The S&P MidCap 400 Index, more commonly known as the S&P 400, is a stock market index from S&P Dow Jones Indices. The index serves as a barometer for the U.S. mid-cap equities sector and is the most widely followed mid-cap index. To be included in the index, a stock must have an unadjusted total market capitalization that ranges from $3.2 billion to $9.8 billion at the time of addition to the index. As of 31 January 2020, the median market cap was $4.67 billion with the market cap of the largest company in the index at nearly $22.2 billion and the smallest company at $1.2 billion. The index's market cap covers nearly 7 percent of the total US stock market. The index was launched on June 19, 1991.