How Short Selling Works: Borrowing, Fees, Unlimited Losses, and Squeezes

Short selling is a way to profit from a falling stock price: you borrow shares you don’t own, sell them now, and later buy the same number back to return to the lender. If the price has fallen, you buy back for less than you sold for and keep the difference. If it has risen, you lose, and because there is no ceiling on how high a price can go, there is no ceiling on the loss.

In short: a short sale reverses the usual order of a trade: sell first, buy later. It requires a margin account, a borrow of the shares (arranged by your broker), ongoing borrow fees, and enough collateral to satisfy margin rules as the price moves. The maximum gain is 100% (if the stock goes to zero), while the potential loss is unlimited. Short sellers also owe the lender any dividends paid while the position is open.

The mechanics, step by step

Open a margin account. Short selling is not allowed in a cash account. You need a margin account and approval from your broker.
The broker locates shares. Before accepting a short sale, a US broker must have borrowed the shares, arranged to borrow them, or have reasonable grounds to believe they can be borrowed and delivered on time. This is the locate requirement in Rule 203(b) of the SEC’s Regulation SHO.
You sell the borrowed shares. The sale proceeds go into your account but are held as collateral. You cannot withdraw them.
You pay to borrow. A borrow fee accrues daily, as an annual percentage of the value of the borrowed shares. You also owe the lender an amount equal to any dividend the company pays.
You close the position by buying the shares back (buying to cover) and returning them.

The lender is usually another customer of the broker (whose shares sit in a margin account) or an institution such as a pension fund or ETF lending out its holdings. The lender can recall the shares; if a replacement can’t be found, you may be forced to buy back at the current price. Lending revenue is one way brokers earn money, covered in how stock brokers make money.

Margin rules for short positions (US)

Short sales are governed by the same rules as borrowing to buy, explained in margin trading explained, with different percentages:

Initial margin (Regulation T): the account must hold 150% of the short sale’s value at the time of the trade. The sale proceeds supply 100%, and you deposit the other 50%.
Minimum equity (FINRA Rule 4210): at least $2,000 in the account to sell short.
Maintenance margin (FINRA Rule 4210): for stocks at $5 or more, equity must be at least the greater of $5 per share or 30% of the current market value of the short position. For stocks below $5, the greater of $2.50 per share or 100% of the market value.

Brokers can and often do set higher “house” requirements, especially for volatile or hard-to-borrow stocks.

A worked example

You short 100 shares of hypothetical Maple Widgets Inc. at $50.

Opening the trade

Sale proceeds: 100 x $50 = $5,000, held as collateral.
Regulation T requires 150% in the account: $7,500. You deposit $2,500 of your own money.
Credit balance in the account: $7,500.

Scenario 1: the price falls to $40 after three months

Buy to cover: 100 x $40 = $4,000.
Gross gain: $5,000 minus $4,000 = $1,000.
Borrow fee at an assumed 3% a year: $5,000 x 3% x (90 / 360) is about $37.50. (Fees are calculated on the current market value daily, so this is an approximation.)
Net gain: about $962, or roughly 38% on the $2,500 you deposited.

Scenario 2: the price rises

Your equity in a short position is the credit balance minus the current value of the shares you owe. Under FINRA’s 30% maintenance rule, you get a margin call when:

$7,500 minus (100 x price) is less than 30% of (100 x price)

Solving: $7,500 is less than 130 x price, so the call comes when the price rises above about $57.69.

At $57.69, the shares you owe are worth $5,769. Equity is $1,731, which is 30% of $5,769.
At $75, the shares you owe are worth $7,500. Your equity is zero. Your $2,500 is gone.
At $100, the shares you owe are worth $10,000. You are $2,500 in debt to the broker, beyond losing your deposit: a $5,000 loss on a $2,500 stake.

The broker will not wait for the last two points. Long before that, it will demand more money or buy shares to close your position at whatever the price is.

Why the loss is unlimited

Price	Value of shares owed	Profit or loss
$0	$0	+$5,000 (the maximum)
$25	$2,500	+$2,500
$50	$5,000	$0
$100	$10,000	minus $5,000
$200	$20,000	minus $15,000
$500	$50,000	minus $45,000

The best case is capped at the sale price. The worst case has no cap.

Borrow fees and hard-to-borrow stocks

Most large-company stocks are cheap to borrow, often well under 1% a year. When many people want to short a stock and few shares are available, the fee rises, sometimes to tens of percent a year or more, and brokers may label the stock hard to borrow. A high fee changes the arithmetic: at 50% a year, a $5,000 short costs about $208 a month just to keep open, so the price has to fall that much each month just to break even.

Short squeezes

A short squeeze happens when a rising price forces short sellers to buy back, and their buying pushes the price higher, forcing more of them to buy back. Margin calls, forced buy-ins, and fear all feed the loop.

The best-known modern example is GameStop in January 2021. The SEC staff’s October 2021 report on the episode found that GameStop’s short interest had reached about 123% of its public float in January 2021 (and around 109% of shares outstanding at the end of 2020), possible because the same shares can be lent, sold, and re-lent. The report also found that short sellers covering contributed to the price rise in some periods, but that their buying was a small fraction of overall buying; much of the surge came from other buyers. The full story is in what happened with GameStop.

Rules on short selling

United States

Locate (Regulation SHO Rule 203(b)): a borrow must be located before the short sale.
Close-out (Rule 204): if a short sale results in a failure to deliver at the clearing agency, the broker must close it out by buying or borrowing shares by the start of regular trading on the settlement day after the settlement date.
Short sale circuit breaker (Rule 201): when a stock falls 10% or more from the prior day’s close, short sales are restricted to prices above the current national best bid for the rest of that day and the next.
Marking: sell orders must be marked “long” or “short.”

Canada

In Canada, short selling is supervised by CIRO under the Universal Market Integrity Rules (UMIR). Amendments approved in December 2024 added an explicit requirement in UMIR 3.3 that, before entering an order that would be a short sale, the dealer must have a reasonable expectation to settle it on the settlement date. Canada does not have a direct equivalent of the US Rule 201 price test. Canadian margin requirements for short positions are set by CIRO; for most stocks at $2 or more, the account must hold a credit of 150% of the short position’s market value (130% for stocks on CIRO’s reduced-margin list), with higher requirements for lower-priced stocks.

Short interest data

Short interest is the total number of shares sold short and not yet covered.

US: under FINRA Rule 4560, firms report short positions twice a month (as of mid-month and month-end settlement dates), and FINRA publishes the data. In 2026 FINRA filed a proposal to move to weekly reporting; as of November 2026, check FINRA’s site for whether it has taken effect. A separate SEC rule requiring large short sellers to report positions monthly (Rule 13f-2 and Form SHO) has been exempted from compliance until January 2, 2028.
Canada: CIRO publishes a Consolidated Short Position Report twice a month, as of the 15th and the last day of each month.

Two common measures: short interest as a percentage of the float (shares available to trade), and days to cover (short interest divided by average daily volume), which estimates how long it would take short sellers to buy back.

What can go wrong

Unlimited loss. The central risk, shown in the table above.
Forced buy-ins. A recall by the lender, or a broker’s risk decision, can close your position at the worst moment.
Rising borrow costs. Fees can jump while you hold the position.
Dividends and corporate events. You owe any dividends. Special dividends, spin-offs, and buyouts at a premium can all cost a short seller.
Gaps. A takeover announcement can move a price 30% or more overnight, and a buy stop order (described in stock order types) won’t fill at your stop price in a gap.
Being right too early. A stock can stay overvalued for longer than a margin account can stay solvent.

The short version

Short selling means borrowing shares, selling them, and hoping to buy them back cheaper. It requires margin, a located borrow, and ongoing fees, and it flips the usual risk profile: gains are capped at 100% while losses have no limit. Margin calls, recalls, and squeezes can force you out precisely when the price is moving against you.