The S&P 500 fell about 20.5% during the 1987 Black Monday session.
On 19 October 1987, the broad U.S. equity index dropped 20.47%, while the Dow Jones Industrial Average lost 22.6%. The S&P 500’s decline was therefore enormous but slightly smaller in percentage terms than the Dow’s. Because the S&P 500 contains 500 large U.S. companies, it is often used as a broad benchmark for the American stock market.
Black Monday followed substantial international market weakness and came after a long bull market. Concerns about valuations, interest rates, trade imbalances and computerized trading all contributed to the panic. The crash led regulators to examine market structure and eventually helped produce circuit breakers designed to pause trading during extreme declines. A common error is to quote the Dow’s 22.6% figure as the S&P 500’s loss; the two indexes measure different baskets and had different percentage declines.