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 comes from.
What does compounding add up to?
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.
Related reading
The 529 Plan: A College Fund That Can Outlive College
A 529 isn't a single-purpose account anymore. It can pay for school, move $35,000 into your kid's Roth IRA, and still be compounding for a grandchild who doesn't exist yet.
Read →The Custodial Roth IRA: The Most Underused Account in America
A 9-year-old with earned income can have a Roth IRA. Five summers of matched lawn-mowing money, $12,500 in all, gets you two thirds of the way to what 35 years of maxing out does.
Read →High Yield Savings Account vs Money Market: Where Should Your Cash Live?
Your emergency fund can do better than 0.01%. A top high yield savings account against a money market fund, and which one I would pick.
Read →