Free calculator
Compound Interest Calculator
Enter the starting amount, monthly contribution, expected return and horizon. The table shows how the balance grows each year.
| Year | Contributed | Interest | Balance |
|---|
How compound growth works
Compound interest means that each period's return is added to the balance, so future returns apply to a larger base. With monthly contributions the future value is FV = P(1+r)n + C × ((1+r)n − 1) / r, where P is the starting amount, C the monthly contribution, r the monthly return and n the number of months.
The model assumes a constant return and monthly compounding, which is a simplification: real markets move, fees reduce returns and taxes depend on the account. Treat the result as a planning range, not a promise.
What moves the result most
Time has the largest effect, then contribution size, then the rate. Starting ten years earlier often beats doubling the monthly contribution, because the early balance compounds for longer.
Frequently asked questions
Does the calculator assume monthly compounding?
Yes. The annual return is divided by 12 and applied to the balance every month, with the contribution added at the end of each month.
Is the return guaranteed?
No. It is an assumption you choose. Real returns vary and can be negative in some years.
How do fees affect the result?
Fees reduce the effective return. A 1% annual fee on a 7% return cuts the final balance materially over long periods, so use the net return.
Can I model withdrawals?
Not on this page. Use a negative monthly contribution only if you understand that the result assumes the balance stays positive.