Compound Interest Calculator

Free · no sign-up · reviewed September 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

Your numbers

What do you want to figure out?

Starting amount

Monthly contribution

Annual return

A long-run assumption, not a promise. US stocks returned about 10% a year before inflation from 1928 to 2025 (S&P 500 with dividends reinvested), which is roughly 7% once inflation is taken out. Any single year can be far above or below it.

Years

Compounding

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

NowYr 12Yr 25

What you put in$160,000
Growth$302,290

Total you contribute: $160,000
Interest / growth earned: $302,290
Final balance: $462,290

Compound Interest Calculator · Far Better Off

https://farbetteroff.com/calculators/compound-interest-calculator

Open that link to get this calculator back with these exact numbers. Estimates for education only — Far Better Off is not a lender or a financial advisor, and these results are not financial advice.

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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