Toolkeel

Compound interest calculator

Watch how money grows when interest earns interest. Add a monthly contribution to see the effect of saving regularly.

  • Free
  • Private
  • No sign-up
Final balance

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Total contributed

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Interest earned

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Contributed Interest
YearContributedInterestBalance
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How to use it

  1. Enter the starting amount, monthly contribution, rate and number of years.

  2. Choose how often interest compounds.

  3. Read the final balance, then explore the chart and yearly table.

About this tool

Compound interest means interest is added to the balance and then earns interest itself. The formula for a lump sum is A = P × (1 + r/n)n×t, where P is the starting amount, r the annual rate, n the number of compounding periods per year and t the number of years. This calculator also adds a contribution at the end of every month.

The "rule of 72" is a quick check: divide 72 by the annual rate to estimate how many years money takes to double (about 12 years at 6%). Real returns vary from year to year, and inflation, fees and taxes reduce them.

Good to know

  • Assumes a constant rate; investment returns are not guaranteed.
  • Does not include inflation, fees or taxes.
  • For information only, not financial advice.

Frequently asked questions

What is compound interest?

Interest calculated on both the original amount and the interest already earned, so growth speeds up over time.

How much difference do monthly contributions make?

Often more than the starting amount. Try the calculator with and without a contribution to compare.

Does compounding frequency matter?

A little. Monthly compounding gives slightly more than yearly at the same rate; the difference grows with higher rates and longer periods.

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Last updated 2026-10-20 by the Toolkeel team. How we build and check our tools.