What Is the S&P 500? A Plain-English Guide for New Investors

If you have spent any time reading about investing, you have seen the phrase “the S&P 500” used as shorthand for the stock market itself. Financial news reports whether it was up or down. Index funds are built to match it. Professional investors are judged on whether they beat it. For something so central, it is rarely explained clearly. Here is what it actually is.

The Basic Definition

The S&P 500 is an index — a measurement, not a fund. Specifically, it tracks the combined stock value of about 500 of the largest publicly traded companies in the United States. When those companies are collectively worth more than they were yesterday, the index goes up. When they are worth less, it goes down.

It is maintained by a company called S&P Dow Jones Indices. A committee there decides which companies are included, using rules based on size, profitability, US domicile, and how much of the company’s stock is actually available for public trading.

Because the 500 companies span technology, healthcare, banks, retailers, energy, manufacturing, and more, the index is treated as a reasonable proxy for the entire US large-company stock market.

How the Index Is Weighted

The S&P 500 is market-cap weighted. Each company’s influence on the index is proportional to its total market value — share price multiplied by number of shares.

This means the largest handful of companies move the index far more than the smallest ones. The biggest company in the index might make up 6 or 7 percent of it, while a company near the bottom might account for less than a tenth of a percent. A big day for the largest technology firms can pull the whole index up even if most of the other 490-plus companies are flat.

This is different from the Dow Jones Industrial Average, which tracks only 30 companies and weights them by share price, and from an “equal weight” version of the S&P 500, where every company counts the same regardless of size.

Why Investors Care About It

Three reasons the S&P 500 gets so much attention:

It is the default benchmark. When a mutual fund manager or a financial advisor talks about their performance, they are almost always comparing it to the S&P 500. Decades of data show that most professional managers fail to beat it over the long run, after fees.

It is investable cheaply. You cannot buy the index, but you can buy a fund that holds all 500 companies in the same proportions. These index funds often charge a few dollars per $10,000 invested per year, which is close to free by historical standards.

It has a long track record. The index in its modern form dates to 1957, and comparable data goes back further. That history is what lets people cite a long-run average return, even though any single year can look nothing like the average.

What Returns to Expect

Over long stretches, the S&P 500 has returned roughly 10 percent per year before inflation, or something closer to 7 percent after inflation is subtracted. Reinvested dividends are a meaningful part of that total.

The critical caveat: that average is almost never what a single year looks like. The index has gained more than 30 percent in a year and lost more than 30 percent in a year. It has had multiple stretches where it went nowhere for the better part of a decade. The long-run average only emerges if you actually stay invested through the bad stretches, which is the part most people find hard.

This is why a strategy like dollar-cost averaging — investing a fixed amount on a fixed schedule — is so commonly paired with index investing. It removes the need to guess whether today is a good day to buy.

How to Actually Own It

For most people, exposure to the S&P 500 comes through one of these:

• A 401(k) or workplace plan. Nearly every plan offers an S&P 500 index fund or a broader US total-market fund. If your employer matches contributions, that match is effectively free money on top of the market return — see how a 401(k) match works.
• An IRA or taxable brokerage account. Open an account with a major brokerage and buy their S&P 500 index fund or ETF directly.
• A target-date fund. These hold the S&P 500 (and other indexes) as part of an age-based mix that shifts more conservative over time. Our guide to target-date funds explains the structure.

Before You Invest a Dollar in It

Index investing rewards people who can leave the money alone for years. That is only possible if a surprise expense does not force you to sell at a bad time. Have a cash cushion in place first — building a $1,000 starter fund is the minimum, and a fuller emergency fund is better — so your S&P 500 holdings can ride out a downturn untouched.

The Bottom Line

The S&P 500 is an index that measures the combined value of about 500 of the biggest US companies, weighted by size. It is the benchmark the investing world measures against, it can be owned cheaply through an index fund in almost any account, and it has historically returned about 10 percent a year before inflation over long periods. The catch is entirely in the word “long” — the average only shows up for investors who stay put through the years that look nothing like it.