Compound Interest Calculator
See what a starting balance plus monthly contributions grows to at any rate and horizon, how much of the result is your money versus interest, and a year-by-year table.
Enter what you start with, what you add each month, a return, and a horizon. The calculator compounds monthly and separates what you contributed from what the money earned on its own.
The formula
Each month the balance grows by the monthly rate (annual rate divided by 12), then the contribution is added. That is the standard future-value-of-an-annuity calculation with a lump sum, computed step by step so the year-by-year table is exact rather than approximated.
What rate to use
Use a nominal rate if you want the balance in future dollars, or a real (after-inflation) rate if you want today's purchasing power. A diversified US stock index has returned roughly 10% nominal and about 7% real over long periods; bonds and savings accounts less. The calculator does not subtract taxes or fees; a 0.5% annual fee on a 7% return leaves 6.5%, which over 30 years is a materially smaller number.