In short: Compound interest earns interest on previous interest: A = P(1 + r/n)^(nt), so savings grow faster the more often interest compounds and the longer you wait.
About Compound Interest Calculator
Compound interest means earning interest on previous interest. Over long periods it is the difference between a balance that grows in a straight line and one that curves upward. Regular contributions add to the effect, because each deposit starts compounding as soon as it is made.
Enter a starting amount, a monthly contribution, an annual rate and a number of years. The calculator compounds at the frequency you choose and adds contributions at the end of each month, then shows the final balance, how much you put in and how much came from interest, along with a year-by-year chart and table.
How to use Compound Interest Calculator
- Enter your starting amount and how much you will add each month.
- Enter an expected annual rate and the number of years.
- Choose how often interest compounds.
- Read the final balance and see how interest overtakes contributions over time.
Examples
10,000 to start, 200 a month at 5% compounded monthly for 20 years grows to roughly 109,000, of which 58,000 is your money and the rest interest.
Run the same numbers for 30 years instead of 20: the balance more than doubles, mostly from interest.
Frequently asked questions
What formula is used?
Balance is updated month by month: interest is applied at the chosen compounding frequency (using the equivalent monthly rate) and the contribution is added at month end.
What rate should I use?
Use the rate your savings account pays, or a conservative long-term estimate for investments. Past returns do not guarantee future ones.
Does more frequent compounding make a big difference?
Less than people expect. At 5%, monthly compounding gives an effective annual rate of 5.12% versus 5% for yearly.
Is inflation included?
No. To estimate real (inflation-adjusted) growth, subtract the expected inflation rate from the annual rate.
Updated November 2026 · Free, no signup · How we build and test tools