Compound Interest Calculator
See how an initial deposit and regular contributions grow over time.
About this calculator
Compound interest is interest earned on interest. It turns small, consistent saving into significant wealth over time and is the single most important concept in personal finance. This calculator lets you model exactly what your savings or investment will be worth after any number of years.
How it works
Every compounding period — annually, quarterly, monthly or daily — the calculator adds interest on the current balance, then folds it back into the principal. Next period’s interest is then calculated on the new, larger balance. That feedback loop is what creates exponential growth.
Compounding frequency matters less than you’d think. Going from monthly to daily compounding makes a tiny difference at typical interest rates. Time and contributions matter far more than how often the interest is paid in.
Regular contributions massively amplify the result. Adding a small amount each month, every month, for decades produces vastly more than a single lump sum because each contribution gets the benefit of compounding from the moment it’s made.
Inflation eats into nominal returns. A 7% return at 3% inflation is really about a 4% real return. Always think in real (inflation-adjusted) terms when planning long-horizon goals.
Formula
FV = P · (1 + i)^n + C · ( (1 + i)^n − 1 ) / i
FV = future value
P = initial principal
C = contribution per period
i = rate per period = annual rate / compounds-per-year
n = total number of periods = years × compounds-per-yearExamples
10,000 invested for 20 years at 7%
Left alone for 20 years at 7% compounded monthly, 10,000 grows to about 40,387 — roughly four times the original amount.
Result: Final balance ≈ 40,387 — Interest earned ≈ 30,387
Same lump sum + 200 per month
Adding 200 each month for 20 years pushes the same starting amount to about 144,580 — more than triple the lump-sum-only result.
Result: Final balance ≈ 144,580 — Interest earned ≈ 86,580