Compound Interest Calculator
Watch your money grow. See the real power of compound interest over time.
By the Numbers
~10.3%
S&P 500 avg return
Long run, w/ dividends
~7.4%
After inflation
Real S&P return
9 yrs
To double at 8%
Rule of 72
$0
Cost to start
Time is the real input
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How It Works
Enter your starting money
Add your initial investment and the amount you plan to contribute every month.
Set rate and time
Choose an expected annual return, a time horizon, and how often interest compounds.
See the growth curve
Watch a year-by-year chart reveal how your contributions and interest stack up over time.
$300/month for 30 years
Starting from $0, contributing $300/month, by annual return.
| Annual return | Total contributed | Final balance | Interest earned |
|---|---|---|---|
| 4% | $108,000 | $208,000 | $100,000 |
| 7% | $108,000 | $352,000 | $244,000 |
| 10% | $108,000 | $621,000 | $513,000 |
Rounded, monthly compounding. Returns are not guaranteed.
The Complete Guide to Compound Interest Calculator
The eighth wonder of the world Compound interest means earning returns not just on the money you invested, but on the returns that money already generated. Each period your interest earns its own interest, and over decades the snowball produces growth that feels almost unbelievable. Over the last century the **S&P 500 has averaged about 10.3% a year** with dividends reinvested — roughly **7.4% after inflation**.
Simple vs compound Invest $1,000 at 10% *simple* interest and you earn $100 every year, forever. With *compound* interest, year two earns 10% on $1,100, year three on $1,210, and so on. The gap starts small and becomes enormous: over 40 years compounding can be worth several times the simple-interest result.
Time is the most powerful input Three things drive your result — how much you invest, your rate of return, and **time** — and time matters most because compounding accelerates the longer it runs. Someone who invests for 10 years in their twenties and then stops can finish ahead of someone who invests for 25 years starting at 40. The earliest dollars do the heaviest lifting.
Contributions feed the snowball You can't control markets, but you control how much you add. Consistent monthly contributions supercharge compounding and give you dollar-cost averaging — buying more shares when prices are low. On the chart, the interest area eventually dwarfs your contributions, but only if you keep feeding it early.
APR vs APY and the rule of 72 **APR** is the simple stated rate; **APY** includes the effect of compounding, so it's the truer measure — compare savings by APY and loans by APR. A handy shortcut is the **rule of 72**: divide 72 by your rate to estimate years to double. At **8% money doubles in ~9 years**; at 12%, about 6.
Where to harness it Tax-advantaged accounts — 401(k)s, IRAs, Roth accounts — let gains compound without yearly taxes. A low-cost, diversified index fund inside them is the classic vehicle. The same force works against you on debt: a 22% credit card compounds too.
How we calculate this
- Each month we add your contribution, then apply interest at your chosen compounding frequency.
- Final balance = future value of your initial amount + future value of the contribution stream.
- Interest earned = final balance − everything you contributed.
Official sources & data
Figures reviewed June 2026. Estimates only — not financial advice.
Frequently Asked Questions
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