The Nasdaq Composite is calculated by combining the market values of its component stocks and scaling that total into an index number. Larger companies carry more weight, so their price changes usually matter more than equal percentage moves in smaller companies. The result is a running measure of a broad collection of Nasdaq-listed stocks. Understanding that calculation explains how the index can rise even while many individual stocks fall, and why its points are different from dollars in a bank account.
Start with the collection being measured. Nasdaq describes the Composite as an index of thousands of stocks listed on its stock market, spanning companies of different sizes. Technology has a prominent role, but the collection also reaches into other industries. The exchange and the index serve different purposes: the exchange provides a market for securities, while the index measures a defined group. A company's presence on that market helps determine whether its shares belong in the Composite. Nasdaq's overview explains its scope.
Next comes market capitalization, often shortened to market cap. It connects a share's price with the number of shares represented. A company with a high price for one share is not necessarily larger than a company whose shares cost less. Imagine one business with ten million shares priced at $100 each: its market value is $1 billion. Another has one hundred million shares priced at $20, giving it a $2 billion value. The second company is twice as large by this measure despite its cheaper individual share price.
In a simplified index containing only those two hypothetical companies, their combined value would be $3 billion. If the first company's price rose 10% while the second stayed unchanged, the combined value would gain $100 million, or about 3.33%. If the second company instead rose 10% while the first stayed unchanged, the gain would be $200 million, or about 6.67%. These are teaching examples, not actual Nasdaq readings. They show why counting how many stocks went up cannot, by itself, reproduce a market-cap-weighted index's movement.
The full calculation applies that idea across the component securities. Nasdaq's published Composite methodology describes multiplying each security's index shares by its latest sale price, adding the results, and dividing by an index divisor. That final step puts an enormous combined market value onto a manageable scale. An index level of 20,000, for example, would mean 20,000 index points; it would not mean that the average share costs $20,000. The points express the result of a calculation with its own scale.
The divisor also helps an index remain comparable as its makeup changes. Adding a company should not look like an investment gain merely because another market value entered the calculation. S&P Dow Jones Indices explains how divisor adjustments preserve continuity when changes other than ordinary share-price movements would otherwise distort an index. Percentage changes then put point moves into perspective. In a hypothetical move from 20,000 to 20,200, the 200-point gain is 1%. The same 200 points from a starting level of 10,000 would represent 2%.
The Nasdaq-100 is a different collection. Nasdaq defines it around 100 of the largest nonfinancial companies listed on its market, including domestic and international businesses. The broader Composite and this more selective index therefore do not have identical membership. A similar name on a screen does not make two measurements interchangeable. When two Nasdaq indexes move by different percentages, their different collections and weighting rules provide a place to begin understanding the gap, without assuming that either number is wrong.
An index is also separate from an investment product built to follow it. The SEC's Investor.gov explains that index funds seek to track a market index, sometimes by holding its securities and sometimes through a sample. The index itself is a measurement. Reading the Nasdaq Composite becomes easier once its three basic ingredients are visible: the stocks included, the influence assigned to each, and the scale used to report their combined movement. A single headline number then becomes a useful description of a particular basket, with clear limits on what it can tell you.
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