Compound interest means you earn interest on your interest. Over long periods it does most of the work. Enter a starting amount, a monthly contribution and a rate to watch it grow.
How to use this calculator
- Enter what you're starting with.
- Add how much you'll contribute each month.
- Enter the yearly interest rate or expected return: a high-yield savings account might pay 3–4%, while stock market averages are historically around 7–10% before inflation.
- Choose the number of years and how often interest compounds.
How it's calculated
A = P(1 + r/n)nt + PMT × ((1 + r/n)nt − 1) ÷ (r/n)
P is the starting amount, r the yearly rate, n compounding periods per year, t years and PMT the regular contribution. The calculator simulates it month by month, so it handles any compounding frequency with monthly deposits.
Example: $10,000 plus $200 a month at 7% compounded monthly grows to about $144,600 after 20 years. You contributed $58,000, and the rest is interest.
Frequently asked questions
What is the rule of 72?
Divide 72 by your interest rate to estimate how many years it takes to double your money. At 6%, money doubles in about 12 years; at 9%, about 8 years.
Does compounding frequency matter much?
A little. Daily compounding earns slightly more than monthly, and monthly more than yearly, but the rate and the time invested matter far more.
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Estimates for general information only, not financial advice. See how we calculate.