To be included in the index, a stock must have a total market capitalization that ranges from $850 million to $3.7 billion. These market cap eligibility criteria are for addition to an index, not for continued membership. As a result, an index constituent that appears to violate criteria for addition to that index is not removed unless ongoing conditions warrant an index change.
As of 31 December 2021[update], the index's median market cap was $1.58 billion and covered roughly three percent of the total US stock market. These smallcap stocks cover a narrower range of capitalization than the companies covered by the Russell 2000 Smallcap index which range from $169 million to $4 billion. The market valuation for companies in the SmallCap Index and other indices change over time with inflation and the growth of publicly traded companies. The S&P 400 MidCap index combined with the SmallCap 600 compose the S&P 1000, and the S&P 1000 plus the S&P 500 compose the S&P 1500. The index was launched on October 28, 1994 and its ticker symbol is either SML or ^SML.