What Does the S&P 500 Index Measure and How Is It Calculated?

The S&P 500 Index measures the value of the stocks of the 500 largest corporations by market capitalization listed on the New York Stock Exchange or Nasdaq. The intention of Standard & Poor's is to have a price that provides a quick look at the stock market and economy.

The S&P 500 Index is indeed the most popular measure used by financial media and professionals, while the mainstream media and the general public might be more familiar with the Dow Jones Industrial Average (DJIA). The S&P 500 is meant to provide a barometer of the U.S. stock market and economy, covering approximately 80% of available market capitalization as opposed to the 30 stocks in the Dow Jones.

The S&P 500 lists publicly traded companies and is often considered the primary benchmark for U.S. equities, and many mutual funds and ETFs are available that track the index. Because of its influence, it is important to understand how this index is constructed and what exactly it measures in the market.

How the S&P 500 Index Is Calculated

The S&P 500 Index is a free float-adjusted market-cap-weighted index. Being float-adjusted, the index is continuously recalculated based on the number of shares available for trading. Because it is constructed by market cap, the larger a company is, the greater weight it will represent in the S&P.

S&P 500 Index = Weighted Market Cap of All S&P 500 Stocks ÷ Index Divisor

The free-float adjusted market capitalizations for all constituent stocks are summed to obtain the total market capitalization of the S&P 500. It is then divided by an index divisor, which is a proprietary figure developed by S&P.

The divisor is adjusted when there are stock splits, special dividends, or spinoffs that could affect the value of the index. The divisor ensures that these non-economic factors do not affect the index.

S&P 500 Pitfalls

One result of being float-adjusted is that the index is weighted toward large-cap companies. The weighted average market capitalization of each component is determined by dividing the market cap of the company by the index's total market cap. Apple's weighting is determined by taking its market capitalization and dividing it by the total index market cap.

For example, as of July 9, 2023, Apple (AAPL) has the largest market cap among stocks, at about $3 trillion. The total market cap of all the companies in the index is $39.13 trillion as of June 30, 2023. This puts Apple's weight in the index at approximately 7.70%, given the different dates. Compare that to the likes of Adobe (ADBE), which has just a $221.19 billion market cap, making Adobe's weighting in the S&P 500 a mere 0.60%.

This leads to the mega-cap stocks having an outsized impact on the index. Sometimes, this index structure can mask strength or weakness in smaller companies if larger-cap companies are diverging. In other ways, this index structure better represents the overall economy compared to indexes where the weighting is determined by an equal share or price-weighted.

S&P 500 Benefits

The S&P 500 is considered an effective representation of the economy due to its inclusion of 500 companies, which covers most of the U.S. industries. In contrast, the DJIA is made up of 30 companies, leading to a more narrow reflection. Further, the DJIA is a price-weighted index, so the largest weighted components are determined by their stock price rather than some fundamental measure.

The DJIA is limited to 30 stocks and the movement of a stock in the DJIA can have a greater impact than that of the S&P 500. The largest weighted stock in the S&P 500 likely has a smaller weight than the largest weighted stock in the DJIA.

The Bottom Line

The S&P 500 is a popular benchmark of U.S. large-cap stocks. Compared to more narrow indices like the Dow Jones, which contains just 30 stocks and is constructed using a price weighting, the S&P 500 is broader in scope and its float-adjusted market cap weighting makes it more representative of the market.

However, this is not without its drawbacks. Because of its methodology, very large stocks become more influential components of the index and drown out or diminish the influence of this broad diversification.