Compound Interest Calculator
See how your money compounds: enter a starting amount, optional monthly contributions, rate and time — get the final balance and a year-by-year growth table.
| Year | Balance | Contributed | Interest earned |
|---|
This calculator is for illustration only and assumes a constant rate of return. Actual investment returns vary and are not guaranteed.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original deposit and on previously earned interest — “interest on interest.” Over long periods this snowball effect dramatically outpaces simple interest: $10,000 at 7% for 30 years grows to about $76,123 with annual compounding, versus just $31,000 with simple interest.
How do I calculate compound interest?
The formula is A = P(1 + r/n)^(nt), where A is the final amount, P your principal, r the annual rate as a decimal, n the compounding frequency per year, and t the time in years. For monthly contributions, each deposit compounds separately — the calculator above handles all of this for you.
What is the Rule of 72?
The Rule of 72 is a quick mental math trick: divide 72 by your annual return rate to estimate how long it takes money to double. At 8% per year, money doubles roughly every 9 years; at 6%, every 12 years. It’s remarkably accurate for rates between 4% and 12%.