How is the S&P 500 generally weighted among its constituent companies?

The S&P 500 is generally weighted by market capitalization among its constituent companies.

This means companies with larger total market values have greater influence on the index’s movements than smaller constituents. In practice, the index uses a float-adjusted market-capitalization methodology, so shares that are readily available for public trading are emphasized rather than every share a company has issued.

Market-cap weighting differs from the Dow Jones Industrial Average, which is price-weighted. A high share price alone does not give an S&P 500 company a larger weight; the company’s size and investable share float matter instead. This distinction is a frequent source of confusion when comparing major U.S. indexes.

The S&P 500 was introduced in 1957 and is maintained by S&P Dow Jones Indices. Although its name refers to 500 companies, it can contain more than 500 stocks when some companies have multiple share classes represented.