An at-the-market offering is a company quietly selling brand-new shares straight into the open market, at whatever the stock trades for, through a broker acting as its sales agent. There is no big overnight deal, no fixed price and no roadshow. The company files a prospectus supplement, signs a sales agreement, and then drips stock into the order book for weeks or months. Traders call it "the ATM," and it is one of the most common reasons a small or mid-cap stock keeps hitting supply on every rally. The good news: it leaves a paper trail, and the dilution is easy to measure once you know which filing holds which number.
What an ATM is, in the SEC's own words
The legal definition sits in Securities Act Rule 415(a)(4). The rule text defines an "at the market offering" as "an offering of equity securities into an existing trading market for outstanding shares of the same class at other than a fixed price."
The same paragraph says a registrant's own ATM of equity "must come within paragraph (a)(1)(x)," which covers securities registered on Form S-3 (or F-3) and sold "on an immediate, continuous or delayed basis" by or on behalf of the registrant. Translation: no effective shelf registration, no ATM.
So an ATM has three moving parts:
- The shelf. An effective S-3 (or an automatic S-3ASR for larger issuers) that registers a dollar amount of securities the company may sell later.
- The sales agreement. Called a sales agreement, an equity distribution agreement or an "ATM Equity Offering Sales Agreement." It names the agent banks and the commission, usually a few percent of gross proceeds.
- The 424B5. A prospectus supplement filed under Rule 424(b)(5) that turns on a specific program: "up to $X" of common stock, sold "from time to time" through the named agents.
The paper trail: where an ATM shows up
- S-3 or S-3ASR (capacity). This registers the shelf. It sets the ceiling, not the sales.
- 8-K, Item 1.01 (the agreement). Applied Optoelectronics ($AAOI) filed one on February 26, 2026, reporting it "entered into an Equity Distribution Agreement" with Raymond James and Needham.
- 424B5 (program size, agents, commission). Search the document for "at the market," "sales agreement" and "commission."
- 10-Q and 10-K (what was actually sold). Look in the equity footnote or the liquidity section for shares sold, average price, gross and net proceeds, and capacity remaining.
- Cover page (the running total). Every 10-Q and 10-K states shares outstanding as of a recent date. Compare consecutive cover pages.
The 424B5 tells you what the company can sell. The 10-Q tells you what it did sell. Read both. For a refresher on why the cover-page number is not the same as the tradable float, see Float vs Outstanding Shares.
Three 2026 ATMs, straight from the filings
Here are three real US-listed programs from 2026: a fast-moving tech stock, a regulated water utility and a small-cap IoT company. Every figure comes from the company's own 424B5, 10-Q or 10-K.
| Company | Program (424B5) | Agents | Commission | Reported sales |
|---|---|---|---|---|
| Applied Optoelectronics ($AAOI) | $250M on Feb 26, 2026, raised to $500M on Mar 12; a second $600M program on May 14 | Raymond James, Needham | Up to 2.0% | 7,775,523 shares, March to June 2026; $1,049.8M gross, $21.0M agent compensation, $1,028.8M net (Q2 10-Q) |
| Middlesex Water ($MSEX) | Up to $110M (agreement May 2025; agent change Feb 20, 2026) | BofA Securities, Robert W. Baird, Huntington Securities | Up to 2% | 518,564 shares in the first nine months of 2026 at a $55.90 weighted average; $28.6M net; $51.0M left at Sept 30, 2026 (Q3 10-Q) |
| Lantronix ($LTRX) | Up to $30M (May 8, 2026) | Needham, Canaccord Genuity | 3% | About 1.8M shares by June 30, 2026; $13.03M gross, $390,000 commissions, about $12.34M net; about $17M left (FY2026 10-K) |
AAOI is the clearest picture of how fast an ATM can move. The March 12, 2026 424B5 amendment says that by that date the company had already sold 2,476,307 shares for about $249,999,983, which used up the original $250M in about two weeks. The stock closed at $53.69 on February 26 and at $127.01 on March 11, and the company doubled the program the next day. Its Q2 10-Q breaks out monthly sales at weighted average prices of $104.03 in March, $101.72 in April, $181.24 in May and $197.26 in June.
Middlesex is the opposite tempo: a utility funding capital spending a slice at a time: 518,564 shares over nine months, against roughly 18.5 million outstanding.
The dilution math
Two formulas do most of the work:
- Share dilution = new ATM shares sold ÷ shares outstanding before the program.
- Program size vs. market cap = program dollars (or proceeds) ÷ (shares outstanding × share price on the launch date).
| Company | Shares before (source date) | ATM shares sold | Share dilution | Program ÷ market cap (price date) |
|---|---|---|---|---|
| $AAOI | 75,198,817 (Feb 25, 2026) | 7,775,523 (through June 30) | 10.3% | $250M ÷ about $4.04B at the $53.69 Feb 26 close = 6.2% |
| $MSEX | 18,521,000 (Dec 31, 2025, rounded to thousands) | 518,564 (nine months to Sept 30) | 2.8% | $110M ÷ about $1.01B at the $54.47 Feb 19, 2026 price in the 424B5 supplement = 10.9% |
| $LTRX | 39,816,930 (Mar 31, 2026) | about 1.8M (through June 30) | about 4.5% | $30M ÷ about $232M at the $5.83 May 7 close = 12.9% |
The market-cap ratio is a ceiling. The share ratio is what actually happened. They diverge because price drives everything in an ATM. A fixed dollar target means fewer new shares when the stock is high and more when it is low. At AAOI's $53.69 February 26 close, $1M of stock is about 18,600 shares. At June's $197.26 average, the same $1M is about 5,070 shares. That is exactly why issuers tend to run their ATMs hardest into strength.
Cross-check with the cover pages. AAOI went from 75,198,817 shares (February 25, 2026, per the 424B5) to 84,569,237 (August 3, 2026, Q2 10-Q cover). That is 9,370,420 more shares, of which the ATM accounts for 7,775,523; the rest came from other issuance. Lantronix shows why the cover count matters even more: its 10-K reports an underwritten offering of about 4.8M shares on May 29, 2026, alongside the ATM, and its cover count rose to 46,705,312 by August 20, 2026.
One more data point: an ATM's sale price is not a floor. AAOI closed at $59.27 at the November 4, 2026 close, well below its May and June ATM averages.
The common mistake: confusing shelf capacity with shares sold
A shelf is a permission slip. A 424B5 is a loaded program. Only a 10-Q, 10-K or a company announcement tells you how much of it was used. Traders routinely read a "$250 million shelf" headline as $250 million of stock about to hit the tape. That is not what the filing says.
Powerus ($PUSA) is a live example. Its October 19, 2026 S-3 registers up to $250,000,000 of securities plus a 7,409,209-share resale by selling stockholders, and an SEC EFFECT notice shows the S-3 effective October 28, 2026. The plan-of-distribution section says the company "may engage in at the market offerings" under Rule 415(a)(4), which is standard shelf boilerplate. Through November 4, 2026, its EDGAR filing index showed no 424B5 and no sales agreement. That is capacity, not an ATM. For the merger backstory, see PUSA stock after the merger.
The reverse mistake also happens. AAOI's March 12 424B5 illustrates dilution "assuming" 3,936,698 shares sold at $127.01. That is the filing's illustration, not a sales report. The actual number came later, in the 10-Q.
Watch, too, for share counts that hide outside the ATM, such as warrants exercisable for a fraction of a cent. Our pre-funded warrants explainer covers those.
ATM checklist for traders
- Is there an effective S-3 or S-3ASR? Check EDGAR for the registration and its EFFECT notice.
- Is there a 424B5 using "at the market" language? Note the program size, date, agents and commission.
- Was there an 8-K Item 1.01 for the sales agreement, or for an amendment raising the size?
- In the latest 10-Q or 10-K, how many shares were sold, at what average price, and how much capacity is left?
- Compute shares sold ÷ prior shares outstanding, and program dollars ÷ market cap on the launch date.
- Compare consecutive cover-page share counts to catch all issuance, not just the ATM.
More explainers live in the Analysis section of I'M A STOCK TRADER.
Bull/Bear Verdict
Bull Case: The ATM is one of the few dilution events a company has to report after the fact in a standard table. A trader who reads the 424B5 and then the next 10-Q knows the program size, the agent's cut, how much was sold and how much capacity is left. AAOI's 2026 filings show the math cleanly: about 10% more shares, sold mostly into a rising price.
Bear Case: The 10-Q lags. AAOI's March sales showed up in a 10-Q filed May 7, 2026, so most live supply is invisible while it happens unless the company files an interim update, as AAOI's March 12 amendment did. A shelf headline can also trigger a selloff over capacity that is never used, while a quiet, steady drip like Middlesex's barely registers. Neither the headline nor the silence tells you the size of the supply.
FAQ
Is an at-the-market offering the same as a secondary offering?
No. In an underwritten offering, the shares are priced and sold in one transaction. An ATM sells small amounts into the existing market at prevailing prices over time. Lantronix did both in 2026: a $30M ATM agreement dated May 8 and an underwritten sale of about 4.8M shares on May 29.
Where do I find how many shares a company sold under its ATM?
In the next 10-Q or 10-K, usually in the stockholders' equity note or the liquidity section. Middlesex's Q3 2026 10-Q reports 253,537 shares sold in the quarter and 518,564 year to date, with $51.0 million remaining.
Can any company run an ATM?
Rule 415(a)(4) requires that an issuer's own ATM of equity fall within paragraph (a)(1)(x), which covers securities registered on Form S-3 or F-3. A company without an effective shelf cannot run one.
What does the sales agent earn?
It's set in the sales agreement and repeated in the 424B5. Both AAOI and Middlesex disclosed up to 2% of gross sales price, and Lantronix disclosed 3% of gross proceeds.