Tool
Compound interest calculator
Compounding is how money earns its own returns. Put your numbers into this compound interest calculator and watch what time does to them. No account, no sign-up — the math runs entirely in your browser.
Your numbers
Your future value
- Total contributed
- $0
- Growth
- $0
- Years to double (Rule of 72)
- —
Compound growth assumes contributions happen monthly and the same return every year. Real markets bounce around — this is a planner's estimate, not a promise.
Frequently asked questions
How is compound interest calculated?
Your money earns a return, and then that return starts earning too. The rough version: future value = starting amount × (1 + annual return) ^ years, with any monthly contributions added in and growing from the month you add them. This calculator runs that math with your numbers — the bar below the result shows how much came from you and how much came from compounding.
What is the Rule of 72?
Divide 72 by your annual return and you get roughly how many years it takes your money to double. At 7% a year, that is about 10 years. It is a shortcut, not a promise: real doubling lands near that number, never exactly on it. The result panel shows the doubling time for whatever return you typed in.
Does this calculator assume monthly contributions?
Yes. Anything in the monthly contribution field is added once a month and starts earning from that month onward, which is how most people actually save — paycheck to account, the same day each month. Set it to zero and the calculator runs on your starting amount alone.
Is compound interest the same as an average investment return?
No, and that is where a lot of projections quietly mislead. The return is the input you choose; compounding is what happens to it afterward. A smooth 7% every year is a planner’s assumption, not history — broad market funds have had years like 20% and years well below zero. Try a lower number in the return field and see how much of the projection was time rather than cleverness.