Compound Interest Calculator
Free · no sign-up · reviewed July 2026
Compound interest is when your money earns returns, and then those returns earn returns too. Over years, that snowball is what builds real wealth.
Use the switch at the top to flip between two questions: “what will my money grow to?” and “how long until I hit a goal?”, like the classic “how long to become a millionaire.” Drag the sliders and watch the chart and insights update instantly.
Drag to adjust
In 25 years you'll have
$462,290
$302,290 of that is growth you never had to earn
💡 Your money more than doubles itself: you put in $160,000 and compounding adds $302,290 on top, that's the snowball effect.
2.9×
Growth multiple
of what you put in
~10 yrs
Money doubles every
rule of 72
Year 17
Growth beats deposits
compounding takes over
- What you put in$160,000
- Growth$302,290
- Total you contribute
- $160,000
- Interest / growth earned
- $302,290
- Final balance
- $462,290
The 2-minute guide
What compounding actually is
Compounding means your earnings start earning too. Year one you make a return on your money; year two you make a return on your money plus last year's return, and so on. It starts slow and then snowballs, which is why the growth curve above bends upward instead of rising in a straight line.
Time is the secret ingredient
The longer money compounds, the more of your final balance comes from growth rather than your own deposits. Starting five years earlier can matter more than saving a bit more each month, because those first dollars have the most time to multiply.
Be realistic about the return
A diversified stock index fund has historically returned around 7% a year after inflation over the long run, but any single year can be up or down a lot. Use a sensible rate, treat the result as a projection rather than a promise, and don't count on the best-case number.
Consistency beats timing
You don't need to be a stock-picking genius. Contributing the same amount automatically every month, through good years and scary ones, is what quietly builds the balances you see here. Set it up once and let compounding do the heavy lifting.
Frequently asked questions
How does the “time to reach a goal” mode work?
Pick a goal amount (say $1,000,000), your starting balance, how much you add each month, and an expected return. The calculator grows your money period by period until it reaches the goal, then tells you how many years and months that takes.
What return rate should I use?
For a diversified stock-market index fund, many people use 6–7% as a long-run, after-inflation estimate; a savings account might be 4–5% today. Returns are never guaranteed. This is a projection, not a promise.
What is the Rule of 72?
A quick shortcut: divide 72 by your annual return to estimate how many years it takes your money to double. At 8%, that's about 9 years. This calculator shows your doubling time automatically.
Why does starting early matter so much?
Because compounding needs time. Money invested in your 20s has decades to double and re-double, so early contributions often out-earn much larger amounts added later.
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