. When EUR/USD is 1.10, one Euro costs 1.10 US dollars. Floating rates are determined by currency markets—supply and demand across millions of transactions. Major currencies like USD, EUR,