Quick answer
A stock market index is a single number that measures the performance of a chosen basket of stocks meant to represent a market or sector [1]. How each stock is weighted — by share price, by company size or equally — decides how much each one moves the index.
Key points
- An index is a measuring stick for a group of stocks, not something you buy directly.
- A price-weighted index, like the Dow, gives more influence to stocks with higher share prices.
- A market-cap-weighted index, like the S&P 500, gives more influence to larger companies.
- An equal-weighted index gives every stock the same weight, whatever its size.
- Index funds try to copy an index, but fees and tracking error mean their returns can differ.
#What does a stock market index actually measure?
Investor.gov, the investor education site of the U.S. Securities and Exchange Commission (SEC), defines a market index as "a measurement of the performance of a specific 'basket' of stocks considered to represent a particular market or sector of the U.S. economy" [1]. In plain terms, an index takes many share prices and turns them into one number you can follow from day to day.
The number itself is not a price you pay. If an index stands at 5,000 points, that does not mean anything costs $5,000. What matters is the change: if the index rises from 5,000 to 5,100, the basket it tracks gained 2% by that index's rules. The rules — which stocks are in, and how much each one counts — are written down by the index provider in a document called the methodology.
Investor.gov's own example is the Dow Jones Industrial Average, which it describes as an index of 30 major U.S. company stocks often called "blue chip" securities [1]. You can read the full profile in our guide to the Dow Jones Industrial Average.
#What are the main ways an index can be weighted?
Weighting means how much each stock counts toward the index. S&P Dow Jones Indices, the company that calculates both the S&P 500 and the Dow, describes several schemes in its index-literacy material [2]. The three a beginner meets most often are below.
| Method | What decides a stock's weight | Well-known example |
|---|---|---|
| Price-weighted | Its share price | Dow Jones Industrial Average |
| Market-cap-weighted (often float-adjusted) | The company's total market value — share price × shares available to trade | S&P 500 |
| Equal-weighted | Nothing — every stock gets the same weight | Equal-weight versions of broad indexes |
Price-weighted
In a price-weighted index, a stock with a $300 share price counts six times as much as a stock priced at $50, even if the $50 company is far larger. S&P Dow Jones Indices gives its own illustration: if one Dow stock traded at $90 and the share prices of all Dow components added up to $4,300, that stock's weight would be $90 ÷ $4,300, or about 2.1% [2]. S&P confirms that "The Dow is price-weighted" [3].
Market-cap-weighted
Market capitalization is the share price multiplied by the number of shares (see market capitalization). In a market-cap-weighted index, the biggest companies have the most influence. Most large indexes go one step further and use float adjustment: they count only shares that are readily available for public trading, leaving out blocks held by founders, executives and other controlling holders [2]. S&P describes the S&P 500 as "a float-adjusted market-cap-weighted index" [3]. Our S&P 500 profile covers it in more detail.
Equal-weighted
In an equal-weighted index, "each security has the same weight, regardless of its market cap" [2]. A small company and a giant count the same. Because prices drift apart over time, an equal-weighted index has to be reset back to equal weights on a schedule set in its methodology.
#How much does weighting change the result?
A lot. The same price move in the same stock can barely register in one index and dominate another. The example below uses three made-up companies so the arithmetic is easy to follow.
Worked example
One 10% jump, three different index moves
A hypothetical index holds three stocks. Stock A trades at $300 with 10 million shares ($3.0 billion market cap). Stock B trades at $50 with 200 million shares ($10.0 billion). Stock C trades at $20 with 50 million shares ($1.0 billion). Total market cap is $14.0 billion. Then each stock, one at a time, rises 10%.
- Weight of A: price-weighted / cap-weighted / equal
- 81.1% / 21.4% / 33.3%
- Weight of B: price-weighted / cap-weighted / equal
- 13.5% / 71.4% / 33.3%
- Weight of C: price-weighted / cap-weighted / equal
- 5.4% / 7.1% / 33.3%
- A rises 10% → index change (price / cap / equal)
- +8.11% / +2.14% / +3.33%
- B rises 10% → index change (price / cap / equal)
- +1.35% / +7.14% / +3.33%
- C rises 10% → index change (price / cap / equal)
- +0.54% / +0.71% / +3.33%
Stock A — a mid-sized company here, but with by far the highest share price — moves the price-weighted index more than eight times as much as stock B. In the cap-weighted index, the biggest company, B, does the heavy lifting. In the equal-weighted index, every stock pulls the same.
Hypothetical companies. Weights computed in Python: price weight = price ÷ sum of prices; cap weight = market cap ÷ total market cap; equal weight = 1 ÷ 3. Index change = weight × 10%.
Index move when stock B (the largest company) rises 10%
#Why do indexes use a divisor?
If an index simply added up share prices, a routine event like a stock split would make it jump or drop for no economic reason. S&P Dow Jones Indices notes that each of its indexes has its own divisor, which "is adjusted regularly to keep the level of its index constant" when such changes happen [3].
Using the three stocks above in a price-weighted index: the prices add up to $370, and dividing by 3 gives an index level of 123.33. Now stock A splits 3-for-1, so its price becomes $100. The prices now add up to $170. To keep the level at 123.33, the divisor is changed from 3 to about 1.378 (170 ÷ 123.33). Nobody gained or lost money in the split, so the index should not move either — the divisor makes sure it does not.
#Can you invest in an index directly?
No. An index is a calculation, not a product. What you can buy is an index fund — Investor.gov describes it as "a type of mutual fund or exchange-traded fund that seeks to track the returns of a market index" [4]. Some index funds hold every stock in the index; others hold a sample [4].
A fund's return and its index's return are not identical. Investor.gov notes that an index fund may underperform its index because of fees and expenses, trading costs and tracking error [4]. Our guide to index funds explains how to compare them.
How to read an index headline
Name the index
"Stocks fell" usually means one specific index. Find out which one — the Dow, the S&P 500, the Nasdaq Composite or another.
Check how it is weighted
A price-weighted index can be pushed around by a few high-priced stocks; a cap-weighted one by a few very large companies.
Look at the percentage, not the points
A 500-point move means very different things for an index at 5,000 and one at 40,000. The percentage change is comparable; the point change is not.
Compare it with your own holdings
Your portfolio is not the index. If you own different stocks or funds, your result will differ.
Common beginner mistakes
Comparing indexes by their point level
An index at 40,000 is not "bigger" or "better" than one at 5,000. Each starts from its own base value and uses its own divisor, so only percentage changes can be compared.
Assuming "the market" means every stock
Even a broad index covers a selected basket. A small-company index like the Russell 2000 can move quite differently from a large-company index on the same day.
Ignoring weighting
In a market-cap-weighted index, a handful of very large companies can drive most of the move. The index can rise even while most of its members fall.
Expecting a fund to match the index exactly
Fees, trading costs and tracking error mean an index fund's return will normally differ from the index it follows [4].
What's the bottom line?
A stock market index turns many share prices into one number, and its weighting rule decides which stocks matter most. Price weighting favors high share prices, cap weighting favors large companies and equal weighting treats every member alike. Before reacting to an index headline, check which index it is and how it is built — then see how interest rates affect stocks for one of the big forces that moves them.
Frequently asked questions
Is the Dow the same as the stock market?
No. The Dow Jones Industrial Average tracks 30 large U.S. companies and is price-weighted. It is one widely quoted index, not the whole market.
Who decides which stocks are in an index?
The index provider. It publishes a methodology that sets the entry rules, the weighting method and how often the index is reviewed, so anyone can check how the number is built.
Which weighting method is the right one?
None is right in every case. Each answers a different question: cap weighting shows how the total value of a market changed, equal weighting shows how the average stock did, and price weighting shows how a set of share prices moved, adjusted for splits by the divisor.
Do index points have a money value?
No. Index points are a unit of measurement. Only the percentage change between two dates tells you how the basket performed.
Sources
Grade A = primary source (regulator, government agency, official rulebook or the index provider's own documents). Numbers in brackets in the text point here.
- U.S. SEC — Investor.gov. Market Index (glossary) (2026). Accessed 2026-10-03.A
- S&P Dow Jones Indices. Methodology Matters (2026). Accessed 2026-10-03.A
- S&P Dow Jones Indices. Icons: The S&P 500 and The Dow (2026). Accessed 2026-10-03.A
- U.S. SEC — Investor.gov. Index Funds (2026). Accessed 2026-10-03.A
This page is general education, not personal financial, tax or legal advice. Figures in worked examples are hypothetical and calculated before taxes and fees unless stated. Rules and limits change; check the linked primary sources for the current version. How we check every page.
