The S&P 500’s maximum single-day percentage loss during the 1987 crash was 20.47%, recorded on October 19, 1987.
That session was Black Monday, when selling spread rapidly across global markets. The S&P 500 decline was enormous by modern market standards, although the Dow Jones Industrial Average’s 22.6% fall is more frequently quoted because the Dow is the most familiar U.S. benchmark.
The crash followed months of rising prices and growing concern about interest rates, trade imbalances, currency movements, and market valuations. Computerized trading strategies, especially portfolio insurance, were widely examined as mechanisms that could amplify selling once prices began to fall.
The S&P 500 is a capitalization-weighted index of large U.S. companies, whereas the Dow is a price-weighted index of 30 major companies. Those different designs mean their percentage losses are not identical. Confusing the two indexes is the main reason the 20.47% figure is often incorrectly attributed to the Dow.