How to use this compound interest calculator
Enter your starting amount, a monthly contribution, your expected annual interest rate, the number of years, and how often interest compounds. The projection updates instantly, splitting your future balance into what you put in versus what compounding earned.
What is compound interest?
Compound interest is interest earned on your interest. Each period, the interest you earn is added to your balance, and the next period's interest is calculated on that larger amount. Over time this creates exponential, snowballing growth — which is why Einstein supposedly called it the eighth wonder of the world.
Why starting early wins
Time is the most powerful input. Because growth compounds on itself, money invested in your 20s and 30s has decades to multiply — often ending up as a far larger share of your balance than money added later. Even small, consistent contributions become substantial given enough time.
Frequently asked questions
What is compound interest?
Interest earned on both your original money and the interest it has already earned — so your balance grows faster and faster.
How is it calculated?
A = P(1 + r/n)nt, plus the growth on your ongoing contributions, which this calculator adds in for you.
How often should interest compound?
More often is better — daily beats monthly beats annual — though rate, contributions, and time matter far more. You can choose the frequency above.
Simple vs compound interest?
Simple interest grows in a straight line on the principal only; compound interest grows exponentially because it earns on accumulated interest too.
Related calculators & guides
This calculator is for educational and planning purposes only and provides estimates, not financial advice. Real returns vary and are not guaranteed. See how we calculate.