Free Compound Interest Calculator
Compound interest is the most powerful concept in long-term investing. Enter your starting balance, monthly contribution, expected return, and time horizon — the calculator shows you the exact end value, total contributions, and total interest earned.
S&P 500 historical avg ~10% nominal, ~7% real
Future value
$343,235
After 20 years at 8.00% compounded monthly
Total contributions
$130,000
Total interest earned
$213,235
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How to use this calculator
Formula used:
FV = P(1+r)^n + PMT × [((1+r)^n − 1) / r]
Where:
- P = starting balance
- r = period interest rate (annual rate ÷ compounds per year)
- n = total periods (years × compounds per year)
- PMT = contribution per period
For typical use: pick monthly compounding (the default), enter the annual % you expect to earn, and a monthly contribution that fits your budget. The result is the future value of the account.