Compound Interest Calculator

Compound interest is interest earned on both your original deposit and the interest it has already generated - money making money on its own. Enter a starting amount, an optional monthly contribution, an expected annual return and a time horizon, and this calculator projects your balance year by year. Everything runs in your browser: nothing is uploaded, stored or tracked.

Your numbers

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$
%
yrs
Advanced: inflation
%
Future balance —
Total contributions—
Interest earned—
Value in today’s money—
ContributionsInterest earned

Results update as you type and are estimates for education only — they don't account for taxes, fees or your personal situation, and nothing here is financial advice. Your inputs stay on this device.

How it works

  1. Enter your initial deposit - the amount you are starting with today.
  2. Add a monthly contribution if you plan to keep investing regularly (this is where most long-term growth comes from).
  3. Set the expected annual interest rate. Long-run diversified stock returns have historically averaged around 7-10% before inflation; savings accounts are far lower.
  4. Choose how often interest compounds and how many years you will stay invested, then read your projected balance, total contributions and total interest.

The formula

Future value = P(1 + r/n)^(nt) + PMT x [((1 + r/n)^(nt) - 1) / (r/n)], where P is the starting principal, PMT the contribution per compounding period, r the annual rate, n compounding periods per year and t the number of years. Contributions are assumed to arrive at the end of each period.

Frequently asked questions

What is compound interest in simple terms?
It is interest calculated on your balance including previously earned interest, not just your original deposit. Each period your interest earns interest of its own, which is why balances grow slowly at first and then accelerate dramatically over long periods.
How often should interest compound for the best result?
More frequent compounding grows money slightly faster: daily beats monthly, which beats annually. In practice the difference between monthly and daily compounding is small - your contribution amount, rate and time horizon matter far more.
What annual return should I assume?
For a high-yield savings account use the advertised APY. For a diversified stock index portfolio, long-term historical averages sit roughly between 7% and 10% per year before inflation, but returns vary a lot year to year and are never guaranteed. Testing a conservative and an optimistic scenario is wise.
Does this calculator account for taxes or inflation?
No - results are gross, pre-tax nominal values. Inflation typically erodes purchasing power by 2-3% per year in the US and Europe, so pair this tool with our Inflation Calculator to see results in today's money.
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