Dow vs. S&P 500 vs. Nasdaq: What Each Stock Index Measures and Why They Can Move So Differently

The three numbers you hear every evening are built in very different ways. Knowing how helps you read market news without overreacting.

What is the difference between the Dow, the S&P 500 and the Nasdaq?

The Dow Jones Industrial Average tracks 30 large U.S. companies and is weighted by share price. The S&P 500 tracks 500 leading U.S. companies and is weighted by the market value of their available shares. The Nasdaq Composite tracks more than 3,000 companies listed on the Nasdaq exchange, weighted by total market value.

A stock market index is a scorecard. It takes a group of stocks, combines their prices with a set formula and turns the result into one number that rises and falls through the trading day. When a news anchor says "the market was up today," they almost always mean one of these three scorecards.

The catch is that each index answers a slightly different question. The Dow asks how 30 famous companies are doing. The S&P 500 asks how the biggest slice of the U.S. stock market is doing. The Nasdaq Composite asks how every company that lists its shares on the Nasdaq exchange is doing. Because the questions differ, the answers can differ too, sometimes by a lot.

How does the Dow Jones Industrial Average work?

The Dow is the oldest of the three. Charles Dow began his daily average of 12 industrial stocks in May 1896, according to S&P Dow Jones Indices, the company that runs both the Dow and the S&P 500 today. Eight stocks were added in 1916 and 10 more in 1928, bringing the total to 30, where it has stayed ever since.

Today the Dow is "a price-weighted measure of 30 U.S. blue-chip companies," and it "covers all industries except transportation and utilities," according to its official index page. Blue-chip simply means a large, well-established company. Transportation and utility companies have their own separate Dow averages.

There is no strict formula for joining the Dow. A committee picks the members using judgment rather than a fixed set of rules, S&P Dow Jones Indices explains.

What does price-weighted mean?

Price-weighted means a stock's influence depends on its share price, not on how big the company is. To calculate the Dow, S&P Dow Jones Indices adds up the share prices of all 30 stocks and divides by a number called the divisor. The divisor is adjusted whenever something like a stock split or a member swap would otherwise make the index jump for reasons that have nothing to do with the market.

Here is a simple example of why this matters. Imagine two Dow stocks: Company A trades at $600 a share and Company B at $60. If both rise 10%, Company A gains $60 a share and Company B gains $6. Because the Dow adds up prices, Company A's move pushes the index 10 times as much as Company B's, even if Company B is the larger business by total value. A big, high-priced stock having a bad day can drag the Dow down while the rest of the market is fine.

How does the S&P 500 work?

The S&P 500 "includes 500 leading companies and covers approximately 80% of available market capitalization," according to its official index page. Market capitalization, or market cap, is a company's share price multiplied by its number of shares. The index has been calculated in its current form since March 1957.

Unlike the Dow, the S&P 500 has clear rules for getting in. Companies must meet standards for size, the share of stock available to trade, trading volume and financial health, and a committee then chooses among eligible companies with a balance of industries in mind, S&P Dow Jones Indices explains.

What does float-adjusted market-cap weighting mean?

The S&P 500 "is a float-adjusted market-cap-weighted index." In plain English, bigger companies count more. The "float" is the portion of a company's shares that is actually available for the public to buy and sell, so a company's weight reflects the stock investors can really trade.

Here is how that plays out. Imagine a company worth $3 trillion and another worth $30 billion. In a market-cap-weighted index, the first company has about 100 times the influence of the second. So a 1% move in the giant company moves the S&P 500 far more than a 10% move in the small one. That is why a handful of the largest companies can drive the whole index for months at a time.

What does the Nasdaq Composite track?

The Nasdaq Composite is the broadest of the three by count. It covers "more than 3,000 companies," according to Nasdaq. To be included, a company's stock must be listed exclusively on the Nasdaq Stock Market, and there are no limits on industry, size or the share of stock available to trade, according to the index's methodology document. Exchange-traded funds (ETFs), preferred stocks and warrants are generally left out.

Like the S&P 500, the Composite is weighted by market cap. But it uses a company's total shares rather than just its float, and it has no caps on how big any single company can get. Because many of the world's largest technology companies list their shares on Nasdaq, the Composite tends to lean heavily toward tech.

You will also hear about the Nasdaq-100. It is a separate, narrower index of 100 of the largest non-financial companies listed on Nasdaq. Unlike the Composite, it uses limits on how much weight any one company or group of top companies can carry, Nasdaq explains. When people say "the Nasdaq" on the news, they usually mean the Composite.

Why can the three indexes move in different directions?

Different members and different weighting rules mean the indexes can tell different stories on the same day. The week ended Oct. 16, 2026, is a clear example. Worries about heavy borrowing by AI companies hit technology stocks hard. For the week, the Nasdaq Composite fell 4.5%, the S&P 500 fell 1.9% and the Dow rose 0.7%, according to the Associated Press. Same market, same week, three very different results.

The Nasdaq fell the most because it is packed with the tech companies at the center of the sell-off. The S&P 500 fell less because tech is a large part of it but shares space with banks, health care, energy and consumer companies. The Dow held up better, a sign that its 30 members, chosen across most industries and weighted by price, were less exposed to that week's biggest losers.

This pattern isn't new. S&P Dow Jones Indices notes that the Dow has sometimes lost less than the S&P 500 in downturns, such as 2009, and gained less in strong years, such as 2013 and 2019. The two still overlap a lot: Dow companies generally make up between 25% and 30% of the S&P 500's market value.

Points versus percent

One more source of confusion is points. On Friday, Oct. 16, 2026, the Dow fell 175.28 points, the S&P 500 fell 84.62 points and the Nasdaq fell 572.99 points, the AP reported. In percentage terms, that was 0.3%, 1.1% and 2.1%. Because the Dow sits above 50,000 and the S&P 500 sits in the thousands, point moves aren't comparable across indexes. Percent changes are the fair way to compare them, and a 500-point drop in the Dow today is a much smaller move than it was when the index was at 10,000.

Which index matters most for your 401(k)?

The index that matters most to you is the one your own funds track. Many index funds and ETFs are built to follow one of these benchmarks, or a broader total-market index, and each fund's fact sheet names its benchmark. Our guide to index funds explains how they work, and our list of index funds for beginners shows the kinds of funds people often start with.

Here is a quick example of why the benchmark matters. Using the week ended Oct. 16, 2026, a hypothetical $10,000 in a fund that tracks the Nasdaq Composite would have fallen about $450. The same amount in an S&P 500 fund would have fallen about $190, and in a Dow fund it would have risen about $70. Over a single week, those gaps look dramatic. Over many years, what matters more is how diversified your money is, what you pay in fees and whether you stay invested.

Some investors use the S&P 500 as their main yardstick because it covers about 80% of available U.S. market value. Others watch the Nasdaq for a read on technology, or the Dow for a quick snapshot of big, established companies. None of them is the "right" one. They are different lenses on the same market.

How can you read index headlines without overreacting?

Start with the percent change, not the points. Then check the time frame. A 2% drop in one day feels big, but the S&P 500 was still up 11.9% for 2026 as of Oct. 16, according to the AP, even after a rough week. Context like that keeps one bad day in perspective.

Next, ask which index the headline is about. "Stocks plunge" might mean the Nasdaq fell sharply while the Dow barely moved. Finally, compare the move with your own mix of investments. If you hold a broad index fund, your account will usually move closer to the S&P 500 than to the Nasdaq. If you hold a few individual tech stocks, your account may swing even more than any index.

Market drops are a normal part of investing. Our guide to investing during a market downturn covers how many long-term investors think about rough patches.