What Is The S and P 500 Index, And How Does It Work?

Jul 22, 2022 By Susan Kelly

On the New York Stock Exchange or Nasdaq, the S&P 500 index is based on the market value of the 500 biggest publicly traded companies. If you want to know how the stock market and economy are doing, you may use the S&P 500.

While mainstream media and the public may more recognize the Dow Jones Industrial Average, the S&P 500 index is more commonly used by the financial media and financial experts. A gauge of the US stock market and economy, the S&P 500 covers around 80 percent of accessible market capitalization, whereas the 30 equities in the Dow Jones only cover 30 percent of the available market capitalization.

Methods Used to Compute the S&P 500 Index

Free float-adjusted weights are applied to the S&P 500 index based on market capitalization. In a float-adjusted index, the number of shares that can be traded is constantly updated. Because the S&P is based on market capitalization, the bigger a firm is, the more weight it will have. The total market capitalization of the S&P 500 is calculated by summing the free-float adjusted market capitalizations of each stock in the index.

The Importance of the S&P 500

The S&P 500 is a well-known stock market index. It includes some of the country's major corporations. Because the United States has the world's largest economy by a wide margin, the S&P 500 is often regarded as a barometer of global economic health. The S&P 500 can be affected by the rise and fall of some of the world's largest corporations.

Still, it generally takes the price fluctuations of whole sectors to impact the index significantly. Since the index is so broad and diverse, big events such as a change in the federal funds rate or a conflict that affects commerce across markets only cause substantial daily movements.

Risks of S&P 500 Investing

There are several advantages to the float-adjusted index, one of which is that large-cap businesses are given a larger share of the weight. The weighted average market capitalization of each component is calculated by dividing the company's market capitalization by the index's total market capitalization.

With a market capitalization of $2.86 trillion in Q2 2022, Apple is the most valuable stock. The index's total market capitalization is $40.3 trillion. 67 7 percent of the index's weight belongs to Apple. Consider Adobe, which has just $216.7 billion in market value, making Adobe's weight in the S&P 500 a measly 0.

5% of its total market value.

Benefits of the S&P 500

The S&P 500 is often regarded as an accurate measure of the economy since it includes 500 firms from around the United States and a wide range of industries. With 30 firms, the Dow Jones Industrial Average (DJIA) provides a more focused view of the economy. In addition, because the DJIA is a price-weighted index, the most heavily weighted components are selected by their stock price rather than by any fundamental criteria. There are just 30 equities in the DJIA index, making the DJIA's fluctuation more significant than the S&P 500's.

How Does the DJIA Differ from the S&P 500?

The DJIA is a price-weighted index, whereas the S&P 500 is a market capitalization-weighted index. The Dow Jones uses the average price of each of its 30 constituent stocks as the numerator rather than adding up the market capitalizations of each. If any stock moves by only a single point, then the index will move by the same number of points. Similar to the S&P 500, the DJIA employs a proprietary divisor.

How Do You Calculate an Equal-Weighted Index?

Each stock in a portfolio or index fund is given equal weight in an equal-weighted index, regardless of size. An equal dollar amount of each stock is used as a starting point to figure out how many shares are needed. For example, the number of shares required for a $1,000 equal-weighted index is calculated by dividing $1,000 by the current share price.

Is The S&P 500 A Value Or A Price Index?

However, the S&P 500 index is neither price- nor value-weighted. While market capitalizations of firms included in the S&P are considered when calculating its weighting, this is done by considering the number of shares that may be traded openly.


The S&P 500 index is a widely used gauge of the performance of large-cap companies in the United States. According to the S&P, which is comprised of 500 stocks and uses a market-capitalization weighting, it's far more accurate than a narrower index like the Dow Jones, which has only 30 firms and uses a price-weighting.

However, because of the technique, extremely large stocks become more powerful in the index and drown out or lessen the effect of this vast diversity. This is not without its disadvantages, though. About 30% of S&P 500 market capitalization is concentrated in the top 10 stocks.

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