The IPO Opening Auction

Every other auction on an exchange sets a price for a stock that already has one — yesterday's close is a good starting guess for today's open. An IPO has no yesterday. The listing exchange has to run its opening auction with genuinely no reference price, relying entirely on the orders that show up and, in practice, on an underwriter or designated market maker who has spent weeks building a book of indicative demand before the bell ever rings.

How the price actually gets set

In the weeks before listing, the underwriter runs a roadshow and collects indications of interest from institutional investors — "I'd buy 500,000 shares somewhere between $18 and $22." This is not a binding order, just a signal used to gauge demand and set the IPO's offer price, typically the night before listing. On the morning of the listing itself, the exchange (or, in the US, the lead market maker) opens an order book and lets real, live orders accumulate for anywhere from a few minutes to several hours before the first trade prints, exactly like the mechanics in Why Markets Use Call Auctions: orders sit unmatched, an indicative price and imbalance are published and updated as new orders arrive, and the auction only uncrosses once the book looks stable enough to trade.

A first trade, walked through

Say underwriters priced an IPO's offer at $20 the night before — that's the price at which shares were allocated to institutional investors in the offering itself. At the open the next morning, the following orders build in the book:

Side	Price	Size
Buy	Market	2,000,000
Buy	$24 limit	1,500,000
Buy	$21 limit	3,000,000
Sell	$22 limit	4,000,000
Sell	$25 limit	1,000,000

The designated market maker watches this book build and looks for the price that clears the most shares — matching the logic of any call auction. Here, a print around $22 pairs the market buys and the $24 and $21 limit buys (6.5 million shares of demand at or above $22) against the 4 million shares offered at $22, leaving 2.5 million shares of buy-side imbalance unfilled at the open. The stock opens for trading at $22 — 10% above the $20 offer price — and the leftover buy interest simply becomes the first minutes of continuous trading, pushing the price up further until it finds a level where sellers are willing to show up.

That 10% jump is the "IPO pop": if underwriters had priced the offer at $22 instead of $20 the night before, the company would have raised more money for the same number of shares sold, which is exactly why IPO pricing is contentious — a large pop is good for the institutional investors who got allocated shares at the offer price, and arguably a cost to the company that left money on the table.

Why this differs from a normal reopening

A normal stock's opening auction (or reopening after a halt) has a last trade to reference, so the indicative price rarely strays far and imbalance is usually modest. An IPO's opening auction has no such anchor, so the indicative price can swing sharply as large institutional orders enter and leave the book in the minutes before the print — which is one reason exchanges give IPOs extra time and, in the US, hand the process to a single designated market maker responsible for judging when the book is ready to open, rather than opening purely mechanically at a fixed clock time.

Alternatives that skip this process entirely, like Dutch Auction IPOs and Direct Listings And The Reference Price, exist precisely because critics argue the traditional book-built opening leaves too much pricing power, and too much of the pop, in the underwriter's hands rather than the market's.