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Results are hypothetical illustrations based on the numbers you enter and simple assumptions: a constant return, with no taxes, fees or inflation unless stated. They are not predictions or guarantees; real returns vary from year to year and can be negative, and investing can lose money. Educational content only. Not financial advice. See our Disclaimer.
Quick answer: $500 a month invested for 30 years at an 8% average annual return grows to about $745,180: $180,000 you put in plus $565,180 of growth. At the same 8%, $100 a month becomes about $149,036 and $1,000 a month about $1.49 million. Type your own numbers above; results are hypothetical, not guaranteed.
Use this to calculate compound interest on monthly contributions: type an amount, a number of years and a return, and the chart updates as you go.
How this calculator works
Every month you add your contribution, and the whole balance earns one month of growth (your annual return divided by 12). Twelve of those make a year; repeat for as many years as you choose. That is all compounding is — growth earning growth — and the chart shows why it looks slow for a decade and then takes off.
Example: $500 a month for 30 years at 8% means you put in $180,000 and end with about $745,000. The other $565,000 is growth you never had to earn at work.
What return should I use?
The S&P 500 has averaged roughly 10% a year with dividends reinvested over the long run (about 7% after inflation). We default to 8% as a middle ground. Nobody can promise any number; try 5% and 10% to see the range.
Three things that move the result most
- Years. Doubling the time does far more than doubling the money. Starting 10 years earlier often beats contributing twice as much.
- Consistency. The calculator assumes you never skip a month. Automating the transfer is the single biggest “little move”.
- Fees. A 1% annual fee on an 8% return takes roughly a quarter of your final balance over 30 years. Low-cost index funds exist for this reason.
From the channel
Run the same numbers above: $100 a month for 30 years.
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Open calculator →Questions people ask
Is this compound interest calculator free?
Yes. No sign-up, no limits. Change any number and the link in your address bar updates so you can share the exact scenario.
Does it include inflation and taxes?
No. Results are in today’s dollars at the return you enter. Use about 7% instead of 10% to approximate an after-inflation stock-market return, and remember taxes depend on the account type (a Roth IRA or 401(k) grows tax-advantaged).
What does “monthly compounding” mean here?
Your annual return is divided by 12 and applied every month to the whole balance, after that month’s contribution is added. Most savings and investment calculators use the same convention.
How much do I need to invest monthly to reach $1,000,000?
At 8% it takes roughly $670 a month for 30 years, $290 a month for 40 years, or about $1,700 a month for 20 years. Our Millionaire calculator solves this for any age and target.
What is the formula for compound interest with monthly contributions?
Future value = P × (1 + r/12)^(12t) + PMT × [((1 + r/12)^(12t) − 1) ÷ (r/12)], where P is what you start with, PMT is the monthly contribution, r is the annual return and t is years. Example: PMT $500, r 8%, t 30, P $0 gives about $745,180.