Building Multi-Currency Financial Calculators: Compound Interest, Amortization and Tax Logic

Building Multi-Currency Financial Calculators: Compound Interest, Amortization and Tax Logic Implementing a financial calculator sounds straightforward until you actually try to handle real-world edge cases. I learned this the hard way when building calculators that needed to work across multiple currencies and tax systems. The Core Challenge: Precision Finance doesn't tolerate approximation. A rounding error of $0.01 across thousands of transactions isn't negligible—it's audit-fail territory. JavaScript's floating-point math is... let's say "optimistic." Try this: 0.1 + 0.2 === 0.3 // false! Result: 0.30000000000000004 For financial calculations, you need either: Decimal libraries (Decimal.js, Big.js) Store everything in cents (integers) Both (paranoid mode) I went with both for critical calculators. Compound Interest: The Real Math The simple formula everyone learns: A = P(1 + r/n)^(nt) Where: P = Principal r = Annual interest rate n = Compounding periods per year t = Time in years B