What Is a Short Squeeze? How Short Covering Can Push a Stock Higher

A short squeeze is a rise in a heavily shorted stock that pushes short sellers to buy shares back to cap their losses, and that buying adds to the rise itself.

The term is used often and loosely, for almost any sharp move in a stock with a large short position. This page covers the mechanics behind a squeeze, the measurements people read to judge how exposed a stock is, why borrowing costs matter, how a short squeeze differs from a gamma squeeze, what the SEC staff found when it examined GameStop in January 2021, and what public data can and cannot show while one is under way. It is descriptive throughout.

How a squeeze works

A short seller borrows shares, sells them, and owes the same number of shares back. The only way to close the position is to buy them, which is covered in what short selling is. That obligation is what makes a squeeze possible. A buyer who owns a stock can lose at most what was paid for it; a short seller's loss grows with every dollar the price rises, with no ceiling.

The SEC staff describe the chain in their report on early 2021 trading. If the price of a shorted stock jumps, short sellers face margin calls requiring them either to post additional collateral or to exit the position. Those who exit do it by buying the stock, which puts more upward pressure on the price, which can force other short sellers out, and so on. The staff note two further effects: while short sellers are out of the market, the price can keep rising unchecked by the people who think it is too high, and because a short seller can lose more than the capital put in, the risk can deter new short selling in the stock.

The same report gives the scale of the collateral involved: broker-dealers typically require a short seller to post at least 50% of the shorted position in a margin account, on top of the cash from the sale. How margin turns a price move into a forced sale or a forced purchase is covered in what leverage and margin are.

What makes a stock exposed to one

No single number identifies a squeeze in advance. People reading for exposure usually look at four measurements together, each describing a different part of the setup.

• Short interest as a percent of float. The share of the freely tradable stock that is sold short. For scale, the SEC staff found that short interest as a share of shares outstanding is often less than 2.5% for large non-financial stocks and still tends to be less than 13% for small ones; a percent of float is never lower for the same position, because the float is never larger than shares outstanding.
• Days to cover. The short position divided by average daily trading volume. A large figure means covering would take many days of normal volume, so buying to cover is a bigger share of the market for the stock.
• Float size. A small float means fewer shares to buy back from and fewer to borrow.
• The cost to borrow and utilisation. Short sellers pay a lending fee to borrow shares, and a high fee makes a position more expensive to hold the longer it stays open. Utilisation, the share of lendable stock already on loan, shows how close the supply of borrowable shares is to running out.

These describe how crowded a short position is and how hard it would be to unwind. They do not say whether anything will force the unwinding, and plenty of heavily shorted stocks never see a squeeze.

Why borrowing costs matter

The lending fee is the price of keeping a short position open, and it rises when shares are scarce relative to the demand to borrow them. The SEC staff report gives the GameStop figures: the cost to borrow GME was greater than 100% during the second quarter of 2020, had declined to under 50% by June 2020, and was around 25% in January 2021. For comparison, between January 2007 and July 2018 only 222 firms had lending fees above 100% at any point, and the average fee was 1.5%.

A high fee works on short sellers slowly, as a running cost, while a margin call works quickly. A lender can also ask for its shares back, and a short seller who cannot borrow them elsewhere has to buy. All three push in the same direction, toward closing the position.

Short squeeze versus gamma squeeze

A short squeeze is buying by short sellers closing losing positions. A gamma squeeze is buying by options dealers adjusting the hedges on call options they sold. Both are purchases that respond to a rising price rather than to new information about the company, and both can run in the same stock at the same time, which is why they are often confused.

They stop differently. A short seller who covers is done: the position is closed and that buying ends. An options hedger keeps adjusting for as long as the options stay open and the price keeps moving. The measurements are different too. Short interest describes the first mechanism. Open interest and modeled dealer gamma describe the second.

The GameStop case: what the SEC staff found

GameStop in January 2021 is the best-known case described as a short squeeze, and it is also the one most often called a gamma squeeze. The SEC staff examined it in its Staff Report on Equity and Options Market Structure Conditions in Early 2021, published on 14 October 2021.

The short position was extreme by any measure the staff used. GME's short interest hovered around 100% of shares outstanding from 2019 to early 2021 and reached 109.26% on 31 December 2020, and the report puts short interest as a percent of float at 122.97% in January 2021. Short interest can pass 100% because shares bought from a short seller can be lent and sold short again.

The staff found that short covering played a part. During some discrete periods, GME had sharp price increases at the same time as known major short sellers covered their positions after significant losses, and during those times covering likely contributed to the price increases. But the staff also found that such buying was a small fraction of overall buy volume, and that GME's price stayed high after the direct effects of covering would have waned. Its conclusion: "it was the positive sentiment, not the buying-to-cover, that sustained the weeks-long price appreciation of GameStop stock." Its summary was that a short squeeze did not appear to be the main driver of events, and a gamma squeeze less likely; on the options side it wrote that it "did not find evidence of a gamma squeeze," which the gamma squeeze page covers.

The staff worked from regulatory data, including the Consolidated Audit Trail, which identifies the accounts behind trades and is not public. That is the gap the next section is about.

What public data can and cannot show

Public data shows the conditions for a squeeze better than the squeeze itself.

• Short interest arrives late. FINRA collects it for two settlement dates a month and publishes it on the seventh business day after each, so covering during a fast move shows up only as a lower short interest figure a week or more later.
• Daily short volume is not covering. FINRA's daily short volume file counts sales marked short, part of them from market makers, and nothing in it marks a purchase as covering. It cannot show who is buying to cover.
• Who bought is not public. The SEC staff could identify accounts with large short positions in regulatory data. Public data cannot, so a price rise alongside falling short interest does not reveal whose buying drove it.

Kresmion's short interest page, part of the app and open with a free account, carries the inputs described above for each settlement: FINRA's short position, its change against the prior settlement, days to cover, a percent of float where one can be derived, and the price move since the settlement date. The percent of float is built from SEC filings and published as a lower bound. Kresmion does not score or forecast squeezes. Cost to borrow and utilisation are not shown, because Kresmion does not license that data. Its source line reads "Source: FINRA. Short interest data owned by FINRA."

Key takeaways

Frequently asked questions

What triggers a short squeeze?

A rising price in a stock with a large short position. As losses grow, short sellers face margin calls or decide to cap the loss, and both mean buying shares. What starts the initial rise varies: news about the company, a wave of buying from other investors, or a change in the cost of keeping short positions open.

What short interest is considered high?

There is no official line. The SEC staff found that large non-financial stocks often have short interest below 2.5% of shares outstanding and small ones tend to stay below 13%, and that few stocks exceed 50% on any date. GameStop's 109.26% of shares outstanding at the end of 2020 was far outside that range.

Can high short interest predict a short squeeze?

No. High short interest, many days to cover and a small float describe a crowded position. They do not say whether anything will force short sellers to buy, and many heavily shorted stocks never see a squeeze.

How do you know when short sellers are covering?

Only after the fact, and only roughly. A drop in short interest between two settlement dates shows that, on net, more short positions were closed than opened during that period, but the figure is published about a week and a half after the settlement date and does not say when within the period the covering happened.

Was GameStop a short squeeze?

Partly. The SEC staff found that short sellers covering likely contributed to some of the sharpest price increases, but that buying to cover was a small fraction of overall buy volume and that positive sentiment, not covering, sustained the weeks-long rise.

This page is information, not investment advice.

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Source: U.S. Securities and Exchange Commission, Staff Report on Equity and Options Market Structure Conditions in Early 2021 (14 October 2021), sections 3.2 and 3.4 with footnotes 61, 74, 75 and 84, https://www.sec.gov/files/staff-report-equity-options-market-struction-conditions-early-2021.pdf ; FINRA, About Equity Short Interest (reporting and publication schedule), https://www.finra.org/finra-data/browse-catalog/equity-short-interest ; Kresmion short interest page, inputs shown and float method. Source: FINRA. Short interest data owned by FINRA.

Kresmion Research.