About the Index Fund Calculator
This index fund calculator projects how a low-cost index fund — such as an S&P 500 or total stock market fund — could grow over time. Enter a starting amount, how much you will add each month, an expected annual return, the fund’s expense ratio and the number of years. It shows the projected balance after fees, how much of it is your own contributions versus investment growth, what the fund fees cost you, and the balance in today’s money after inflation.
It is designed for new investors building a passive portfolio, people planning regular 401(k), IRA or brokerage contributions, and anyone curious about what steady monthly investing can add up to. The year-by-year chart makes the compounding effect visible: in later years, growth usually outpaces what you contribute.
Returns are assumed to be constant and compounded monthly, with contributions at the end of each month. Real markets swing from year to year — the S&P 500 has averaged roughly 10% a year nominal over long periods, with many negative years — so try a more conservative rate such as 6–8% as well.
With the default inputs, the projected balance (after fees) is $1,319,647.79. Change any value above to recalculate instantly.
How to use the index fund calculator
- 1Enter what you are investing today and how much you will add each month.
- 2Enter an expected annual return — try both a long-run average (around 10%) and a conservative rate.
- 3Enter the fund’s expense ratio from its fact sheet.
- 4Choose how many years you will stay invested and an inflation rate.
- 5Review the projected balance, growth versus contributions and the value in today’s dollars.
Formula and method
The fund’s net return is the expected market return R minus the expense ratio ER. That net annual rate is divided by 12 and compounded monthly. The initial investment P grows for all n months, and each monthly contribution C is added at the end of the month and compounds for the remaining months — the future value of an ordinary annuity.
The cost of fees compares this with the same plan earning the full return R. The inflation-adjusted value divides the final balance by (1 + inflation)^years to express it in today’s purchasing power. Taxes and trading costs are not included.
- P
- Initial investment
- C
- Monthly contribution
- R
- Expected annual return before fees
- ER
- Fund expense ratio
- n
- Number of months
Worked examples
$10,000 plus $500 a month for 30 years at 10%
Contributing $190,000 in total into a 0.03% index fund earning 10% a year before fees grows to about $1.32 million. Fees cost only about $8,970 over 30 years because the expense ratio is so low. In today’s dollars, with 3% inflation, that is roughly $544,000 of purchasing power.
Conservative 7% return, $300 a month for 20 years
Starting with $5,000 and adding $300 a month for 20 years ($77,000 in total) at a 6.95% net return grows to about $175,327. About $98,300 of that is investment growth.
Frequently asked questions
What return should I expect from an index fund?+
The S&P 500 has returned roughly 10% a year on average over long periods including dividends, or around 6–7% after inflation. Individual years vary widely, so many planners use 6–8% for projections.
Is it better to invest a lump sum or monthly?+
Historically a lump sum invested immediately has beaten spreading it out more often than not, because markets tend to rise. Monthly investing reduces regret and timing risk and is the natural choice when investing from each paycheck.
What is a good expense ratio for an index fund?+
Major S&P 500 and total-market index funds and ETFs charge roughly 0.02–0.10% a year. Paying much more than 0.20% for a plain index fund is usually unnecessary.
How much will $500 a month be worth in 30 years?+
At a 10% annual return compounded monthly, $500 a month grows to about $1.13 million in 30 years; at 7% it is roughly $610,000. Use the calculator with your own rate and fees for a precise figure.
Does this include dividends?+
Yes, if you enter a total return. Historical index averages of about 10% include reinvested dividends. If you plan to spend dividends rather than reinvest them, use a lower return.
Results are estimates for educational purposes and are not financial advice. Rates, fees and terms vary — confirm with your lender or a licensed advisor before making decisions.