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 September 18, 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.

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 September 18, 2026, SPY returned 9.3% a year with dividends and 7.4% on price alone. Over 28.7 years, money multiplied by 12.9 in one case and by 7.8 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.

In euros: what the exchange rate does

SPY trades in dollars. Someone who saves in euros converts them to dollars to buy and back to euros to sell, so the result depends on two things: what the S&P 500 did and what the dollar did against the euro. The calculator applies the European Central Bank reference rate, published every business day since January 4, 1999: each contribution is converted at the rate of its date and the final value at the rate of the end date. There was no euro before 1999, so in euros the earliest start is February 1999. Real periods with 10,000 invested up front:

From 2002 to 2007 SPY returned +6.0% a year in dollars, but the euro went from $0.8813 to $1.4721 and in euros the result was -2.7% a year. From 2010 to 2024 the dollar rose and the currency added: €96,148 versus $69,338. Over the whole life of the euro, from January 29, 1999 to September 18, 2026, it returned +8.6% a year in dollars and +8.5% in euros.

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.

Guides: Investing in the S&P 500 · Sharpe ratio, max drawdown and CAGR · all guides

Data: SPY closes and dividends from Yahoo Finance, updated every trading day (last close: September 18, 2026); euro-dollar reference rate from the European Central Bank (latest: September 18, 2026). Our own calculation. Not financial advice.