Compound Interest Calculator

Compound interest pays interest on the interest already earned, rather than only on the original amount. Over a year or two the difference is negligible. Over decades it is the difference between a modest return and a transformed one, which is why it gets talked about the way it does.

This calculator shows both figures side by side — what you would get with compounding and what plain simple interest would have produced — because that comparison is the whole point and almost no calculator shows it. At 10% for ten years, compounding roughly doubles the interest earned compared with simple interest on the same money.

Frequency matters, though less than people assume. Moving from yearly to monthly compounding on a ten-year deposit adds a meaningful amount; moving from monthly to daily adds very little on top. The formula is A = P(1 + r/n)^(nt), where n is how many times a year interest is added, and you can switch between yearly, half-yearly, quarterly, monthly and daily to see the effect.

You can also add a monthly contribution, which is how most people actually save. Those deposits are compounded monthly regardless of the headline frequency, because that is when the money genuinely arrives in the account.

Nothing you enter leaves your browser, and the result is an estimate — real accounts have tax, fees and rates that change.