Compound Interest Calculator
See how an investment grows over time with compound interest and regular monthly contributions.
Future value
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Total contributions
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Total interest earned
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| Year | Contributions to date | Interest earned to date | Balance |
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What compound interest actually does
Compound interest is interest earned on interest — each period's gains get added to the balance and start earning their own return the next period. Over long stretches of time that compounding effect grows much faster than simple interest, which is why starting early tends to matter more than the exact monthly amount.
Enter a starting amount, a monthly contribution, an expected annual rate, and a time horizon to see the future value, and how much of it came from your own contributions versus interest earned. The chart and table below break that down year by year.
A note on the rate
This calculator assumes a constant annual return compounded monthly, which is a simplification — real investments fluctuate year to year. Use it to compare scenarios (higher contribution vs. more time vs. a different rate assumption), not as a guaranteed outcome.