Compound Interest Calculator

See how a starting amount and regular monthly contributions grow with compound interest. Choose your rate, time horizon and compounding frequency to project your savings.

Enter a starting balance, an optional monthly contribution, an annual interest rate, the number of years and how often interest compounds. The calculator projects the final balance, how much you contributed and how much is interest earned.

Final balance
Total contributed
Interest earned

The Compound Interest Calculator shows the magic of compounding: interest earns interest, so even modest regular contributions snowball over time. It projects the future value of a lump sum plus monthly savings.

The formula

FV = P(1 + i)N + PMT × ((1 + i)N − 1) / i

where P is the starting amount, PMT the contribution per compounding period, i the rate per period (annual rate ÷ periods per year) and N the total number of periods. More frequent compounding grows the balance slightly faster.

Examples

  • $5,000 + $250/month at 7% for 20 years → ~$139,000
  • Doubling your time horizon often more than triples the result
  • Daily vs monthly compounding makes only a small difference

Frequently asked questions

Is the rate guaranteed?

No. Investment returns vary year to year and can be negative. Use a realistic long-term average and treat the result as a projection, not a promise.

Does it account for inflation or tax?

No. This is a nominal projection. Inflation reduces real spending power and tax may be due on the interest. Adjust the rate down for a more conservative real-terms view.

Just for the giggles, much love.
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