Compound interest is the interest you earn on both your original money and on the interest it has already earned. Over years, that snowball is what turns steady saving into a meaningfully larger balance. Use the calculator to see how a starting amount and a monthly contribution grow at a given rate.

How this calculator works

It compounds monthly. Each month your balance earns one twelfth of the annual rate, then your monthly contribution is added. Future value combines two parts: your starting balance growing on its own, plus the growing stream of monthly contributions. The ledger bar under the result shows how much of the total is your own money versus interest earned.

An example

Start with $1,000, add $200 a month, and assume a 7% annual return for 20 years. You contribute $49,000 of your own money, and compounding does the rest — the longer the horizon, the larger the interest slice becomes relative to what you put in.

Good to know

Contributions are added at the end of each month.
The rate is a steady annual estimate — real returns vary year to year.
Results ignore taxes, fees, and inflation.

This tool is for education, not financial advice.