Compound Interest Calculator
See how savings and investments grow with compounding and regular contributions.
About this compound interest calculator
Compound interest means you earn interest on your interest. Each period's earnings are added to the balance, so the next period's interest is calculated on a larger amount. Over long periods this snowball effect does most of the work — which is why starting early often matters more than the amount you save.
This calculator combines an initial deposit with regular monthly contributions and lets you choose how often interest compounds. The year-by-year table shows how much came from your own deposits versus growth.
How to use the Compound Interest Calculator
- Enter your starting balance and how much you will add each month.
- Enter the expected annual rate of return or interest rate.
- Choose the number of years and the compounding frequency.
- Read the future balance and the growth table.
The compound interest formula
A = P(1 + r/n)nt + contributions grown at the same rate
P is the starting amount, r the annual rate, n the compounding periods per year and t the years. With monthly contributions, the calculator converts the rate to an equivalent monthly rate and adds each deposit at the end of the month, then grows the balance month by month.
Worked example
$10,000 invested at 7% compounded monthly, plus $200 a month for 10 years, grows to about $54,714. You deposited $34,000, so roughly $20,714 is interest. Leave it for 20 more years and the interest portion overtakes your deposits several times over.
Tips for better results
- Use a conservative rate for investments — returns vary year to year and are not guaranteed.
- Compounding frequency matters less than rate and time: daily vs monthly compounding changes the result only slightly.
- Remember inflation. Use the inflation calculator to see what a future balance is worth in today's dollars.
Frequently asked questions
What is the rule of 72?
Divide 72 by the annual rate to estimate how many years it takes money to double. At 8%, money doubles in about 9 years.
Does more frequent compounding make a big difference?
Only a little. At 5%, yearly compounding gives an effective 5.00% and daily compounding about 5.13%.
Are contributions added at the start or end of each month?
At the end of each month, which is the common convention for savings plans.
This calculator provides estimates for planning only and is not financial advice. Confirm exact figures with your lender or a licensed professional.
Last reviewed: October 2026 · Results are calculated in your browser.