The Dow Jones Industrial Average lost nearly 89% from its 1929 peak to its 1932 trough.
The Dow reached a closing peak of 381.17 on September 3, 1929. It eventually fell to 41.22 on July 8, 1932, a decline of roughly 89% from peak to trough. The market therefore continued falling for years after the initial October 1929 crash.
The collapse occurred alongside bank failures, falling prices, reduced production, unemployment, and a severe contraction in international trade. Historians and economists describe the Great Depression as the result of many interacting forces rather than the stock crash alone.
The Dow did not regain its 1929 closing peak until November 1954. The nearly 89% figure describes the full peak-to-trough decline, not the percentage lost on one particular day such as Black Tuesday.