What Is the S&P 500?

Key Takeaways:

• The S&P 500 tracks 500 large U.S. companies and represents approximately 80% of the total U.S. stock market by value
• Companies are selected by a committee at S&P Dow Jones Indices based on size, earnings, liquidity, and sector representation
• Since its launch in 1957, the S&P 500 has delivered an average annual return of approximately 10%, though individual years vary significantly
• The S&P 500 is broader and more representative of the U.S. economy than either the Dow Jones Industrial Average or the Nasdaq Composite

What Is the S&P 500?

The S&P 500, formally known as the Standard and Poor's 500, is one of the most widely followed financial indexes in the world. When financial media refer to "the market," they are almost always referring to movements in the S&P 500.

The index tracks 500 of the largest publicly traded companies in the United States and represents approximately 80% of the total value of the U.S. stock market. Most investors and fund managers use it as the standard benchmark for measuring portfolio performance.

Standard and Poor's introduced the S&P 500 in its current form in 1957. Its ticker symbol is SPX. The value of the index is expressed in points, not dollars. When the S&P 500 is quoted at a given level, that number represents a weighted average of the prices of its constituent companies, adjusted by an internal factor. It is not a price tag.

Because the index itself is not directly investable, investors typically gain exposure through index funds or exchange-traded funds (ETFs) that replicate its composition.

How Are Companies Selected for the S&P 500?

A committee at S&P Dow Jones Indices determines inclusion in the S&P 500, using a combination of quantitative criteria and qualitative judgment. To be eligible, a company generally must be headquartered in the United States, have a market capitalization above a defined threshold, and report positive earnings over recent quarters. It must also maintain sufficient liquidity and carry a meaningful public float.

The committee reviews the index regularly, adding and removing constituents to keep it representative of the large-cap U.S. market. When a company joins the index, funds that track it must buy its shares, which can create short-term price movement in the newly included stock.

How Is the S&P 500 Calculated?

The S&P 500 uses a market-capitalisation-weighted methodology. Each company's influence on the index is proportional to its total market value, not its share price.

Market capitalization is calculated using each company's freely traded shares, excluding shares held by insiders or subject to trading restrictions, multiplied by the current share price. This is known as float-adjusted market capitalisation.

Companies with larger market capitalisations carry more weight, meaning their price movements have a greater effect on the index as a whole. For example, a company whose float-adjusted market cap represents 5% of the index's total will move the index five times more than a company representing 1%. As of early 2026, the ten largest companies account for more than one-third of the index's total value, with technology companies dominating the top positions, according to S&P Dow Jones Indices.

A calculation known as the divisor converts the sum of all float-adjusted market capitalisations into the index's point value. S&P periodically updates the divisor to account for corporate events such as stock splits or changes in index composition, ensuring those events do not artificially distort the index level.

Why Does the S&P 500 Matter?

The S&P 500 serves three main functions in financial markets.

Market benchmark. It is the most widely used standard for evaluating investment performance. Portfolio returns are routinely compared against the S&P 500 to determine whether a given strategy has outperformed or underperformed the broader market.

Economic indicator. Because the index includes companies from all major sectors, including technology, healthcare, financials, consumer goods, and energy, movements in the S&P 500 broadly reflect overall economic conditions and investor sentiment about the U.S. economy.

Foundation for financial products. A large ecosystem of financial instruments is built around the S&P 500, including index funds, ETFs, and futures contracts, which allow investors and institutions to gain broad market exposure or manage portfolio risk.

How Does the S&P 500 Compare to Other Major Indexes?

The S&P 500 is one of three indexes most commonly cited as a measure of the U.S. stock market. Understanding how they differ clarifies what each one actually measures.

The Dow Jones Industrial Average tracks only 30 companies and uses a price-weighted methodology, meaning companies with higher share prices, not larger market values, have more influence on the index. That makes it a narrower and less representative measure of the overall market.

The Nasdaq Composite tracks more than 3,500 companies listed on the Nasdaq exchange, with a heavy concentration in the technology sector. It tends to be more volatile than the S&P 500 because of that sectoral concentration.

Of the three, the S&P 500 is generally considered the most representative measure of the U.S. equity market, broad enough to capture meaningful economic diversity while focused on large, established companies.

How To Invest in the S&P 500?

The two most common approaches are:

Exchange-traded funds (ETFs) track the S&P 500 and trade on exchanges throughout the day like individual stocks. They are generally accessible through any standard brokerage account and tend to carry low management fees.

Index mutual funds track the same index but are structured differently. They are priced once per day after the market closes, and purchases execute at that end-of-day price rather than a real-time market price.

Both structures hold the same constituent companies in the same market-cap weighted proportions as the index, and both aim to deliver performance that closely mirrors the S&P 500 before fees. 

The Bottom Line

The S&P 500 is a float-adjusted, market-capitalisation-weighted index of 500 large U.S. publicly traded companies, representing approximately 80% of the total value of the U.S. stock market. It functions as a benchmark for investment performance, an indicator of broader economic conditions, and the foundation for a wide range of financial instruments including index funds and futures contracts. Since its launch in 1957, it has delivered an average annual return of approximately 10%, though individual years vary significantly.