$100 a month in the S&P 500: what it would have become

These are dollars: the series is SPY, the oldest exchange-traded fund tracking the S&P 500, and it trades in dollars. Further down, the same sums in euros at each date's exchange rate.

The last column is the annual return on your money (the IRR), which is not the index's return. From August 31, 2006 to August 31, 2026 the index rose 11.28% a year with dividends; the money paid in returned 13.82%, because much of it went in after the 2008-2009 crash and bought cheap. Over ten years it is the other way round: 15.28% for the index and 16.00% for the money.

Dividends are not a detail: over twenty years, without reinvesting them the result falls from $114,805 to $91,517. What each company in the index pays today is in S&P 500 dividends.

• Twenty-five years (from August 2001). In February 2009 it had 91 contributions, $9,100, and a balance of $6,348: 30.2% below the money paid in and 41.0% below its October 2007 peak ($10,767). It closed 26 months below the money paid in, the last in August 2009.
• Twenty years (from August 2006). The worst close, also February 2009: $3,100 paid in and a $1,853 balance, 40.2% less. It stayed below the money paid in for 29 month-end closes, through August 2010: almost four straight years of contributing without a gain.
• Ten years (from August 2016). Only the first two months closed below the money paid in, and by a few dollars. The scare came in 2022: the account went from $11,154 in December 2021 to $9,276 in September 2022, 16.8% less, with $7,400 paid in by then.

As for the index: between October 9, 2007 and March 9, 2009 SPY with dividends fell 55.20%; in 2020, 33.70% in 33 days, from February 19 to March 23. What that maximum drawdown means, in the guide to the Sharpe ratio and drawdowns.

Thirteen of the 225 ten-year windows ended below the money paid in; all of them ended between October 2008 and June 2010. The worst, March 1999 to February 2009, turned $12,000 into $8,163. The best, January 2012 to December 2021, turned it into $28,851. That is the spread: same amount, same index, a 3.5x difference depending on the month you started.

The windows overlap (they share months), so they are not 225 independent experiments: the series covers only twenty-eight and a half years. And none of these figures says what the index will do over the next ten.

For comparison: the same $24,000 invested all at once in August 2006 would have been worth $203,630 in August 2026, against $114,805 paying it in monthly. It is not a fair comparison — someone contributing monthly did not have that money at the start — but it explains the difference: money that goes in early spends more time invested, and more time exposed.

The euro was worth $1.1596 on August 31, 2026. Over the twenty years the euro fell against the dollar, and that added: €127,396 against the $114,805 of the dollar run. Over ten years it took away a little. And over twenty-five there was everything: someone who started in 2001, with the euro at $0.92, watched it climb to $1.60 in July 2008, and in euros the account stayed below the money paid in for 65 month-end closes, against 26 in dollars.

The exchange rate is neither a detail to ignore nor a return to expect: over twenty-five years it has added and subtracted depending on the stretch. The rest — which index and which product — is in S&P 500, Nasdaq-100 or MSCI World.

• Broker fees. Trading, custody and currency exchange. Paying in 100 a month, a fixed fee per trade weighs far more than paying in 1,000.
• Taxes. On sale, a resident of Spain pays 19% to 30% on the gain as savings income in 2026; dividends, if the product distributes them, are taxed each year. That is in the tax guide.
• The actual product. SPY is the longest series available, but a retail investor in Europe usually buys a European index fund or ETF, with its own annual cost and small tracking differences.
• Inflation. Everything is in the euros and dollars of each moment, with no adjustment for rising prices.

Change the dates, the lump sum and the monthly contribution in the S&P 500 calculator; it also has a compound-interest tab to project forward at whatever return you choose. Concepts, in the glossary (CAGR, total return). More guides, in the guides.

How much would $100 a month in the S&P 500 be worth after 20 years?

With $100 a month in SPY, dividends reinvested, from the close of August 31, 2006 to August 31, 2026: $24,000 paid in and $114,805 at the end, an annual return on the money (IRR) of 13.82%. For someone paying in €100 a month, converted at the ECB rate on each date, €127,396. Before broker fees and taxes. It is what happened in that period, not a forecast.

Can you lose money investing monthly in the S&P 500?

Yes. Of the 225 ten-year windows starting each month since January 1998, 13 ended below what was paid in. The worst, March 1999 to February 2009: $12,000 paid in and $8,163 at the end. None of the 105 twenty-year windows ended in a loss, but twenty-eight years of history do not guarantee the next twenty.

Why is my return different from the index's return?

Because each contribution spends a different amount of time invested: the first one twenty years, the last one a month. The index return (CAGR) measures how much the index rose; the return on your money (IRR) accounts for when each euro went in. Over the twenty years to August 2026 the index returned 11.28% a year and the money paid in 13.82%, because much of it went in after the 2008-2009 crash.

Do these figures include dividends, fees and taxes?

They include dividends, reinvested on the ex date. They do not include broker fees, custody, currency exchange or taxes. SPY does carry its own 0.09% annual fee. Without reinvesting dividends, the $114,805 of the twenty-year run drops to $91,517.

What if I start right before a crash?

It takes years to recover. Someone who started in September 2006 saw, in February 2009, $3,100 paid in worth $1,853, and the balance stayed below the money paid in for 29 month-end closes, the last in August 2010. Paying in every month also buys in the cheap months, which is where much of the later return came from. That is no guarantee it happens again.