A company files an IPO with a set of shares in mind. Then something happens that every issuer hopes for, and few are fully ready for. Investors want more than what is on offer. 

This is not rare. A hot sector, a recognizable brand, or simple scarcity can push demand well past the planned deal size. When that happens, the offering does not just move ahead as filed. Underwriters and the issuer have a short list of tools to handle it, and the choice between them depends on timing, size, and how much room is left in the existing paperwork. 

This post walks through what actually happens behind the scenes when an IPO order book fills up faster than expected. 

Why IPO Demand Sometimes Runs Hotter Than Expected 

A few conditions tend to show up together when an IPO ends up being oversubscribed. 

A recognizable brand draws retail and institutional interest at the same time. 

Comparable companies in the same sector recently listed and performed well. 

The deal is priced conservatively, leaving room for upside that investors want to capture early. 

A limited float means fewer shares are available relative to the number of interested buyers. 

None of these guaranteed oversubscription on their own. Together, they explain why some IPOs end up many times oversubscribed while others barely clear their minimum. 

How Underwriters Gauge Demand Before Pricing Day 

Book Building 

Underwriters do not guess at demand. During the roadshow, they collect indications of interest from institutional investors through a process called book building. Each investor states how many shares they want and at what price is within the proposed range. 

If the order book fills up fast, especially near the top of that range, it is usually the first sign that demand is running ahead of supply. 

Pricing Above the Range 

When the book is oversubscribed by a wide margin, underwriters sometimes price the deal above the original range. This captures some of the excess demand without changing the number of shares on offer. It is the simplest response, but it has a limit. It does not help if investors want more shares, not just a higher price. 

The Underwriters' First Tool: The Over-Allotment Option 

This is where the greenshoe, formally called the over-allotment option, comes in. It lets underwriters sell up to 15 percent more shares than the original offering size, then decide within 30 days whether to buy those shares from the issuer at the original offer price. The clause is named after the Green Shoe Manufacturing Company, the first issuer to include it in an underwriting agreement, according to Wikipedia. 

It is the only price stabilization method the SEC permits after pricing. It also absorbs a meaningful chunk of excess demand without touching the registration statement, since the extra shares are already accounted for in the original filing. 

Snap Inc is a clear real-world example. When the company priced its 2017 IPO at $17 per share, demand was strong enough that underwriters fully exercised their option to buy an additional 30,000,000 shares of Class A common stock, according to Snap's own investor relations announcement. 

When the Greenshoe Is Not Enough 

The greenshoe only works inside the boundaries of the existing registration statement. If the issuer needs more than that 15 percent buffer, or if the original filing does not leave enough room, the company needs a separate filing to register the additional securities. 

Under normal conditions, registering new securities with the SEC takes time. Every offer of securities in the US must be registered or qualify for an exemption, and that process usually involves SEC review before the registration becomes effective, as outlined on the SEC's own investor education site. That kind of delay is the last thing an issuer wants in the middle of a hot deal. 

This is where filing what is known as a SEC MEF filing becomes useful. It is a short registration filing made under Rule 462(b) that becomes effective immediately upon filing, provided the issuer meets a specific set of conditions. Instead of repeating the company's full disclosure, it incorporates the earlier registration statement by reference and adds more securities to the same offering. 

The 20 Percent Boundary 

This route is not unlimited either. The added securities, combined with their price, cannot exceed 20 percent of the maximum aggregate offering price listed in the earlier filing. It is built for a quick top up, not a full redesign of the deal. 

What This Means for Investors 

Heavy oversubscription tells you something, but not everything. A few questions are worth asking before reading too much into it. 

Has the company increased the deal size, and through which mechanism? 

Does the increase change dilution for existing shareholders? 

Is the demand mostly institutional, or is retail interest driving it too? 

What does the updated prospectus say about the use of proceeds? 

Investors who want to track this directly can pull the underlying filings from EDGAR or use a structured data source like Quantillium, which organizes SEC filing data for faster review. 

Strong demand is a signal of investor confidence at that moment. It says nothing about how the stock performs six months later. 

Bottom Line 

When IPO demand runs past expectations, the response usually follows a sequence. Underwriters price within or above the range first. If that is not enough, the over-allotment option absorbs more demand inside the original filing. If the issuer still needs more room, a fast registration filing fills the gap without restarting the entire process.