Compound Interest Calculator
See how your money grows with compound interest. Enter a starting amount, monthly contributions, rate, and time to project the future balance.
Compound interest earns returns on both your principal and previously earned interest. Estimates assume a constant rate; real returns vary.
How to use this calculator
Enter your initial amount, an optional monthly contribution, the expected annual rate, the number of years, and how often interest compounds. You'll see the future balance split into what you contributed and what compounding earned.
How compound interest works
Each period, interest is calculated on your current balance and added back in, so the next period earns interest on a larger base. The longer the money stays invested, the more dramatic the snowball effect.
Worked example
$10,000 plus $200/month at 7% compounded monthly for 20 years grows to roughly $140,000 — of which about $58,000 is contributions and the rest is compound growth.
The formula
A = P(1 + r/n)nt, plus the future value of recurring contributions.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original principal and the interest already added to it. Over time this 'interest on interest' makes savings grow faster than simple interest.
What's the compound interest formula?
A = P(1 + r/n)^(nt), where P is principal, r the annual rate, n the number of compounding periods per year, and t the years. Contributions are added each period on top of this.
How does compounding frequency matter?
More frequent compounding (daily vs. annually) earns slightly more because interest is added and starts earning sooner. The difference grows with higher rates and longer time horizons.
Why do regular contributions matter so much?
Each contribution has its own runway to compound. Adding a fixed amount monthly, even a small one, can dwarf the starting principal over decades thanks to compounding.