When a stock is chosen for the S&P 500, S&P Dow Jones Indices announces it at 5:15 p.m. Eastern Time, usually about two weeks before the change takes effect, and index funds that track it typically buy the shares at the close of the last trading day before the change takes effect [4][8]. To qualify, a US company needs an unadjusted market value of at least $22.7 billion (the threshold in S&P's July 2026 methodology, reviewed quarterly) and positive earnings, among other tests, but the final choice rests with an index committee [4]. In 2026, Marvell Technology (MRVL) and Bloom Energy (BE) each rose 9.6% on the first trading day after their announcements, while Illumina (ILMN) fell 3.3%; research shows the average "index effect" has fallen from 7.4% in the 1990s to less than 1% in the last decade [1][2][3][11].
What does it take to join the S&P 500?
The S&P 500 is not simply the 500 largest US companies. S&P Dow Jones Indices sets eligibility rules in its US indices methodology, and a company must pass all of them to be considered [4]:
| Rule | Requirement for an S&P 500 addition |
|---|---|
| Market capitalization | Unadjusted company market cap of at least $22.7 billion, and a float-adjusted market cap of at least half that threshold |
| Domicile | US-domiciled company |
| Public float | Investable weight factor (share of shares available to investors) of at least 0.10 |
| Earnings | Positive GAAP net income from continuing operations in the most recent quarter and over the last four quarters combined |
| Liquidity | Annual dollar value traded at least 0.75 times float-adjusted market cap, and at least 250,000 shares traded in each of the six months before evaluation |
| Seasoning | Initial public offerings must trade for at least 12 months before being considered |
Source: S&P Dow Jones Indices, S&P U.S. Indices Methodology, July 2026 [4].
The market-cap floor moves with the market. It rose from $20.5 billion to $22.7 billion on July 1, 2025; S&P Dow Jones Indices reviews the ranges at the start of every calendar quarter, and the July 2026 methodology still lists $22.7 billion [4][5]. The rules apply to additions only: a member that later falls below them is not automatically removed [5].
Who decides?
An Index Committee of S&P Dow Jones Indices staff meets monthly to review candidates, corporate actions and the index's makeup, and its discussions are confidential because changes can move markets [4]. Sector balance, compared with the wider US market, is also considered [4]. Meeting every rule therefore makes a company eligible, not guaranteed.
When are stocks added to the S&P 500?
Most changes happen at the quarterly rebalance, after the close on the third Friday of March, June, September and December, with the new members counted from the following Monday [4]. In 2026, each quarterly announcement came on the first Friday of the month [6][7][8]:
| Announced | Effective (before the open) | Added to the S&P 500 | Removed |
|---|---|---|---|
| March 6, 2026 | March 23, 2026 | Vertiv (VRT), Lumentum (LITE), Coherent (COHR), EchoStar (SATS) | Match Group, Molina Healthcare, Lamb Weston, Paycom Software |
| June 5, 2026 | June 22, 2026 | Marvell Technology (MRVL), Flex (FLEX) | Pool Corp, The Campbell's Company |
| September 4, 2026 | September 21, 2026 | Bloom Energy (BE), Illumina (ILMN), Everpure (P) | Molson Coors, The Trade Desk, Builders FirstSource |
Source: S&P Dow Jones Indices press releases [6][7][8].
Changes also happen between rebalances, usually when a member is acquired. Ciena (CIEN) joined on February 9, 2026, replacing Dayforce after its acquisition by Thoma Bravo [9]. Many additions are promotions from the S&P MidCap 400: Lumentum, Coherent and EchoStar in March, and Illumina and Everpure in September, all came from the mid-cap index, while the removed companies moved down to the S&P SmallCap 600 [6][8]. Under the methodology, the next quarterly rebalance is after the close on Friday, December 18, 2026.
Why does the stock price move?
Index funds that track the S&P 500 must own every member in proportion to its weight, so an addition creates forced buying. S&P Dow Jones Indices estimates that about $23 trillion was indexed or benchmarked to the S&P 500 at the end of 2025, of which about $16 trillion was indexed, or passively managed [10]. Those funds generally buy at the close of the last trading day before the change takes effect.
The trading volume shows it. Marvell traded 251.0 million shares on June 18, 2026, the last session before its June 22 inclusion (June 19 was a market holiday), against 50.6 million the day before. Bloom Energy traded 73.5 million shares on September 18, about five times the 14.2 million of September 17, and Illumina 32.6 million, about nine times its 3.7 million [1][2][3].
| Stock | Close on announcement day | Next trading day | Announcement close to last close before inclusion |
|---|---|---|---|
| Marvell Technology (MRVL) | $263.47 (June 5) | +9.6% (June 8) | +17.9% (to $310.58 on June 18) |
| Bloom Energy (BE) | $252.87 (September 4) | +9.6% (September 8) | +5.0% (to $265.63 on September 18) |
| Illumina (ILMN) | $218.22 (September 4) | -3.3% (September 8) | +9.8% (to $239.62 on September 18) |
Source: DataPorium daily closes; moves are not adjusted for the overall market [1][2][3].
The examples show how uneven the effect is. Marvell and Bloom Energy jumped on the first day, while Illumina fell before rising into the inclusion date. Other news, sector moves and the market's direction all mix with the index effect, and an announcement on a Friday evening, as all three were, gives traders a weekend to position.
Is the S&P 500 index effect shrinking?
Yes, on average. Robin Greenwood and Marco Sammon found that the abnormal return from being added to the S&P 500, the gain beyond what the market would explain, fell from an average of 7.4% in the 1990s to less than 1% in the past decade, even though the share of stock market assets linked to the index rose sharply [11]. Deletions followed the same pattern: large negative abnormal returns in the 1990s, but an average return of only 0.1% between 2010 and 2020 [11].
The authors investigate several drivers [11]. Features of today's market that point the same way are easy to see in the 2026 changes. Many new members are promotions from the S&P MidCap 400, so mid-cap index funds sell what S&P 500 funds buy, partly offsetting the demand [6][8]. Candidates are easier to anticipate from the published rules, so traders can buy before the announcement. And index funds trade at the official closing price, where large orders can be matched against other investors willing to sell.
Joining the S&P 500 brings guaranteed buying from index funds, but in recent decades much of that demand has been anticipated, offset or absorbed before the effective date.
What it means for shareholders
- The announcement is the event: the largest price move usually comes on the first trading day after the 5:15 p.m. announcement, not on the effective date [4].
- The inclusion day is about volume: index buying concentrates at the close before the effective date, as the 251.0 million Marvell shares on June 18 show [1].
- Membership is not permanent: seven of the 2026 removals listed above moved down to the S&P SmallCap 600 [6][8].
Daily prices and volume for each company are on its DataPorium page, such as Marvell's, and the companies of the index can be compared with the DataPorium stock screener.
Key takeaways
- An S&P 500 addition needs an unadjusted market cap of at least $22.7 billion, positive GAAP earnings and 12 months of trading, but a committee makes the final choice [4].
- Quarterly changes take effect after the third Friday of March, June, September and December; in 2026 they were announced on March 6, June 5 and September 4 [4][6][7][8].
- About $16 trillion was indexed to the S&P 500 at the end of 2025, out of $23 trillion indexed or benchmarked [10].
- Marvell and Bloom Energy rose 9.6% the day after their 2026 announcements, while Illumina fell 3.3% [1][2][3].
- The average abnormal return from an addition fell from 7.4% in the 1990s to less than 1% in the last decade [11].
Frequently asked questions
What are the requirements to be added to the S&P 500?
A US company needs an unadjusted market cap of at least $22.7 billion, a float of at least 10%, positive GAAP earnings in the latest quarter and the last four quarters combined, adequate trading volume and 12 months of trading since its IPO [4].
When does the S&P 500 rebalance?
Quarterly, after the close on the third Friday of March, June, September and December. Under that schedule, the next rebalance is after the close on December 18, 2026 [4].
Do stocks go up when added to the S&P 500?
Often, but less than they used to. The average abnormal return has fallen below 1% in the last decade, and in 2026 Illumina fell 3.3% the day after its announcement while Marvell rose 9.6% [1][3][11].
Why are S&P 500 changes announced after the market closes?
S&P Dow Jones Indices treats index changes as potentially market-moving and announces additions and deletions at 5:15 p.m. Eastern Time, after regular trading ends [4].
Sources & References
- [1] DataPorium Marvell Technology (MRVL) stock page (daily prices and volume)
- [2] DataPorium Bloom Energy (BE) stock page (daily prices and volume)
- [3] DataPorium Illumina (ILMN) stock page (daily prices and volume)
- [4] S&P Dow Jones Indices, S&P U.S. Indices Methodology (July 2026)
- [5] S&P Dow Jones Indices, S&P Dow Jones Indices Announces Update to S&P Composite 1500 Market Cap Guidelines (July 1, 2025)
- [6] S&P Dow Jones Indices, Vertiv Holdings, Lumentum Holdings, Coherent, and EchoStar Set to Join S&P 500 (March 6, 2026)
- [7] S&P Dow Jones Indices, Marvell Technology and Flex Set to Join S&P 500 (June 5, 2026)
- [8] S&P Dow Jones Indices, Bloom Energy, Illumina, and Everpure Set to Join S&P 500 (September 4, 2026)
- [9] S&P Dow Jones Indices, Ciena Set to Join S&P 500 (February 4, 2026)
- [10] S&P Dow Jones Indices, Indexed Asset Survey (assets as of December 31, 2025)
- [11] Greenwood and Sammon, The Disappearing Index Effect, Journal of Finance 80(2), 2025 (IDEAS/RePEc listing)