Compound Interest Calculator

Compound interest is interest earning interest: each period's growth is added to the balance, and the next period's growth is calculated on that larger amount. The result is exponential rather than linear — slow-looking at first, then dramatic. It's the mechanism behind long-term investing, and equally behind long-term debt.

This calculator handles the realistic case most simple tools skip: a starting lump sum plus regular monthly contributions. Set the annual rate, the time period, and how often interest compounds (yearly, half-yearly, quarterly or monthly), and it computes the final value with a month-by-month simulation — accurate even when your contribution schedule and the compounding frequency differ. The result splits into what you put in versus what compounding added, with a visual bar that makes the growth share unmistakable.

Try this experiment: enter any amount at 8% for 10 years, note the interest, then change it to 30 years. The interest doesn't triple — it grows roughly tenfold. That non-intuitive leap is why every finance guide repeats the same advice: starting early matters more than starting big. Works in eight currencies, entirely in your browser.