What Is the S&P 500? A Beginner's Guide

The S&P 500 is a stock market index that tracks roughly 500 of the largest companies in the United States. It's the single most-watched gauge of how the U.S. stock market is doing.

What it actually is

The S&P 500 is a market index — a number that summarizes the combined value of a group of stocks. In this case, the group is about 500 of the biggest U.S. public companies, spanning technology, banks, retailers, healthcare, energy and more. When that basket of companies rises in value, the index goes up; when it falls, the index goes down.

How the S&P 500 is built

Not every company counts equally. The S&P 500 is weighted by market value — bigger companies have more influence on the index than smaller ones. A company's size here is its market cap (share price times the number of shares).

• The largest companies can move the index noticeably on their own.
• The smallest members have only a tiny effect.
• The list is reviewed regularly, so companies are added or removed over time as they grow or shrink.

Why it's the benchmark

The S&P 500 became the default yardstick for U.S. stocks for a few reasons:

• It covers a broad mix of industries, so it reflects the wider economy better than a narrow list.
• Its 500 companies make up a large share of the entire U.S. stock market's value.
• Professional investors and funds routinely measure their own performance against it — beating "the S&P" is a common goal (and a hard one).

That's why financial news constantly quotes it, and why "the market" often means the S&P 500 in casual conversation.

How beginners use it

You can't buy the index itself — it's just a measurement. But you can buy funds designed to mirror it, holding the same 500 companies in the same proportions. These come in two common wrappers: index mutual funds and ETFs. In one purchase, you effectively own a tiny slice of all 500 companies, which spreads your money across the market rather than betting on a single stock.

If you're weighing which wrapper suits you, our guide on index funds vs ETFs breaks down the practical differences. And note the S&P 500 is U.S.-focused — it's a different index from tech-heavy benchmarks like the Nasdaq.

The bottom line

The S&P 500 is a weighted index of about 500 of the largest U.S. companies, and it's the market's go-to benchmark. You can't buy it directly, but funds that track it let beginners own a broad slice of the U.S. market in a single, diversified purchase. When you hear "the market did X today," this is usually the number people mean.