The S&P 500 is a stock market index that tracks 500 of the largest publicly traded companies in the United States. It’s widely considered the single best gauge of how the U.S. stock market is performing, and it serves as the benchmark that most professional money managers try (and usually fail) to beat. Since its launch in 1957, the index has delivered an average annual return of about 10%, making it the foundation of many Americans’ retirement and investment strategies.
How the Index Works
The S&P 500 is weighted by market capitalization, meaning each company’s influence on the index is proportional to its total market value. A company worth $3 trillion moves the index far more than one worth $100 billion. If you imagine the index as a pie, the biggest companies get the biggest slices.
This has practical consequences. As of early 2026, the top 10 companies alone make up a substantial share of the entire index. Nvidia, Apple, Microsoft, Amazon, Alphabet (Google’s parent company, which has two share classes in the top 10), Broadcom, Meta, Tesla, and Berkshire Hathaway sit at the top. When these giants have a good or bad day, you feel it in the index even if the other 490 companies barely moved.
The index is maintained by a committee at S&P Dow Jones Indices, not by a formula that automatically adds or removes companies. The committee decides which companies belong, guided by a set of eligibility rules but ultimately using judgment about whether a company represents the large-cap U.S. equity market.
What It Takes to Join
Not every large company automatically gets in. To be eligible, a company must meet several requirements. It must be domiciled in the U.S. and listed on a major U.S. stock exchange like the NYSE or Nasdaq. It must be structured as a corporation (limited partnerships, LLCs, and SPACs are excluded, among other structures). And it must file standard financial reports with the SEC.
The financial bar is high. A company needs a total market capitalization of at least $22.7 billion (the threshold as of mid-2025, which is reviewed quarterly and can change). Its shares also need to be liquid enough that investors can actually buy and sell them easily. The index requires a minimum trading volume of 250,000 shares per month for each of the six months before evaluation, and a liquidity ratio that ensures the stock isn’t thinly traded relative to its size.
There’s one more filter: at least 50% of a company’s shares must be available for public trading, not locked up by insiders, governments, or other large holders. This “float-adjusted” requirement ensures the index reflects what regular investors can actually buy.
Sector Breakdown
The S&P 500 covers all 11 major sectors of the U.S. economy, but the allocation is far from even. As of March 2026, information technology dominates at 32.9% of the index. Financials come in second at 12.6%, followed by communication services (10.3%), consumer discretionary (9.9%), and health care (9.5%). Industrials hold 9.0%, while consumer staples, energy, utilities, materials, and real estate each account for 5.3% or less.
This means buying the S&P 500 gives you broad exposure to the U.S. economy, but you’re making a bigger bet on tech than on any other sector. That concentration has boosted returns during tech-led rallies and dragged on performance during tech sell-offs.
Historical Returns
Over its full history since 1957, the S&P 500 has returned roughly 10% per year on average when you include both price gains and reinvested dividends. More recent windows have been somewhat higher: the 40-year average through December 2025 stands at 11.5%, and the 10-year average through that same date is 14.8%.
Those are averages, not guarantees. In any given year, the index can swing wildly. It has dropped more than 30% in some years and gained more than 30% in others. The longer you stay invested, the more those swings tend to smooth out toward that long-run average. A $10,000 investment growing at the historical 10% average would roughly double every seven years, reaching about $80,000 after 30 years with dividends reinvested.
How to Invest in It
You can’t buy the S&P 500 directly because it’s an index, not an investment product. But hundreds of funds are designed to mirror its performance, and they come in two main forms.
Index mutual funds pool your money with other investors and buy all 500 stocks in proportion to their index weights. They price once per day after the market closes. If your 401(k) offers an S&P 500 option, it’s almost certainly a mutual fund. The Fidelity 500 Index Fund (FXAIX), for example, charges just 0.015% per year, which works out to $1.50 annually on a $10,000 investment.
Exchange-traded funds (ETFs) do the same thing but trade throughout the day like individual stocks. You can buy or sell shares at any point during market hours. ETFs also tend to be slightly more tax-efficient in taxable accounts because of how they’re structured, generally avoiding the capital gains distributions that mutual funds occasionally pass along. The most heavily traded S&P 500 ETF, the SPDR S&P 500 ETF (SPY), charges 0.09%, though competitors offer even lower fees.
Because every S&P 500 fund holds the same stocks in the same proportions, the cheapest fund almost always delivers the best results. A difference of 0.05% in fees sounds trivial, but over 30 years on a six-figure portfolio, it adds up to thousands of dollars. When choosing between funds, compare expense ratios first and pick the lowest one available to you.
Why Market Cap Weighting Matters
The market cap weighting system means the S&P 500 is not a snapshot of 500 equally important companies. It’s a reflection of where investors have placed the most money. A company like Walmart, with a market cap around $786 billion, might carry a weight of only about 0.8%. A company four times its size would have roughly four times the influence on the index’s daily movement.
This concentration has sparked debate. Critics point out that a handful of mega-cap tech stocks can drive the index’s returns, reducing the diversification you’d expect from owning 500 companies. Supporters argue that weighting by market cap reflects reality: these companies are the biggest because millions of investors collectively decided they’re worth the most.
If concentration concerns you, equal-weighted versions of the S&P 500 exist. These give each of the 500 stocks the same slice, so a $50 billion company counts as much as a $3 trillion one. Equal-weighted funds tend to give more exposure to mid-sized companies and less to the largest names, which can lead to meaningfully different returns in any given year.
What the Index Doesn’t Cover
Despite its reputation as “the market,” the S&P 500 only includes large U.S. companies. It excludes mid-cap and small-cap stocks, international companies, and private businesses. It also leaves out bonds, real estate (beyond REITs), and commodities. Investors who want true diversification across the full investment landscape typically pair an S&P 500 fund with funds covering international stocks, smaller U.S. companies, and bonds.