The S&P 500 lost approximately 33.9% from its February 2020 high to its March 2020 low during the 2020 stock-market crash.
The index closed at a record high on February 19, 2020, after which fears about the rapidly spreading COVID-19 pandemic caused an unusually fast sell-off. By March 23, the S&P 500 had fallen about one-third from that high, placing the decline among the steepest bear markets in modern U.S. history.
Trading was interrupted several times by circuit breakers as prices fell rapidly. The Federal Reserve cut interest rates, resumed large-scale asset purchases, and supported financial markets, while the U.S. government enacted major emergency spending programs. These responses helped produce a powerful rebound after the March low.
The 33.9% figure measures the move between the closing high and closing low. Intraday prices and alternative measurements can produce slightly different percentages, which is why sources may round the decline differently.