The S&P 500 fell 20.5% during the October 19, 1987 crash.
The decline was part of the broader Black Monday collapse, when the Dow Jones Industrial Average dropped 22.6%. The S&P 500 represented a broad basket of large U.S. companies, so its loss provides a different measure of the crash from the narrower, price-weighted Dow.
The 1987 fall followed a powerful rise in U.S. share prices and was accompanied by international selling. Portfolio insurance strategies, derivatives, investor fear, economic concerns, and market structure all played roles in the rapid decline. Analysts continue to debate how much weight each factor deserves.
Index percentages are not interchangeable. The Nasdaq Composite, Dow, and S&P 500 cover different groups of securities and use different calculation methods. Comparing their results requires checking both the index definition and the exact trading date.