The Dow Jones Industrial Average’s largest single-day percentage loss was 22.6 percent, recorded during the 1987 crash.
The decline occurred on Monday, 19 October 1987, a date remembered as Black Monday. The Dow dropped 508 points, a fall that represented 22.6 percent of its previous closing value. Because the index level was much lower than it is today, the point loss and percentage loss tell different parts of the story; the percentage figure is the standard historical comparison.
The causes included overvalued shares, rising interest-rate concerns, international market weakness, and computerized selling strategies. A common mistake is to describe the event as a 500-point crash without noting that percentage comparisons are more meaningful across eras. The Dow recovered much of the loss relatively quickly, unlike the prolonged decline after 1929.