Compound Interest Calculator

A compound interest calculator computes growth where interest earns interest: A = P(1 + r/n)^(nt) — principal P at annual rate r, compounded n times a year for t years. $10,000 at 7% for 30 years becomes $76,123 — the last decade alone adds more than the first two combined, which is compounding's whole point.

Einstein's (apocryphal) "eighth wonder of the world" is really just exponential arithmetic: growth applies to an ever-larger base. The practical corollaries — starting early beats contributing more, and high-rate debt compounds against you by the same math — fall straight out of the formula.

Enter your starting amount, annual rate, time horizon and (optionally) regular monthly contributions, and see the final balance with total contributions versus total interest earned — the split that makes compounding visible.

The numbers worth internalizing: at 7% (a typical long-run stock-index real return), money doubles roughly every decade — the Rule of 72 (72 ÷ rate = doubling years) gives the shortcut. Contributions change everything: $200/month at 7% for 30 years ends near $243,000, of which only $72,000 was deposited. And the dark mirror: credit-card debt at 24% compounds the same way against you — a carried $5,000 balance doubles in three years untouched.

Play with the start-age scenario: from 25 to 65 versus 35 to 65 at the same monthly amount typically ends near double — the 10 extra years of compounding, not the extra deposits, do the work.