Compound Interest Calculator
See how compound interest grows your money over time with regular contributions. Results update instantly.
Written and reviewed per our independent editorial methodology.
This free compound interest calculator shows how your money grows as interest earns interest on itself, month after month. Enter your starting balance, what you add each month, your APY and a time frame, and the results update instantly, no sign-up required.
How this compound interest calculator works
The calculator takes your interest rate and compounding frequency, converts them to an exact per-period rate, and grows your balance while adding your contribution each month. The chart splits your total into what you put in versus interest earned, so you can see exactly how much the bank is paying you.
Your real return depends on the rate staying competitive. A national-average account and a high-yield account look identical on day one but drift worlds apart over a few years, which is why the APY you choose matters more than any other input.
How fast will your money double? The Rule of 72
The Rule of 72 estimates doubling time: divide 72 by your rate. Reinvested interest and time do the heavy lifting. If you are not sure what to aim for, start with our guide on how to choose a high-yield savings account and the basics of what APY means. You can also cross-check the concepts with the U.S. Consumer Financial Protection Bureau.
The compound interest formula we use
The classic compound interest formula, where r is your rate, n your compounding frequency and t the years, plus your monthly contributions:
A = P(1 + r/n)nt + contributions
Example: $10,000 at 7% compounded monthly for 10 years, plus $500 a month, grows to about $106,639, and $36,639 of that is pure interest.
