S&P 500 calculator: what you would have made, and compound interest
Two calculators in one. The first uses what actually happened: the daily price of SPY, the oldest fund tracking the S&P 500, with dividends reinvested, from 1998 to November 4, 2026, in dollars or in euros. The second is a compound interest calculator: you choose how much, for how long and at what return.
Simulation with historical data, in US dollars or euros. Past performance does not guarantee future results. This is not an investment recommendation and does not take your circumstances into account.
With $10,000 up front and $100 a month (203 contributions) in the S&P 500 (SPY, dividends reinvested) from January 2010 to November 4, 2026:
Series: SPY (SPDR S&P 500 ETF Trust), daily closes from December 1, 1997 to November 4, 2026, with each dividend reinvested at the close of its ex-date. In euros, at the European Central Bank reference rate of each date (the latest earlier one if none was published that day), from January 4, 1999 to November 3, 2026; in dollars, unconverted. SPY's own expense ratio is already inside its price; broker fees, taxes and inflation are not deducted. The lump sum and each contribution go in at the last close of the previous month. The worst fall is SPY's with dividends, in the chosen currency, peak to trough on daily closes.
If you put in $10,000 at the start and $100 a month for 20 years and the money always earned 9.3% a year:
Hypothetical projection: it assumes the same return every year, which is not how markets move. Contributions at the start of each month; monthly compounding equivalent to the annual rate. Before fees and inflation.
What data this calculator uses
The S&P 500 is an index: a list of about five hundred large US companies, each weighted by its stock market value. You cannot buy an index; you buy a fund that tracks it. This page uses the oldest one, SPY, which has traded since 1993. The site stores its daily closes since December 1, 1997 and its quarterly dividends, each on the date it comes off the price.
The calculator reinvests each dividend on that same day, which is how a "total return" index is built. It matters more than it looks: from December 31, 1997 to November 4, 2026, SPY returned 9.3% a year with dividends and 7.4% on price alone. Over 28.8 years, money multiplied by 13.0 in one case and by 7.9 in the other.
Because SPY is a real fund, its expenses are already inside its price, so it trails the official total return index slightly: by a tenth of a point a year on average from 2010 to 2024, by our comparison with the index's published annual returns. Everything is in US dollars; investors in another currency would also have had the exchange rate move, which is not applied here.
How compound interest works
Compound interest means that what you earn one year also earns the next. For a single amount, the final value is the amount times (1 + return) to the power of the number of years. With monthly contributions, each one grows for the months it has left.
A round-number example: $100 a month for 30 years at 7% a year adds up to $36,000 paid in and ends at about $117,606. More than two thirds of the end result is interest on interest. With half the time, 15 years, the result is not half but $31,286. Time matters more than the amount.
The projection runs exactly that calculation with the return you choose. By default it uses SPY's measured average, but that is the average of one particular period: a different starting point gives a different number, and good and bad years do not arrive evenly.
Lump sum or a little every month
When you invest monthly, your money has not been in the market the whole time. That is why the calculator shows two returns: the S&P 500's over the period (CAGR), and your money's (IRR), which accounts for when each dollar went in. With $100 a month from January 2010 to December 2024, the index returned 13.8% a year and the money paid in, 14.2%. Neither approach is better in general; the calculator lets you compare both over the same period.
What the calculator does not deduct
- The exchange rate, except for euros. If you invest in pounds or any other currency, your result would also depend on how that currency moved against the dollar over the period. The site only has the euro-dollar series, so for other currencies it neither applies it nor estimates it.
- Your fees, beyond SPY's own expense ratio. Many investors outside the US cannot buy SPY itself and use a local fund tracking the same index, with its own costs.
- Withholding on dividends. Funds domiciled outside the US usually have tax withheld in the US on the dividends they receive, so the effect of reinvesting them would be somewhat smaller than shown here.
- Taxes on dividends or gains. The projection can apply a flat rate you choose; the historical mode does not.
- Inflation. All figures are nominal.
The falls are part of the result
An annual average hides the path. Since 1997, SPY's worst fall with dividends was 55.2%, from the high in October 2007 to the low in March 2009. In 2020 it fell 33.7% between February 2020 and March 2020. Only someone who stayed invested through those months got the final figures above. Past performance does not guarantee future results, and nothing on this page is an investment recommendation.
Frequently asked questions
How much would $10,000 invested in the S&P 500 ten years ago be worth today?
With dividends reinvested, $10,000 invested in SPY at the close of November 2016 would have been worth $40,779 on November 4, 2026: 15.2% a year. Without reinvesting dividends, $34,802. Before broker fees and taxes.
What if I invest $100 a month in the S&P 500?
Nobody knows what will happen. As a reference from the past: $100 a month in SPY over the last 20 years (240 contributions since December 2006) adds up to $24,000 paid in and would have ended at $112,897, an annual return on that money (IRR) of 13.8%. Along the way, SPY fell as much as 55.2% from a high (October 2007 to March 2009). The projection tab lets you try any return you like.
What has the average annual return of the S&P 500 been?
Measured with SPY and its dividends reinvested, 9.3% a year from December 31, 1997 to November 4, 2026, in dollars. Price only, without dividends, 7.4%. It changes a lot with the period: someone who started just before a crisis has had years of less. It is a past average, not a forecast.
How does the projection handle taxes?
If you enter a tax rate, it is applied as a flat rate to the gain, as if you withdrew everything at once and had paid no tax along the way. Real tax rules depend on your country and account type; this is only an approximation.
Is this investment advice?
No. It is a calculator that shows what happened with one specific fund in the past, and what a formula gives with a return you choose. It does not know your circumstances, recommends no product, and past performance does not guarantee future results.
Guides: Investing in the S&P 500 · $100 a month in the S&P 500 · S&P 500, Nasdaq-100 or MSCI World · Sharpe ratio, max drawdown and CAGR · all guides
Data: SPY closes and dividends from Yahoo Finance, updated every trading day (last close: November 4, 2026); euro-dollar reference rate from the European Central Bank (latest: November 3, 2026). Our own calculation. Not financial advice.