Compound Interest Calculator
Enter a starting balance, a monthly contribution, an expected yearly return and a number of years to see what your money could grow to. Results are estimates, not promises.
Last updated: October 2026How it works
Compound interest formula
Future value = P × (1+r)ⁿ + M × ((1+r)ⁿ − 1) ÷ r, where P is the starting balance, M is the monthly contribution, r is the monthly return (annual % ÷ 12 ÷ 100) and n is the number of months. Interest is added every month and contributions are made at the end of each month.
Worked example
Start with $10,000, add $500 a month and earn 7% a year for 20 years. You contribute $130,000 in total and the balance grows to about $300,851, so roughly $170,851 comes from compounding.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original money and the interest it has already earned. Over long periods this snowball effect becomes the biggest driver of growth.
What is the Rule of 72?
Divide 72 by your annual return to estimate how many years it takes to double your money. At 6% it takes about 12 years, and at 8% about 9 years.
Can I use this for a 401(k), IRA or savings account?
Yes, as an estimate. It does not apply contribution limits, employer matching, fees or taxes, so use it to compare scenarios rather than to predict an exact balance.
What return should I use?
Returns are never guaranteed and change from year to year. Try several rates, such as 4%, 7% and 10%, to see a range of outcomes. A savings account usually earns less than a stock market investment.
Does this include inflation or taxes?
No. Real purchasing power is lower after inflation, and taxes depend on the account type. Treat the result as an illustration.