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Compound Interest Calculator
See how a starting balance and regular monthly contributions grow when your returns earn returns of their own.
How compound interest works
Compound interest means you earn returns not only on the money you put in, but also on the gains it has already produced. Over long periods this snowball effect does most of the work: in many long-term projections, the interest earned ends up larger than the total you contributed.
The formula
For a single deposit, future value = P × (1 + r/n)^(n×t), where P is the principal, r the annual rate, n the number of compounding periods per year and t the number of years. This calculator also adds a monthly contribution and compounds it alongside your balance.
How to use this compound interest calculator
Enter your starting amount, how much you plan to add each month, your expected annual return and the number of years. The calculator projects your future balance and splits it into what you contributed and what came from growth, so you can see how much compounding adds over time.
Example: $10,000 invested with $500 added each month at a 7% annual return grows to roughly $700,000 after 30 years, from about $190,000 of total contributions. Try changing the number of years: adding or removing a decade usually changes the result far more than changing the monthly amount, which is why starting early matters so much.
Choosing a return rate: broad US stock indexes have historically returned roughly 7% to 10% a year over very long periods before inflation, but returns vary widely from year to year and are never guaranteed. Running a cautious case (for example 5%) alongside a base case gives a more realistic range. Subtract fund fees from your expected return to see their long-term cost.
Related guides
How compound interest grows your investments · Dollar-cost averaging explained · How to start investing with little money · How inflation affects your investments · Asset allocation by age
Frequently asked questions
What return should I use?
Many investors use 6% to 8% a year for a diversified stock portfolio as a conservative long-run assumption. Actual returns vary from year to year and are never guaranteed.
Does this include inflation?
No. Results are in future (nominal) dollars. To think in today's dollars, subtract an inflation estimate (for example 2.5%) from your return assumption.
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