Which U.S. stock index fell more than 20% in one day during the October 1987 crash?

The Dow Jones Industrial Average fell more than 20% in one day during the October 1987 crash.

On October 19, 1987, the Dow dropped 508 points, equivalent to 22.6% of its previous closing level. That percentage loss remains the largest one-day decline in the index’s history. The event became known as Black Monday, although markets in different countries experienced their own severe declines on nearby dates.

The Dow is a price-weighted index of major U.S. companies. Because it is price-weighted rather than market-capitalization-weighted, its movements are calculated differently from those of the S&P 500 or Nasdaq Composite.

The crash did not produce a repeat of the economic collapse that followed 1929. The Federal Reserve provided liquidity, and the market eventually stabilized. The contrast is a useful reminder that a spectacular one-day stock decline does not automatically imply a depression.