Free tool

Compound interest calculator

See how a starting amount plus regular monthly contributions could grow over time. This is the single most powerful idea in investing — small amounts, left to compound, add up.

Starting amount (£)

Monthly contribution (£)

Annual return (%)

Number of years

After 20 years, you could have

£0

You put in £0 · growth from compounding £0

How compound interest works

Compounding means your growth earns its own growth. In year one you earn a return on your money; in year two you earn a return on your money and on last year's return; and so on. The effect starts slowly and then accelerates — which is why starting early matters far more than starting big.

Try this: keep everything the same but change the years from 20 to 30. The extra decade often adds more than doubling your monthly contribution would. That's compounding — and time is the ingredient you can't buy back.

A note on the "annual return" figure

This calculator assumes a steady yearly return for simplicity. Real investments don't grow in a smooth line — some years are up, some are down. Historically, broad stock markets have averaged roughly 5–8% a year over the very long term, but the future isn't guaranteed and your actual returns could be higher or lower (or negative).

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This calculator is for illustration and education only. It is not financial advice or a forecast, and assumes a constant rate of return that real investments do not provide. Investing involves risk, including the possible loss of the money you invest. Figures ignore fees, tax and inflation. Always do your own research or consult a qualified, regulated adviser.