What is the S&P 500? A complete guide for beginners

The S&P 500 is a stock market index that tracks the performance of 500 of the largest publicly traded companies in the United States. Together, those companies represent roughly 80% of the total market capitalization of U.S. equities — which is why investors, journalists, and economists treat it as shorthand for “the market.”

When you hear that “stocks were up today,” the speaker is almost always referring to the S&P 500. It is the most-watched equity benchmark in the world.

Who runs the S&P 500?

The index is owned and maintained by S&P Dow Jones Indices, a joint venture between S&P Global, the CME Group, and News Corp. A small committee at S&P decides which companies belong in the index, using a published methodology that covers things like market size, profitability, share liquidity, and U.S. domicile.

Despite the name, the S&P 500 is not simply the 500 largest companies in America. It is a curated benchmark of leading U.S. companies that the committee considers representative of the broad economy.

How the index is calculated

The S&P 500 is market-cap weighted. That means a company's influence on the index is proportional to its size: bigger companies move the index more. A 5% drop in a $3 trillion company has a much larger effect than a 5% drop in a $30 billion company.

We explain this in more detail in our guide to how the S&P 500 is weighted.

What kinds of companies are in it?

The index spans 11 sectors defined by the Global Industry Classification Standard (GICS): Information Technology, Health Care, Financials, Consumer Discretionary, Communication Services, Industrials, Consumer Staples, Energy, Utilities, Real Estate, and Materials.

You can browse the full list of S&P 500 companies on our tickers list, or explore them by sector.

Why does the S&P 500 matter?

Three reasons it dominates the conversation:

• Coverage. 500 companies cover most of the investable U.S. equity market by value.
• Diversification. Exposure across 11 sectors means no single industry dominates the return.
• Track record.Over long periods, the S&P 500 has produced average annual returns of roughly 10% (nominal) — enough to double an investment about every seven years.

How to invest in the S&P 500

You can't buy “the S&P 500” directly — it's just an index, a calculation. But you can buy index funds and ETFs that hold the 500 underlying stocks in their actual weights. Popular vehicles include:

• VOO— Vanguard S&P 500 ETF
• SPY— SPDR S&P 500 ETF Trust (the original, launched in 1993)
• IVV— iShares Core S&P 500 ETF

All three track the same 500 holdings with very low expense ratios (under 0.1% in most cases).

Common misconceptions

“The S&P 500 is the U.S. economy.”

Not exactly. It's the 500 largest publicly traded companies — many of which earn a meaningful share of their revenue overseas. It excludes private companies, small businesses, and most of the labor market.

“The index never changes.”

It changes constantly. Companies get added and removed several times a year as the committee responds to mergers, spin-offs, and changes in eligibility. We track every change on the S&P 500 changes page.

The takeaway

The S&P 500 is the most important benchmark in finance because it is broad, diversified, transparently maintained, and easy to invest in. Whether you're reading market commentary or building a long-term portfolio, understanding how it works gives you a much better mental model of how U.S. stocks behave.