Compound Interest Calculator
Enter a starting amount, a monthly contribution and an interest rate to see how your money could grow over time.
Compound interest calculator
Assumes contributions are added at the end of each month and the rate stays constant. Taxes and fees are not included.
How this calculator works
Compound interest means you earn interest on your original deposit and on the interest you've already earned.
The calculator applies the standard formula A = P(1 + r/n)nt, where P is your starting balance,
r is the annual rate, n is how many times per year interest compounds and t is the number of years.
Your monthly contributions are added at the end of each month and start earning interest right away.
Tips for using the results
- Time matters most. Doubling the number of years usually does more than doubling the rate, because growth builds on itself.
- Compounding frequency matters less than you think. Daily vs. monthly compounding changes the result only slightly; the rate and time are what move the needle.
- Use a realistic rate. Savings accounts and Treasury bills pay variable rates that change with the economy. Check today's rates for a reference point.
New to the topic? Read What is compound interest? and APR vs. APY.
This content is for educational purposes only and isn't financial, tax or investment advice. Rates change daily; confirm current terms with your bank, broker or TreasuryDirect before making decisions. See our full disclaimer.