Stats › Best and worst days › Black Monday 1987

Black Monday, 19 October 1987

Best days/1987 · single session · 2 sources · data as of 31 Jul 2026

On one Monday in October 1987 the American stock market fell about a fifth. Not over a month, not over a week. In a single session. The S&P 500 lost 20.47% and the Dow lost 22.61%, and nearly forty years later those are still the largest one day declines either index has ever recorded. The gap to second place is not small either. The next worst S&P day is -12.34%, back in 1929.

What makes it strange is what's missing. No bank failure that morning, no war, no default, no policy announcement, nothing of the sort that usually sits in front of a decline like this. People have offered explanations and the best of them are about plumbing rather than fundamentals: portfolio insurance selling into a falling market, order systems that couldn't keep up, futures and cash markets pulling on each other. None of it explains why that Monday. The honest answer to "why" is still that nobody knows.

This page gives you the numbers, where the day sits against the other worst sessions on record, what happened next, and the market structure it left behind. I built it because 1987 is the comparison people reach for whenever markets fall hard, usually without knowing how quickly it was over. The sources are listed at the bottom with dates. If you only read one, read the circuit breaker report, because that's the part that still governs your trading today.