Which U.S. stock index suffered the largest one-day percentage fall during the 1929 Wall Street Crash?

The Dow Jones Industrial Average suffered the largest one-day percentage fall among the major U.S. stock indexes during the 1929 Wall Street Crash.

The Dow lost 12.8% on Black Monday, 28 October 1929, and fell another 11.7% on Black Tuesday, 29 October. These declines made the index the clearest widely reported measure of the panic. The Dow had reached a peak in September before confidence began to deteriorate.

The crash followed years of speculation, widespread use of borrowed money, uneven economic conditions and concerns about industrial profits. The market did not reach its ultimate low until July 1932. A frequent mistake is to treat the crash as a single day; in reality, it was a series of violent declines during October 1929, followed by a prolonged bear market.

The Dow’s percentage loss is not the same as its point loss. Percentage measures allow comparisons across eras, while point changes depend on the index’s level and construction. The S&P 500 did not yet exist in its modern form in 1929.