See how a starting balance and steady monthly deposits snowball over time. Enter your numbers below to get your future value, total contributions and total interest earned — instantly, with no sign-up.
Compound interest is the engine behind almost every long-term investing plan. Unlike simple interest, which only ever pays you on your original deposit, compound interest pays you interest on your interest. Each period the balance grows, and the next period's interest is calculated on that larger balance. Over a few years the difference is modest; over a few decades it becomes the single biggest driver of your final wealth.
This compound interest calculator combines two classic finance formulas. Your starting principal grows by the standard lump-sum compound formula, while your monthly contributions grow by the future value of an ordinary annuity. Adding the two together gives a realistic projection for a savings or investment plan where you put money in regularly.
The future value of a single lump sum is:
A = P × (1 + r/n)n·t
where P is the principal, r is the annual rate written as a decimal (7% = 0.07), n is the number of compounding periods per year, and t is the number of years. For regular deposits, the calculator adds the future value of an annuity, compounding every monthly deposit for the exact time it stays invested. The result is the same math a savings account, brokerage or retirement plan uses to project growth.
Worked example: Put in $10,000 today, add $300 a month, earn 7% a year compounded monthly for 20 years, and you finish with roughly $196,000. You contributed about $82,000 of that — the other ~$114,000 is pure compound interest.
In a savings account, the rate is fixed and the compounding is contractual, so the projection is fairly precise. In the stock market, "compounding" comes from reinvested gains and dividends, and the real-world rate bounces around year to year — some years up 25%, some down 15%. The long-run average for a broad index has historically landed in the high single digits, which is why 7% is a common planning assumption. Use this calculator to model the smooth average, then remember that real markets are bumpy. You can feel that volatility risk-free in our stock market simulator.
Reality check: these are estimates based on a constant rate you choose. Real returns vary, inflation erodes purchasing power, and fees and taxes reduce results. Nothing here is financial advice — for trustworthy basics see the U.S. SEC at investor.gov.
Want to turn the theory into practice? Run the numbers here, then try a longer-horizon strategy in the dividend calculator or model steady monthly investing with the dollar-cost averaging calculator.
Compound interest uses the formula A = P(1 + r/n)^(nt), where P is the principal, r is the annual rate as a decimal, n is compounding periods per year, and t is years. Regular contributions are added with the future value of an annuity, so each deposit grows for the time it stays invested.
More frequent compounding (monthly or daily instead of annually) lets interest start earning its own interest sooner, so the final balance is slightly higher for the same nominal rate. The effect is real but small next to the rate, contribution and number of years.
Yes. It runs entirely in your browser with no sign-up and no data sent anywhere. All results are estimates for education only and are not financial advice.
Yes. Enter a monthly contribution and the calculator compounds your principal and every monthly deposit, then shows total contributions and total interest so you can see how much came from compounding versus your own deposits.