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Compound Interest Calculator

How much will my money grow, and when will I hit $1 million?

$
$
%
Compounding
%
$

Balance after 30 years

$691,150

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You put in
$190,000
Growth
$501,150
In today's dollars
$329,501
Reach $1,000,000
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How it grows

Money you put inGrowth
Year-by-year table
YearPut in so farGrowth so farBalance

How it works

Compound interest means your returns earn returns of their own. Each month the calculator grows your balance by the monthly rate, then adds that month's contribution. Repeat that for every month and small, steady amounts turn into large sums, mostly in the later years.

Balance next month = balance × (1 + monthly rate) + monthly contribution

With monthly compounding the monthly rate is the yearly rate ÷ 12. With yearly compounding we use the monthly rate that compounds to exactly the yearly rate, so contributions made mid-year still earn their share.

Worked example

Start with $10,000, add $500 a month and earn 7% a year, compounded monthly. After 30 years:

You've put in $190,000, and the balance is $691,150. $501,150 of that is growth.

Compounded yearly instead, the same plan ends at $660,849. At 2.5% inflation, $691,150 in 30 years buys what about $329,501 buys today.

The same plan reaches $1,000,000 after 34 years and 10 months.

Things to know

  • Returns aren't smooth. Investments rise and fall year to year. A fixed rate is a planning assumption, not a promise.
  • Fees compound too. A 1% yearly fee comes straight off your return: enter 6% instead of 7% to see what it costs over decades.
  • Think in today's dollars. A million in 30 years won't buy what a million buys now. The inflation figure shows the difference.
  • The rule of 72: divide 72 by your yearly return to estimate the years it takes money to double. At 7%, 72 ÷ 7 ≈ 10.3 years; the exact figure is 10.2.

Questions people ask

What return should I use?

It depends on what you invest in. Savings accounts pay their stated rate. Stock-heavy portfolios have historically returned more over long periods, with big swings along the way. Try a cautious and an optimistic rate and plan around the range.

Does compounding frequency matter much?

Less than people think. Over 30 years in the example above, monthly compounding beats yearly by $30,302. The rate, the time and how much you add matter far more.

How is this different from simple interest?

Simple interest pays only on the original amount. Compound interest also pays on the interest already earned, so the gap widens every year.

Sources

Reviewed September 24, 2026 by The Dollars World team. How we build and check our calculators. This is general information, not financial advice.