The S&P 500 fell 20.47% during the 1987 Black Monday crash.
The decline occurred on 19 October 1987, when markets around the world experienced exceptionally rapid selling. The S&P 500, which represents large US companies across many industries, dropped by more than one-fifth in a single session.
The Dow Jones Industrial Average fell even more in percentage terms that day, losing 22.6%. This difference is why the two figures should not be treated as interchangeable. The S&P 500’s 20.47% figure refers specifically to that index’s closing decline.
After the crash, US regulators introduced or strengthened mechanisms intended to slow extreme market movements. These circuit breakers can temporarily halt trading when benchmark indexes move by specified amounts, giving investors time to assess information.