See how money grows over time when interest earns interest. Enter your starting principal, an optional regular monthly contribution, the annual interest rate, how often interest compounds, and the number of years. The calculator applies the compound interest formula to the principal and the future-value-of-an-annuity formula to your contributions, giving an accurate final balance. It also splits the result into how much you put in versus how much interest you earned, which is the number that shows the real power of compounding over long periods. Changing the rate or the time horizon makes it clear why starting early matters so much. This is an estimate that ignores taxes and inflation, and it all runs locally in your browser.