investing basics

Stock Market Indices Explained: S&P 500, Dow Jones, and Nasdaq

By MoneyMattersDaily Editorial · Published October 9, 2026 · 5 min read

“Stocks fell today” usually means one of three numbers fell: the S&P 500, the Dow, or the Nasdaq. They’re often reported as if they were the same thing, but each one is a different list of companies, calculated in a different way. Knowing the difference makes headlines easier to read and fund choices easier to understand.

Who this is for, and who it isn’t

It’s for: beginners who see index numbers in the news and want to know what they mean, and anyone about to choose an index fund or ETF.

It’s not for: anyone looking for a prediction or a recommendation on which index to buy. This is general education, not personalized investment advice, and investments can lose value.

What a stock market index is

An index is a list of stocks, plus a rule for combining their prices into one number. It’s a scoreboard, not something you can buy. When a headline says “the S&P 500 rose 1%,” it means the combined value of that list, calculated under that index’s rules, went up 1%.

Two things differ from index to index: which companies are on the list and how much each one counts. That second part, called weighting, is where most of the confusion comes from.

The three you hear about most

S&P 500. Tracks roughly 500 large companies listed in the US. A committee selects the companies, and the index is weighted by company size, specifically by the market value of shares available to the public. Bigger companies move the index more. Many investors treat it as a stand-in for large US companies as a whole.

Dow Jones Industrial Average. Tracks just 30 large US companies, also chosen by a committee. It’s price-weighted, which means companies with higher share prices have more influence, no matter how big the company actually is. The index is calculated by adding up the 30 share prices and dividing by a number called the divisor, which is adjusted over time for splits and similar events. With only 30 names, it’s a narrow snapshot, and it’s generally not considered representative of the whole US economy.

Nasdaq. Two indexes get this name:

• The Nasdaq Composite covers almost all the stocks listed on the Nasdaq exchange, which is thousands of securities, weighted by company size. It leans heavily toward technology companies.
• The Nasdaq-100 covers about 100 of the largest non-financial companies on the exchange. Its weighting is a modified size-based version that limits the influence of the very biggest names.

People often mix these two up. When you see “Nasdaq” in a headline, it usually means the Composite, but funds that say “Nasdaq” often track the Nasdaq-100.

Why weighting matters: a worked example

Imagine a tiny index with two companies:

Company B is more than twice as big, but Company A has the higher share price. Now suppose A’s stock rises 5% (from $400 to $420) and B’s falls 5% (from $100 to $95).

• Price-weighted (like the Dow): the sum of prices goes from $500 to $515. The index is up about 3.0%, because A’s high share price dominates.
• Size-weighted (like the S&P 500): the combined value goes from $1,400 billion to $1,370 billion. The index is down about 2.1%, because B is the bigger company.

Same two stocks, same moves, opposite headlines. This is a simplified illustration, but it shows why the Dow and the S&P 500 can disagree on the same day.

What size weighting means for you

Size-weighted indexes like the S&P 500 and the Nasdaq Composite give more influence to the biggest companies. That means a small group of very large companies can drive a large share of the index’s moves, and a fund that tracks it carries the same tilt. The Nasdaq indexes in particular lean toward technology. This isn’t good or bad on its own, but it’s worth knowing that “the index” and “the whole economy” aren’t the same thing.

You can’t buy an index, but you can buy a fund that tracks one

An index is only a calculation. To invest in one, you buy an index fund or ETF that holds the same stocks in similar proportions. That’s what people mean by “investing in the S&P 500.” The fund’s results will be close to the index’s but slightly lower because of fees. For how these funds work and why many beginners choose them, see index funds explained for beginners.

Indexes around the world

Most big markets have their own headline index, built on similar ideas. The FTSE 100 follows large companies listed in London, and Japan’s Nikkei 225 follows large Japanese companies and, as commonly described, is price-weighted like the Dow. India’s Nifty 50 and Sensex follow large Indian companies. Before you assume two indexes are comparable, check what each one includes and how it’s weighted.

How to read index numbers without being misled

• Look at percentage change, not points. The Dow and the S&P 500 have very different index levels, so a “300 point” move means something different on each. A 1% move is a 1% move on any of them.
• Remember the headline number usually leaves out dividends. A fund that tracks the index and pays out dividends can earn a bit more than the plain index level suggests.
• Don’t treat one day as a trend. For how to think about swings, see what stock market volatility means for your savings.
• Know which index you’re looking at. “The market” in a headline could mean any of the three.

Where people actually go wrong

• Thinking the Dow is the whole market. It’s 30 companies.
• Mixing up the Nasdaq Composite and the Nasdaq-100. They’re related but not the same.
• Comparing index points across indexes. Compare percentage moves instead.
• Assuming an index is the same as being diversified worldwide. A US index only covers US companies.
• Chasing last year’s best-performing index. Past performance doesn’t predict what comes next.
• Forgetting that funds have costs. A fund tracks an index, but it charges fees the index doesn’t.

Getting started

If you’re new to investing, you don’t need to pick a favorite index. Learn what each headline number means, then focus on the basics: a plan you can stick to, low costs, and enough time. For a starting point, see how to start investing with little money and index funds explained for beginners.

Crypto has no equivalent of an index of companies behind it, which is part of why it behaves so differently; see what is cryptocurrency: how it works and the risks.

Last updated October 2026. Index membership, weighting rules, and methodology can change, and this post deliberately avoids quoting current index levels. This is general education for a broad audience, not personalized investment advice, and investments can lose value. Check the index provider’s website for current rules before relying on any detail.