Compound Interest Calculator
Free compound interest calculator. Enter your principal, interest rate, time horizon, compounding frequency, and optional monthly contributions to project future value, total contributions, and total interest earned.
This compound interest calculator projects how much a lump sum and a stream of regular contributions could grow over time. Enter your starting principal, the annual interest rate, the number of years, how often interest compounds, and an optional monthly contribution — the results update live in your browser as you change any value.
The lump-sum growth is computed with the compound interest formula A = P(1 + r/n)nt, where P is the principal, r is the annual rate, n is the number of compounding periods per year, and t is the number of years. Monthly contributions are grown as a future value of a series. You also see your total contributions and the total interest earned, so you can see exactly how much of the final balance came from growth versus the money you put in.
Frequently asked questions
How does compounding frequency affect the result?
More frequent compounding (e.g. daily vs. annually) lets interest start earning interest sooner, which slightly increases the future value for the same nominal rate.
Are contributions added before or after interest?
Monthly contributions are added each month and earn compound growth from the month they are deposited through the end of the time horizon.
What is total interest earned?
It is the final future value minus your initial principal and all the contributions you made — in other words, the portion of the balance produced purely by compounding.
Is this calculation done on a server?
No. Everything runs locally in your browser, so your figures are never uploaded anywhere.
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