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Compound interest calculator

Simulate money growth over time with monthly contributions, interest rate, and a clear wealth chart.

Compound Interest Calculator

Rate type

Investment growth

Total invested Final balance

How the calculation works

Each month, the calculator applies the return to the accumulated balance and then adds the monthly contribution. If the rate is annual, it is converted to an equivalent monthly rate using (1 + rate)^(1/12) − 1.

Example

With $1,000 initially, $500 per month, 8% per year, and 20 years, the chart shows how compound interest becomes a larger share of the balance over time — the classic snowball effect.

Frequently asked questions

Can the rate be monthly or annual?

Yes. Choose the rate type and the calculator converts it when needed.

When is the monthly contribution applied?

The monthly contribution is considered at the end of each month, after that month's return.

Does the result include taxes?

No. The calculation does not include taxes, fees, inflation, or investment costs.

What is the difference between simple and compound interest?

With simple interest, returns are always calculated on the original principal. With compound interest, each period's earnings are added to the balance and earn returns themselves — the snowball effect.

What rate should I use?

Use your investment's actual rate. For long-term stock or index fund projections, historical averages of 7–10% per year (real return) are commonly used as estimates.

How long does it take to double money?

Use the Rule of 72: divide 72 by the annual interest rate. At 8% per year, money roughly doubles in 9 years.

Does the chart show compound interest growing?

Yes. The top curve shows the total balance and the dashed line shows the total contributed. The gap between them is the interest earned — it grows faster as the balance grows.

How to use

  1. Enter the initial investment amount (can be zero).
  2. Set the monthly contribution, interest rate, and whether it is monthly or annual.
  3. Select the period in years.
  4. The chart and results update automatically.

See also