S-1 vs S-3 vs the Baby Shelf Rule, With the One-Third Math Worked Out

The short version

Before a US company can sell new shares to the public, it generally has to register them with the SEC. The two registration forms you meet most often are Form S-1 and Form S-3. The S-1 is the long, self-contained form anyone can use. The S-3 is a shorter form reserved for companies that have been reporting to the SEC on time, and it is the form that makes a shelf registration practical: register once, sell in pieces later.

Smaller companies get the S-3 shelf with a catch. If a company's public float is under $75 million, the baby shelf rule (General Instruction I.B.6 of Form S-3) limits how much it can sell off that shelf to one-third of its public float in any 12-month period. That cap is one of the most useful numbers in small-cap dilution research, because it puts a ceiling on how fast a company can raise money through its shelf.

Form S-1: the long form

The S-1 has no eligibility test. It is used for initial public offerings, for follow-on offerings by companies that do not qualify for an S-3, and very often for resale registrations, where the company registers shares that investors already bought in a private deal so those investors can sell them. That last use is common enough to have its own name, the S-1 resale.

Because it is self-contained, an S-1 has to restate the business, risk factors and financial statements, and it has to be updated when that information goes stale. It is a poor fit for selling small amounts over time. In particular, at-the-market offerings under Rule 415(a)(4) must be made on a form like the S-3, so an S-1 cannot host an ATM program for the company's own primary sales.

Form S-3: the short form and the shelf

A company can use Form S-3 for a primary offering if, among other things, it has been an SEC reporting company for at least 12 months and has filed all required reports on time during that period. The S-3 incorporates the company's 10-K, 10-Qs and 8-Ks by reference, so it does not need to be rewritten each quarter.

The practical effect is the shelf. A company files an S-3 registering, say, "up to $100 million" of common stock, preferred stock, warrants and debt. Once the SEC declares it effective, nothing has been sold. When the company actually wants money, it files a short 424B5 prospectus supplement describing that specific takedown and sells. A shelf is permission to dilute, not dilution itself.

How the baby shelf cap is calculated

The cap is built from three inputs:

1. Public float. The aggregate market value of common equity held by non-affiliates. Shares held by officers, directors and controlling holders are excluded. This is the non-affiliate float, which is not the same thing as shares outstanding or the float a trading screen shows.
2. The price used. The instruction lets the company value the float using a price from within 60 days before the sale. Companies commonly pick the highest qualifying price in that window, which maximises the cap.
3. Trailing usage. Everything the company sold under I.B.6 in the prior 12 months counts against the cap, including the sale being made.

A real example: reading the number off a filing

Most baby shelf prospectus supplements show the arithmetic. Planet Green Holdings Corp. filed a 424B5 on October 2, 2026 for an at-the-market program. It stated its public float was "approximately $67,619,766, based on 11,638,514 outstanding shares of common stock held by non-affiliates and a per share price of $5.81, the closing price of our common stock on August 11, 2026."

Check the math: 11,638,514 × $5.81 = $67,619,766. One-third of that is $22,539,922, which is exactly the figure the filing gave as its I.B.6 limit. The filing also stated the company had sold $0 of securities in reliance on I.B.6 during the prior 12 calendar months, so the full one-third was available, and the ATM was sized at "up to $22,539,922." The program size was set by the cap, not by how much money the company necessarily wanted.

A worked example: how the cap moves

Now a hypothetical, Company X, with round numbers.

All figures are hypothetical.

Nothing about the company's shelf changed between the two columns. The registration still says whatever it says. But a 25% drop in the share price cut remaining capacity from $8 million to $3 million, because the cap is recalculated from the current float while the trailing sales stay on the books until they roll out of the 12-month window.

The opposite can happen too. Under the instruction, if the public float rises to $75 million or more after the shelf is effective, the one-third limitation stops applying. If Company X's non-affiliate shares rose in value to $80 million, its shelf would no longer be capped this way.

What counts against the cap and what does not

• Counts: shares sold off the shelf in primary offerings under I.B.6, including ATM sales and registered direct offerings. For warrants and convertible securities sold off the shelf, the instruction measures them by the aggregate exercise or conversion price of the underlying shares.
• Does not count: resale registrations by selling shareholders, private placements that are not sold off the shelf, and shares issued on exercise of warrants that were themselves registered separately.

This is why small companies often pair a registered direct offering with warrants sold in a concurrent private placement: the unregistered warrants do not use shelf capacity.

How to check a company on Signal8

• The live SEC filings feed shows S-1, S-3 and 424B5 filings as they arrive, so you can open the prospectus supplement and find the public float paragraph.
• Each company page has a Dilution tab, for example AAPL's Dilution tab, where Signal8 organises shelf registrations and offering data it has extracted. Dilution analysis focuses on small-cap issuers, so coverage varies.
• The dilution screener lets you compare companies on dilution-related data across the market.

FAQ

What is the baby shelf rule?

It is General Instruction I.B.6 of Form S-3. It lets a company with a public float below $75 million use an S-3 shelf for primary offerings, provided it is listed on a national securities exchange, is not a shell company and has filed its reports on time. In exchange, it can sell no more than one-third of its public float through the shelf in any 12-month period, counting all I.B.6 sales in that period.

Why do baby shelf filings show such specific dollar amounts?

Because the offering size is usually the arithmetic output of the cap. The company multiplies its non-affiliate shares by a recent price, divides by three and subtracts trailing sales. The result is often an odd figure like $22,539,922 rather than a round number.

Can a company with a float under $75 million use an S-1 instead?

Yes. An S-1 has no float requirement, so a company over its baby shelf cap can register an offering on an S-1. The S-1 is longer, takes more time to prepare and review, and cannot host a primary at-the-market program, which is why companies that qualify usually prefer the S-3.

Is public float the same as shares outstanding?

No. Public float only counts shares held by non-affiliates, valued at a market price. Shares held by officers, directors and controlling holders are excluded. It is also different from the tradeable float on a quote screen, which may exclude restricted shares too.