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Compound interest calculator
See how money grows when interest earns interest. Enter a starting amount, a monthly contribution and an expected annual return.
How to use it
Fill in the initial amount, how much you add each month, the annual interest rate and the number of years. Interest is compounded monthly.
Formula and method
Final balance = P × (1 + r)^n + C × ((1 + r)^n − 1) ÷ r, where P is the initial amount, C the monthly contribution, r the annual rate ÷ 12 and n the number of months.
Worked example
10,000 invested at 7% per year with 200 added every month for 20 years grows to about 144,600: 58,000 contributed and about 86,600 of interest.
Frequently asked questions
What is compound interest?
Interest calculated on the initial amount and on the interest already earned. Over long periods it makes savings grow much faster than simple interest.
What is the rule of 72?
Divide 72 by the annual rate to estimate how many years it takes to double your money: at 6%, about 12 years.