Compound interest calculator

The Compound interest calculator projects how a savings or investment balance grows when interest is earned on interest. Start with a principal, add an annual rate and a time horizon, choose how often interest compounds — from daily to annually — and optionally a regular monthly contribution. It returns the final balance, the total interest earned and a year-by-year breakdown, so you can see the curve steepen as the balance grows and compare what a higher rate or an extra deposit does to the end result.

Each period's interest is added to the balance, so the next period earns interest on a slightly larger amount. Over time this speeds up growth — interest on interest.

Daily compounding grows a little more than monthly, which grows more than annually — but the differences are smaller than the rate or the time horizon. Savings products often quote APR and APY, where APY is the rate after compounding.

Contributions are spread across the compounding periods: a $300 monthly deposit becomes $3,600 a year, split evenly. Each period's contribution is added at the end, after that period's interest.