Invested in Index Funds? Here's What You Need to Know About Mega-Cap IPOs
SpaceX and other mega-cap IPOs, such as Anthropic and OpenAI, could impact the makeup of your index funds — but not all portfolios will see an immediate impact.
2026 could be the biggest year ever for initial public offerings (IPOs), with high-profile private companies SpaceX, Anthropic and Databricks among numerous super-sized upcoming IPOs expected to hit the markets over the coming months.
If you're a stock investor looking for fresh meat to sink your teeth into, 2026 is a veritable Fogo de Chão.
But even if most or all of your money is wrapped up in low-cost index funds and you have no real hunger for these shares, you still might be digesting a considerable slice of some of these and other potential mega-cap IPOs.
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And depending on which funds you own, that day might be coming sooner rather than later.
How index funds pick stocks
Index investing is a simple concept. An index is a hypothetical portfolio of stocks built by a set of rules. And an index fund "tracks" that index by owning the same companies in the same weights as that index dictates.
But the rules can be complex and numerous — not just in niche indexes that carve up far-flung corners of the market, but even in ubiquitous indexes such as the S&P 500.
A common misconception is that the S&P 500 represents the 500 largest U.S. companies. Not so. It's 500 of the largest companies, all of which have met a much more exhaustive set of selection criteria than most people realize. Here's what the S&P 500 looks for in prospective components:
Criteria | Description |
Universe | It must be a U.S. company. |
Company type | Its stock must be common equity or a real estate investment trust (REIT). No American Depositary Receipts (ADRs) or American Depositary Shares (ADSs). No exchange-traded funds (ETFs) or closed-end funds (CEFs), for that matter. |
Exchange | Its stock must be listed on an eligible exchange. |
Market capitalization | It must have (1) an unadjusted market cap of $22.7 billion or more, and (2) a "float-adjusted" market cap that's at least 50% of the unadjusted market cap minimum (so, $11.35 billion). |
Float | Its number of shares available for public trading (the "float") must be at least 10% of its total outstanding shares. |
Volume | Its stock must trade a minimum of 250,000 shares in each of the six months before it's evaluated for inclusion, and its ratio of annual dollar value traded to its float-adjusted market cap must be at least 0.75. |
Profitability | It must have reported a generally accepted accounting principles (GAAP) profit in the most recent quarter, as well as in total across the past four quarters. |
IPO "seasoning period" | As it pertains specifically to initial public offerings, the stock must trade for at least 12 months before being considered. |
Sector balance | The S&P 500 lists "sector balance" as one of its criteria, but without a hard threshold. This is something the U.S. Index Committee looks at on a case-by-case basis to make sure the index appropriately reflects the U.S. economy. |
On that final point: the S&P 500 ultimately is not 100% governed by a set of rules. A selection committee ultimately makes calls on which stocks enter and exit the indexes.
Which stocks are eligible are determined by the rules, but the U.S. Index Committee can and does review those stocks, and can even make exceptions for stocks that otherwise wouldn't make the cut. This isn't specific to the S&P 500, either — this applies to a host of S&P Dow Jones indexes.
The U.S. Index Committee can also choose to revise index rules over time, so yesterday's rules might not necessarily apply tomorrow. For instance, they've raised the minimum market cap several times throughout the S&P 500's history.
Most other fund providers don't employ an individual-stock level of human review in their index funds. But they absolutely can use index rule changes to give them the flexibility they need to own mega-cap IPOs … and several have done just that ahead of the SpaceX IPO.
Will SpaceX be in your index fund? Well …
Elon Musk's SpaceX (SPCX) is expected to join the public markets later this week in what will likely be the biggest IPO ever. And many believe that its offering will value the aerospace manufacturing company at $1.75 trillion.
If the SpaceX IPO were most other offerings, we wouldn't expect it to show up in the low-cost funds in your portfolio for at least a couple of months. That's because many indexes have seasoning rules like the S&P 500's that prohibit companies from quickly joining.
This gives IPO companies time to meet liquidity and float thresholds. It also ensures index fund shareholders don't suffer if insiders sell shares in droves when the company's lockup period expires, which is usually 90 to 180 days after the IPO.
But most index providers have "fast-track" or "fast-entry" rules that allow them to waive that requirement, typically for massive IPOs where demand is expected to be high regardless of the risk.
"Fast-entry rules are important because they allow rules-based passive ETFs to keep pace with the market," says Tejas Dessai, director of Thematic Research at Global X, whose Global X NYSE 100 ETF (NYSX), a mega-cap fund, and Global X Space Tech ETF (ORBX), a space fund, have fast-track allowances. "When a major private company becomes public, investors should not necessarily have to wait months for that exposure to appear in a relevant ETF."
Importantly, SpaceX and other offerings would have to meet those indexes' other eligibility rules, Dessai says.
Among the major index makers with fast-track rules?
- Nasdaq recently changed its rules to allow IPOs to join an index as soon as 15 days after listing. That's expected to allow SpaceX to join the Nasdaq-100 Index — the largest 100 non-financial companies on the Nasdaq exchange. If so, owners of Invesco QQQ ETF (QQQ) and its smaller, more cost-effective cousin, Invesco Nasdaq 100 ETF (QQQM), would own a slice of SPCX.
- FTSE Russell updated its methodology to allow companies to join some of its indexes after just five trading days. Funds like the iShares Russell 1000 ETF (IWB) and iShares Russell 1000 Growth ETF (IWF), then, could add shares quickly.
- The Center for Research in Security Prices (CRSP), which already had fast-track rules in place, recently relaxed its float requirement, and new IPOs are eligible for inclusion after just five trading days. That puts the likes of Vanguard Total Stock Market Index Fund ETF (VTI) and Vanguard Growth ETF (VUG) into play.
But far more noteworthy is where S&P Dow Jones Indices didn't loosen up the rules: the S&P 500.
SpaceX is only making a sliver of its overall shares available in its public offering; its float is expected to be about 4% of shares outstanding, which is well below the S&P 500's threshold. SpaceX is also deeply unprofitable, posting a net loss of $4.3 billion during Q1 2026. Add in the 12-month IPO seasoning period, and that's a clear strikeout.
S&P Dow Jones Indices recently reviewed potential changes to several of its indices. It actually let through a few — it relaxed the float rules on a few products with fast-track allowances, most notably the S&P Total Market Index, which anchors the iShares Core S&P Total U.S. Stock Market ETF (ITOT).
However, it kept its float, profitability and seasoning criteria in place for the S&P 500, as well as the S&P MidCap 400, S&P SmallCap 600, and S&P Composite 1500.
That means if you hold any of the S&P 500 ETFs — Vanguard S&P 500 ETF (VOO), iShares Core S&P 500 ETF (IVV), State Street SPDR S&P 500 ETF Trust (SPY), State Street SPDR Portfolio S&P 500 ETF (SPYM) — you won't be holding any fraction of SpaceX stock for at least a year.
The same goes if you own S&P 500 tracking index mutual funds, or even funds that are indirectly tethered to the S&P 500, such as State Street's sector funds.
And we can expect similar fates for Anthropic, OpenAI and other mega-cap listings that are expected to offer small slivers of their total outstanding shares in their IPOs.
How much SpaceX am I about to own?
The good news — at least for anyone worried about the prospects of SpaceX taking over their portfolio — is that in many cases, SPCX might not enjoy a significant weight in your fund.
Not right away, anyway.
Again, SpaceX is expected to fetch a $1.75 trillion valuation. For perspective, $1.9 trillion chipmaker Broadcom (AVGO) is a top-10 holding in Vanguard's VTI, accounting for almost 3% of the index's weight. That's a massive amount of influence relative to all but a handful of VTI's 3,500 components — and by pure market cap alone, SpaceX would qualify for pretty similar treatment.
Whether you're concerned or excited about the prospect of owning SpaceX and/or other mega-cap IPOs in your index fund, now's a good reminder to go to the fund's provider page and look at its fact sheet and prospectus.
However, many index funds (including VTI) are "float-adjusted," which means weights aren't assigned based on total market cap, but the market cap based on the float.
So even though SpaceX would be a $1.75 trillion company, many index funds that include it would have to treat it like a $70 billion company. In Vanguard's total-market ETF, then, SPCX would enjoy a similar weight as $73 billion retailer Ross Stores (ROST) — currently the 157th-largest stock in VTI, accounting for about 0.1% of its assets.
That could change. Morningstar's Dan Lefkovitz points out that "by the end of the lockup period, it's typical for a company's free float to rise to 50% to 60% of its overall value. As the float increases, so will the stock's weighting in the indexes." Which means in a few months, your fund could end up owning a greater share of SPCX … even if the company's value shrinks.
And remember: Different funds have different weighting methodologies. So, whether you're concerned or excited about the prospect of owning SpaceX and/or other mega-cap IPOs in your index fund, now's a good reminder to go to the fund's provider page and look at its fact sheet and prospectus.
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