Compound Interest Calculator
See how your money grows over time when interest compounds and you keep adding to it. Enter a starting amount, a monthly contribution, an expected return, and a time horizon, the calculator shows your future value, how much you contributed, and how much is pure interest.
The one idea behind every long-term plan
Compound interest is the closest thing investing has to a free lunch, and it rewards exactly one thing: time. Money left to compound doesn't grow in a straight line, it grows in a curve that starts almost flat and then bends sharply upward, because you eventually earn interest on years of accumulated interest.
Try it yourself above. Drag the years slider and watch the "interest earned" number pull away from "you contributed." In the early years your contributions do most of the work. Somewhere around the two-decade mark, the interest quietly takes over and never looks back. That crossover is the whole reason "start early" is the most repeated advice in personal finance, and one of the truest.
How to use this calculator honestly
- Use a conservative return. A lower assumption that you beat is far better than a rosy one you miss.
- Remember inflation. A number 30 years from now buys less than it does today. Treat the result as tomorrow's dollars.
- Focus on the contribution. You can't control the market, but you fully control how much you add each month, and early on, that's what matters most.
Common questions
What is compound interest?
Compound interest is interest earned on both your original money and the interest it has already earned. Each period's interest is added to the balance, so growth accelerates over time, and the longer you stay invested, the more of your final balance comes from interest rather than your own contributions.
How is compound interest calculated?
For a lump sum, future value = principal x (1 + r/n)nxt, where r is the annual rate, n is compounds per year, and t is years. With regular contributions, each one compounds from when it's added. This calculator compounds monthly and assumes end-of-month contributions.
What return should I assume?
There's no guaranteed number. Historically a broad US stock index has averaged roughly 7% a year after inflation over long periods, but returns swing widely and the past doesn't predict the future. Use a conservative assumption and treat the result as an estimate, not a promise.
Why do small differences in return matter so much?
Because compounding multiplies. Over 30 years the gap between 6% and 8% isn't "2%", it can be tens of thousands of dollars, since the higher rate compounds on a bigger balance every year. It's also why fees hurt so much: a 1% fee is a 1% lower return, compounded for decades.
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This calculator is for education and general information only, not financial, investment, or tax advice. Results are estimates based on the assumptions you enter and do not predict actual returns.