Compound Interest Calculator
Compound interest is the whole game in investing — your returns start earning returns. Set a starting amount, a monthly contribution, and a time horizon, and watch where the growth actually comes from.
What does compounding actually do?
The eighth wonder of the world, supposedly. Put in your numbers and watch the interest take over.
Balance after 30 years
$618,102
You put in
$181,000
Interest earned
$437,102
You contribute $181,000; the market adds $437,102. At 7% over 30 years, compounding does 71% of the work.
How this works
The calculator compounds monthly: each month your balance earns one-twelfth of the annual rate, then your contribution is added, and the new total earns interest next month. That's the snowball.
The two levers that matter most are time and rate. Doubling your monthly contribution roughly doubles the result; adding ten more years can do much more than that, because the earliest dollars compound the longest.
Real returns aren't a smooth line — markets swing year to year. A broad index fund has historically averaged somewhere around 7–10% nominal over long periods, but any single year can be wildly higher or lower.
// Illustrative only. Not financial advice. Assumes a constant return and steady contributions, which reality won't provide.