The index buys the IPO

2026-10-27

Index providers have changed their rules so that very large new listings do not have to wait for the next annual reconstitution. The result is a predictable wave of passive buying in the first fifteen trading days of a large IPO. For anyone holding pre-IPO paper, or planning to sell into a listing, that wave is now a line in the model.

The rules, briefly

S&P Total Market Index: eligible with five business days' notice once the company has either 10% free float or a float-adjusted value above a tenth of the hundredth-largest US company, currently a little over twelve billion dollars.
MSCI Standard indices: eligible from the tenth trading day. The fast track requires full market value above roughly twenty-nine billion dollars and free float above roughly fourteen billion. Size is assessed on the first or second trading day, including any announced over-allotment.
Morningstar: eligible from the fifth trading day above a total value of about three billion dollars.
Russell 1000: eligible after the fifth trading day if investable value exceeds the Top 500 breakpoint, around seventeen and a half billion dollars, with lock-up restrictions tolerated if they lapse within a year.
Nasdaq-100: since May, a Nasdaq-listed company large enough to rank in the top forty of the index can be added after fifteen trading days. The old 10% float minimum has gone; instead the index weight is scaled to three times the free float, which lets a huge low-float company in at a reduced weight.

What it adds up to

For a very large company listing in mid-November, the estimated passive demand is around 3% of the offered shares from the S&P total-market funds within a week, a further 3% from Russell on day five, about 4% from Morningstar, just under 4% from MSCI on day ten or at its end-November review, and up to 9% from Nasdaq-100 funds if the listing is on Nasdaq. In total, something like a quarter of the shares sold in the IPO are bought by index funds within fifteen days of the offering.

That is demand that does not read the prospectus. It arrives on a schedule set by the index calendar, and it arrives regardless of price.

What it means before the IPO

We spend much of our time on access to late-stage private companies, and the secondary market in their shares. Three consequences follow.

The IPO discount is being arbitraged by the calendar. Underwriters can price knowing that a defined block of passive demand will absorb float in the first three weeks. That supports the deal price and compresses the discount a pre-IPO seller must accept.
Lock-up design now interacts with index rules. A listing with a small initial float and a staged lock-up release can still qualify for several indices, because the providers either waive the float minimum for mega-caps or scale the weight. The timing of lock-up expiries relative to index reviews becomes a deliberate choice.
Listing venue has a price. The Nasdaq-100 fast entry is the largest single source of early passive demand and is available only to Nasdaq-listed companies. That is a tangible argument in the venue decision for the largest candidates.

What it means for the secondary buyer

A pre-IPO position bought at a discount to an expected listing price is, in part, a bet on the index calendar. If the company is large enough to qualify, the first weeks of trading carry a floor of mechanical demand. If it is not, the position relies on fundamental buyers alone. Knowing which side of the thresholds a company sits on, and when its lock-ups lapse relative to the review dates, is now part of the diligence.

Our read

Passive money used to be a slow follower of listings. The rule changes have made it a fast, scheduled, price-insensitive buyer in the first month. For sellers of pre-IPO paper, that is a reason to hold closer to the listing. For buyers, it is a reason to check the size thresholds before the price. For founders, it is a reason to think about venue and float with the index calendar open on the desk.