The Dow's highest single-day percentage fall during the 1987 crash was 22.6%.
On 19 October 1987, the Dow Jones Industrial Average dropped 508 points, a loss equal to 22.6 percent of its value at the previous close. The decline is commonly called Black Monday, and it remains the Dow's largest one-day percentage loss.
The fall was not confined to the United States. Stock markets in Europe, Asia, Australia, and Canada also plunged, demonstrating how increasingly connected financial markets had become. Investors were reacting to economic concerns as well as automated and rules-based selling strategies.
The figure is a percentage, not a point total. That distinction matters because the Dow's point value changes as its index level changes. For example, a 500-point fall represented a much larger percentage in 1987 than it would at a far higher index level decades later. The crash also helped prompt the development of coordinated trading halts, now known as circuit breakers.