The S&P 500 fell 20.47% on October 19, 1987, during the 1987 stock-market crash.
That decline is the index’s largest single-day percentage loss. The fall occurred on the same Black Monday when the Dow Jones Industrial Average dropped 22.6%. Because the S&P 500 represents a broad group of large U.S. companies, its percentage decline provides a different measure from the Dow’s movement.
Several pressures converged in 1987. Stock valuations had risen, interest-rate and currency concerns unsettled investors, and computerized portfolio-insurance strategies encouraged further selling as prices declined. International markets had already weakened, helping transmit the panic across borders.
The 1987 percentage is distinct from the S&P 500’s largest point decline, which occurred much later when the index traded at a far higher level. It is also different from the 2020 crash, when the S&P 500 suffered several exceptionally large daily percentage losses during the COVID-19 market shock.