04 · Calculators

Compound Interest Calculator

Find out what a starting amount and regular monthly deposits could grow to, and how much of that growth is interest.

Final balance

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How to use the compound interest calculator

  1. Enter what you’re starting with and how much you’ll add each month.
  2. Enter the expected yearly interest rate and how many years to look ahead.
  3. Pick how often interest compounds.
  4. Compare total contributed with interest earned, and check the yearly table.

How compounding works

Compound interest means you earn interest on your interest. Each period, interest is added to the balance, and the next period’s interest is calculated on that larger balance. Over long periods this snowball effect usually matters more than the exact rate.

In this calculator, deposits are added at the end of each month and interest compounds at the frequency you choose. Changing from monthly to daily compounding makes only a small difference; time and the rate matter far more.

Keep the estimate realistic

  • Investment returns aren’t guaranteed and vary from year to year. A fixed rate is a simplification.
  • Inflation reduces what the final balance can buy.
  • Taxes and fees aren’t included.

Examples

$10,000 start, $250 a month, 6%, 15 years

About $97,246 · $55,000 contributed · $42,246 interest

$5,000, no deposits, 4% yearly, 10 years

About $7,401

Questions

Is this the same as the “rule of 72”?

The rule of 72 is a shortcut: divide 72 by the rate to estimate how many years money takes to double. This calculator does the exact math instead.

Can I use a negative rate?

Yes, down to −100%, which can help you model losses or fees.

When are contributions added?

At the end of each month. Adding them at the start would give a slightly higher result.