Alphabet’s Class A shares fell 12.6% on October 29, 2026, their worst day since the company listed in 2004, and about $482 billion of market value disappeared in one session. That one result weighed on the S&P 500 and the Nasdaq 100 as well, and the reason sits in how stock indices are calculated. Most big benchmarks give each company a weight based on its size, and Alphabet’s two share classes made up a little over 5% of the S&P 500 going into that week. Once you see how the weights are set, a headline index move starts to tell you which companies did the moving.
Quick answer: How stock indices are calculated depends on the index. Most major ones, including the S&P 500 and the Nasdaq 100, add up the market value of their members and divide the total by a number called the divisor. Larger companies carry more weight, which means a handful of giants can move the whole index. The Dow Jones Industrial Average is the main exception, because it’s weighted by share price.
Trade a Whole Market in One Position
Index CFDs on SelfMSF let you take a position on the S&P 500, Nasdaq 100 and other benchmarks rising or falling, without buying any of the shares inside them [CONFIRM: available indices].
How Stock Indices Are Calculated by Market Value
A company’s market value, or market capitalization, is its share price multiplied by the number of shares in issue. In a market-cap weighted index, each member’s weight is its market value divided by the total for the whole index.
S&P Dow Jones Indices adds one refinement. It counts only the shares available to the public, using what it calls an investable weight factor, and leaves out large blocks held by founders or governments. Under the S&P index mathematics methodology, each company’s share price is multiplied by those adjusted shares, the results are added up, and the total is divided by a number called the divisor to give the index level.
The S&P 500 covers about 80% of the available value of US shares.
The Divisor That Keeps the Index Level Steady
Index providers set the divisor when an index starts, and they change it every time something happens that would move the index for reasons unrelated to trading. When a company joins or leaves the S&P 500, or issues a lot of new shares, the divisor is adjusted to keep the index level exactly where it was a moment earlier.
Without that step, adding a $500 billion company would push the index up overnight even if no share price had moved. The divisor is the reason a chart of the S&P 500 reflects price changes and nothing else, and it’s a large part of how stock indices are calculated consistently over decades.
Price-Weighted Indices Like the Dow
The Dow takes an older approach to how stock indices are calculated. It’s a price-weighted measure of 30 US blue-chip companies. The share prices are added together and divided by the Dow’s own divisor, and the number of shares or the size of the company plays no part.
A company with a $400 share price carries eight times the weight of one priced at $50, even if the cheaper stock belongs to a much bigger business. A stock split cuts a company’s weight in the Dow, because the price falls while the business stays the same size.
A Worked Example: One Index Calculated Two Ways
Here’s a made-up index of three companies, calculated both ways, to show how stock indices are calculated and why the method changes the result.
| Company | Share price | Shares | Market value | Weight by market value | Weight by price |
|---|---|---|---|---|---|
| A | $500 | 200 million | $100 billion | 10% | 76.9% |
| B | $50 | 10 billion | $500 billion | 50% | 7.7% |
| C | $100 | 4 billion | $400 billion | 40% | 15.4% |
| Total | $650 | $1,000 billion | 100% | 100% |
Say Company B, the biggest business, rises 10% to $55 and the others stay flat.
- Market-value index: rises 5%, because B is half the index
- Price-weighted index: rises about 0.8%, because the prices now add up to $655
Now say Company A, the smallest business with the highest share price, falls 10% to $450.
- Market-value index: falls 1%
- Price-weighted index: falls about 7.7%, because the prices now add up to $600
The companies and the moves are identical in both cases, and the two indices still give very different readings, which is why the Dow and the S&P 500 can disagree on a given day.
Why a Few Companies Carry So Much Weight
Market-cap weighting rewards size, and the largest US companies have grown fastest. Apollo’s chief economist found the ten biggest accounted for 55% of the S&P 500’s market value gain between January 2021 and early 2026. At the close on October 29, 2026, the ten largest holdings made up about 37% of the index, and they belonged to nine companies, because both of Alphabet’s share classes made the list.
| Holding | Weight |
|---|---|
| Nvidia | 7.77% |
| Apple | 7.65% |
| Microsoft | 5.92% |
| Amazon | 3.60% |
| Alphabet Class A | 2.50% |
| Broadcom | 2.47% |
| Meta Platforms | 2.07% |
| Alphabet Class C | 2.02% |
| Tesla | 1.71% |
| Eli Lilly | 1.65% |
| Top 10 total | 37.36% |
Weights in the SPDR S&P 500 ETF Trust, which holds the index’s members at their index weights, as of October 29, 2026, from State Street Global Advisors.
The S&P 500 has around 500 members, which means an equal share would be about 0.2% each. Nvidia’s weight was nearly 40 times that. Alphabet’s two share classes still added up to 4.52% after their fall. On October 23 they’d come to 5.16%, according to Stock Analysis, and a 12.6% fall on that weight works out to roughly two thirds of a percentage point off the S&P 500 by itself, before any other stock moves.
This is why index traders watch results from the biggest companies so closely. Our guide to reading company results covers what investors look for in those reports.
How Index Providers Limit Concentration
Some providers adjust how stock indices are calculated to cap their biggest members. The Nasdaq 100 uses a modified market-cap method, and in July 2023 it held a special rebalance after its six largest members reached a combined 51.2% of the index. Under its rules, when members above 4.5% add up to more than 48%, their combined weight is cut back to 40%.
Equal weighting changes how stock indices are calculated altogether. The S&P 500 Equal Weight Index holds the same companies as the S&P 500 and resets each one to a fixed 0.2% every quarter. Comparing the two versions shows how much of a move came from the largest names and how much came from the typical company.
What Index Weighting Means When You Trade an Index
- Big earnings days: results from the top five or six companies can move an index CFD as much as major economic data
- Sector tilt: the Nasdaq 100 leans heavily toward technology, which means it often moves further than the S&P 500 when tech leads
- Breadth: an index can rise while most of its members fall, if a few large companies climb fast
- Different methods: the Dow and the S&P 500 can move in opposite directions on the same day because of the way each is calculated
The indices page lists the benchmarks available on SelfMSF, and our article on bitcoin and tech stocks shows how the same tech-heavy days can reach crypto too.
Key Takeaways
- How stock indices are calculated shapes what an index move tells you, and most major benchmarks weight members by market value.
- The divisor keeps the index level steady when the list of members or their share counts change.
- The Dow is price-weighted, which gives the highest-priced shares the most influence.
- The ten largest holdings made up about 37% of the S&P 500 at the end of October 2026, which is why a few companies can move the whole index.
- Capped and equal-weight versions show how much of a move came from the largest names.
Frequently Asked Questions
How is the S&P 500 calculated?
The S&P 500 is calculated by multiplying each member’s share price by its publicly available shares, adding those values together and dividing by the index divisor. Larger companies carry more weight. S&P Dow Jones Indices adjusts the divisor whenever companies join, leave or change their share count, which keeps the index level consistent over time.
What is a price-weighted index?
A price-weighted index gives each company a weight based on its share price alone, and company size plays no part. The share prices are added up and divided by a divisor. The Dow Jones Industrial Average works this way, which means a $400 stock moves the index eight times as much as a $50 stock.
What is an index divisor?
An index divisor is the number an index’s total value is divided by to give the published level. Index providers adjust it when companies are added or removed, or after events like stock splits, which stops those changes from moving the index by themselves. It keeps the index reflecting price moves alone.
What is an equal-weight index?
An equal-weight index gives every member the same share, regardless of company size. The S&P 500 Equal Weight Index holds the same companies as the S&P 500 at a fixed 0.2% each, reset every quarter. Comparing it with the standard S&P 500 shows how much the biggest companies are driving the market.