The FTSE 100’s largest one-day percentage fall during the 1987 Black Monday crash was 12.2%.
The London market suffered its sharpest decline on October 19, 1987, as selling spread internationally from the United States and other financial centers. The FTSE 100’s fall was severe, although it was smaller in percentage terms than the Dow Jones Industrial Average’s roughly 22.6% drop that day.
The crash demonstrated how quickly modern markets could transmit stress across borders. Investors and regulators subsequently examined trading systems, market liquidity, and mechanisms for slowing extreme declines.
A common error is to give the Dow’s 22.6% figure for every market. That number belongs to the Dow, not the FTSE 100. The 12.2% figure specifically describes London’s benchmark index on Black Monday.