Compound Interest Calculator
Watch a balance grow: starting amount, monthly contributions, rate and time — with a year-by-year breakdown.
This shows the mathematics of a constant rate — real investments fluctuate and can lose value, and savings rates change. It's a learning and comparison tool, not a forecast or financial advice.
About this tool
Compound interest is the reason small, boring, regular saving beats occasional heroics: interest earns interest, and the curve bends upward the longer you leave it alone. This calculator takes a starting amount, an optional monthly contribution, an annual rate and a number of years and shows the final balance, how much of it you actually put in, and how much the compounding did — plus a year-by-year table so you can watch the acceleration.
Try the classic experiment: 20 years at 6% versus 30 years at 6%. The extra decade doesn't add a third — it roughly doubles the interest. Time is the biggest lever in the whole formula.
How to use it
- Enter a starting amount (0 is fine) and an optional monthly contribution.
- Set the annual rate, the number of years and how often interest compounds.
- Read the totals and scan the year-by-year table — everything updates live.
Frequently asked questions
What is compound interest?
Interest earned on both your original money and on the interest it has already earned. That feedback loop is why balances grow slowly at first and then accelerate — the longer the time, the more dramatic the effect.
How does compounding frequency affect the result?
More frequent compounding (monthly vs yearly) earns slightly more, because interest starts earning interest sooner. The difference is real but modest — time and rate matter far more.
How are monthly contributions handled?
Each contribution is added at the end of every month and then compounds along with the rest of the balance for the remaining time. The year-by-year table shows the running effect.
Is this a prediction of investment returns?
No — it's the mathematics of a constant interest rate. Real investments fluctuate and can lose money, and savings rates change over time. Use it to understand the mechanics and compare scenarios, not as a forecast, and note that it isn't financial advice.