The S&P 500 Index, or Standard & Poor’s 500 Index, is a market-capitalization-weighted index of 500 leading publicly traded companies in the U.S. It is not an exact list of the top 500 U.S. companies by market cap because there are other criteria that the index includes. Still, the S&P 500 index is regarded as one of the best gauges of prominent American equities’ performance, and by extension, that of the stock market overall.

The S&P 500 index is a free-float weighted/capitalization-weighted index. As of September 30, 2021, the nine largest companies on the list of S&P 500 companies accounted for 28.1% of the market capitalization of the index and were, in order of weighting, Apple, Microsoft, Alphabet (including both class A & C shares), Amazon.com, Facebook, Tesla, Nvidia, Berkshire Hathaway and JPMorgan Chase. The components that have increased their dividends in 25 consecutive years are known as the S&P 500 Dividend Aristocrats.

• The S&P 500 Index features 500 leading U.S. publicly traded companies, with a primary emphasis on market capitalization.
• The S&P is a float-weighted index, meaning the market capitalizations of the companies in the index are adjusted by the number of shares available for public trading.
• Because of its depth and diversity, the S&P 500 is widely considered one of the best gauges of large U.S. stocks, and even the entire equities market.