What Is a Stock Index?
6 min read · Updated June 30, 2026
When the news says "the market was up today," it is almost always talking about a stock index — a single number that summarizes how a whole group of stocks performed.
Indexes turn thousands of moving prices into one readable figure. Knowing how the major ones are built tells you what a headline move actually means.
What an index measures
A stock index is a basket of selected stocks whose combined value is tracked over time. Rather than follow every company individually, investors watch the index as a shorthand for the market or a slice of it.
Each index has rules for what it includes — a country, an industry, or a size band — so different indexes answer different questions about how stocks are doing.
The major U.S. indexes
The S&P 500 tracks about 500 of the largest U.S. companies and is the most widely used benchmark for the overall market. The Nasdaq Composite leans heavily toward technology and growth companies. The Dow Jones Industrial Average follows just 30 large companies and is the oldest of the three.
Because they are built differently, they can move by different amounts on the same day — a tech-heavy day might lift the Nasdaq more than the Dow.
How indexes are weighted
Most major indexes, including the S&P 500, are weighted by market capitalization — bigger companies count for more. A handful of the largest companies can therefore drive much of the index’s movement.
The Dow is unusual: it is price-weighted, so a company’s share price, not its size, determines its influence. This is one reason the two indexes do not always agree.
How to invest in an index
You cannot buy an index directly, but you can buy a fund that tracks it. Index funds and exchange-traded funds (ETFs) hold the same stocks in the same proportions, aiming to match the index’s return at very low cost.
This is why "buying the S&P 500" is shorthand for purchasing a fund that mirrors it — a simple way to own a broad slice of the market at once.
Frequently asked questions
What is the S&P 500?
The S&P 500 is a stock index tracking about 500 of the largest publicly traded U.S. companies, weighted by market capitalization. It is the most widely used benchmark for the U.S. stock market.
What is the difference between the Dow and the S&P 500?
The S&P 500 holds about 500 companies and is weighted by market value, while the Dow holds just 30 companies and is weighted by share price. The S&P 500 is generally considered a broader measure of the market.
Can you invest directly in a stock index?
Not directly. Instead, investors buy index funds or ETFs that hold the same stocks as the index in the same proportions, closely tracking its performance at low cost.