The S&P 500 is the primary U.S. stock-market index made up of 500 large companies. It is maintained by S&P Dow Jones Indices and is widely used as a benchmark for the performance of large-cap U.S. equities.
The index is weighted by float-adjusted market capitalization. In practical terms, companies with larger publicly available market values have greater influence on the index’s movement than smaller constituents. The index is not simply an equal collection in which every company changes the result by the same amount.
Despite its name, the S&P 500 can contain more than 500 individual stocks because some companies have multiple share classes. Membership is selected using eligibility requirements and an index committee, rather than being determined solely by company size. The index is different from the Dow Jones Industrial Average, which contains 30 stocks and uses a price-weighting method.