The Dow Jones Industrial Average lost nearly 89% from its 1929 peak to its 1932 trough.
The Dow reached a pre-crash high of 381.17 on September 3, 1929. It eventually bottomed at 41.22 on July 8, 1932, representing a decline of about 89.2%.
The fall was not a single continuous plunge. Major selling days in October 1929 were followed by rallies, but the broader decline continued for years. The Dow did not recover its 1929 closing high until November 1954.
This figure is sometimes confused with the percentage loss on an individual crash day. The nearly 89% number describes the entire peak-to-trough bear market, not October 28 or October 29 alone.