The S&P 500 is not simply the 500 biggest US companies
Ask most people what the S&P 500 is and they will say it is the 500 largest companies in America. That is close, but it leaves out the part that matters. The list is chosen. A committee at S&P Dow Jones Indices, the company that runs the index, decides who joins and who leaves.
S&P's own rulebook says the committee is made up of full-time S&P Dow Jones Indices staff. It meets once a month and keeps its discussions confidential, because news that a company is being added can move markets.
The rulebook fixes the count at 500 companies. The index usually holds a few more than 500 stocks, because some companies have more than one class of shares. At the end of September 2026 it held 503, according to S&P's factsheet for that date.
What a company has to show before it can even be considered
The published rules work like an entry exam. Under the July 2026 edition of the rulebook, a company must be based in the US and listed on a US exchange. A newly listed company must normally have traded there for at least 12 months, so a brand-new listing cannot join straight away. The rulebook makes some exceptions to that wait, for instance for certain spin-offs and for companies moving over from an exchange that does not qualify.
It must be worth at least $22.7 billion in total. That bar was set in July 2025. S&P reviews it every quarter, but a review does not mean a change : the bar is updated only when the market has moved enough, by S&P's own measure a difference of at least 10%. There is also a second size test. Counting only the shares freely available to trade, which are the shares the index actually adds, the company must be worth at least half of that bar.
It must make a profit under standard US accounting rules, known as GAAP, both in its latest quarter and across its last four quarters added together. At least 10% of its shares must be freely available to trade, rather than held by founders or other owners who are locked in. And its shares must change hands often enough. One of those tests asks for at least 250,000 shares traded in each of the previous six months.
Why meeting every rule does not guarantee a place
Passing the exam only puts a company in the pool of candidates. The rulebook says that choosing members 'is at the discretion of the Index Committee'. It adds that the committee weighs sector balance, comparing how much of the index each industry takes up with how much of the wider US market it takes up. The committee also reserves the right to make exceptions to its own rules when it sees a need.
That discretion is not a free hand. The committee works inside a long published rulebook on eligibility, exceptions and how companies move between S&P's indexes, and many changes follow from those rules. Where its judgment shows most is in the cases that stand out.
The case people remember is Tesla. By September 2020 it met the criteria, but the committee did not pick it in that month's changes, and CNBC reported it as a snub. The committee does not publish its reasons, so why it waited is not on the record. Two months later, on November 16, 2020, S&P announced that Tesla would join before trading opened on December 21.
The committee's judgment can also take the form of refusing to bend. In June 2026, after asking market participants for their views, S&P decided not to relax the 12-month waiting period, the profit test or the 10% free-trading minimum for very large newly listed companies. It said exceptions should not be granted 'solely based on market capitalization'.
Once a company is in, the entry rules stop applying
This is the part that surprises even people who follow markets. The size, profit and trading rules apply to companies joining, not to companies already inside. The rulebook says the criteria are 'for addition to an index, not for continued membership', and that S&P wants to avoid needless turnover. A member that slips below them is not removed automatically. It is removed only if 'ongoing conditions warrant' it, and that too is the committee's call.
S&P's own figures show the result. At the end of September 2026, the smallest company in the index was worth about $5.7 billion, roughly a quarter of the $22.7 billion a newcomer needs.
There is one more shortcut. A company moving up from S&P's index of mid-sized companies, the S&P MidCap 400, must still clear the $22.7 billion bar for total size, but it does not have to pass the profit and trading tests again, or the second size test on its freely traded shares.
How an index run by rules alone picks its members
Not every index works this way. The Russell indexes, run by FTSE Russell, part of the London Stock Exchange Group, build their membership from a ranking. On a fixed rank day, every eligible US company is ranked by its total market value, and those that pass the trading rules are placed by size. Committees oversee the rules and review how they work, but the list comes out of the ranking rather than out of a decision about each company.
Since 2026 the Russell lists have been rebuilt twice a year, in June and December, instead of once. The next rank day is October 30, 2026, and the new lists take effect after the close on December 11.
So two of the best-known families of US stock indexes answer the question of who gets in differently. For Russell, the ranking decides. For the S&P 500, the rules decide who may be considered, and people working within those rules decide the rest.
Why a place on the list used to be worth money
Funds that track the S&P 500 have to hold whatever is in it. When a company is added, they all have to buy its shares around the same date, whatever the price. In the 1990s that forced buying showed up clearly. Robin Greenwood and Marco Sammon of Harvard Business School found that a stock added to the index then gained on average 7.4% more than the market's overall movement would explain.
Tesla shows how big that buying can be. When its entry was announced, Reuters reported that it would set off a trade of about $51 billion, as index funds sold other holdings to buy Tesla. According to reports at the time, within two weeks the stock had risen nearly 40%, and on December 18, the last trading day before it joined, more than $150 billion of Tesla shares changed hands. Measured against the size of the index, Greenwood and Sammon's working paper counts it as the largest addition ever.
- Greenwood and Sammon, The Disappearing Index Effect (Harvard DASH)
- Greenwood and Sammon, The Disappearing Index Effect (NBER working paper 30748)
- Al Jazeera : Tesla shares up 12% after announcement it will zoom onto S&P 500
- CNBC : Tesla's stock will be added to the S&P 500 in a single step before the open on Dec. 21
- Fortune : Tesla shares fall ahead of first day in the S&P 500
Why the jump has largely disappeared
Tesla's jump was the exception. In the published version of the same study, the average extra gain for an added stock fell to 0.3%, well under 1%, over the most recent decade studied, even though a much larger share of the stock market is now tied to the index than in the 1990s. In the researchers' earlier working paper, leaving Tesla out of 2020 brings that year's average for new members to roughly zero.
The researchers weigh several explanations. The clearest is where new members now come from. Their working paper finds that in the 1990s about 40% of additions moved up from the S&P MidCap 400, and in recent years it has been over 80%. When such a company moves up, funds tracking the S&P 500 must buy it, but funds tracking the mid-cap index must sell it at the same moment, so the net buying is much smaller.
They also find some evidence, which they call mixed, that additions have become easier to guess, so traders buy before the announcement. That means a smaller jump around the announcement does not show that an addition no longer moves the price at all ; some of the move may simply come earlier. And they argue that markets have become better at handling these trades. This is a finding about averages over the past few decades, and the effect has changed size before. It is not a rule for any future addition.
What the committee still decides
So the money story has faded, but the question of who decides has not. A growing share of the US stock market is tied to the S&P 500, and which companies that money holds is settled not by a formula but by a committee of S&P staff. It meets every month, keeps its discussions confidential, and works within published rules it is allowed to step beyond.
The rules tell you who may be considered. They do not tell you who will be chosen, or who will be asked to leave.
