A short seller owes shares, not dollars, and has to carry margin against them for as long as the position is open: FINRA Rule 4210(c) sets maintenance on a short stock position at $5.00 per share or 30 percent of current market value, whichever is greater, for a stock priced at $5 or above, and $2.50 per share or 100 percent of current market value, whichever is greater, below $5. The condition that changes the size of the obligation is the stock's price, which sets both the cost of buying the shares back and the margin required to keep waiting.

The loop is borrow, sell, buy back, return

Investor.gov's long and short page describes a short sale as a sale of stock the investor does not own, normally settled by delivering a security borrowed by or on behalf of the investor, with the position closed out by returning the borrowed security to the lender, typically after purchasing on the open market. The same page names the three places the stock comes from: the firm's own inventory, the margin account of another of the firm's customers, or another lender.

The obligation is denominated in shares. A cash loan is repaid with a fixed number of dollars; a stock loan is repaid with a fixed number of shares at whatever they cost on the day they are bought.

Rule 203 requires a locate before the sale

The SEC's Regulation SHO page sets out three rules that have governed short-sale mechanics since compliance began on 3 January 2005. Rule 200 requires every equity order to be marked long, short or short exempt. Rule 203 requires a broker-dealer, before executing a short sale, to have reasonable grounds to believe the security can be borrowed, documented before the transaction. Marking and locating both happen before the shares are sold, not after.

Example: 100 shares short at $40, covered at $50

Example with illustrative prices, since no public figure fixes a single account's trade. A short seller borrows 100 shares and sells them at $40.00, receiving $4,000. The stock rises to $50.00 and the position is closed by buying 100 shares for $5,000.

  • Proceeds from the sale: 100 × $40.00 = $4,000.
  • Cost to buy the shares back: 100 × $50.00 = $5,000.
  • Result before borrow costs, interest and commissions: −$1,000.

Run the same prices through FINRA Rule 4210(c). At $40.00 the maintenance requirement is the greater of $5.00 × 100 = $500 and 30 percent of $4,000 = $1,200, so $1,200. At $50.00 it is the greater of $500 and 30 percent of $5,000 = $1,500, so $1,500. The requirement rose $300 while the position lost $1,000, and both had to be funded from the same account equity.

The margin requirement moves against the position

That direction is the point. A long position's maintenance requirement, set by the same rule at 25 percent of the current market value of margin securities long in the account, shrinks as the position loses value. A short position's requirement grows, because 30 percent is charged on a market value that is rising. Losses and collateral demands arrive together, and the liquidation that follows a shortfall is covered in a margin call can end in liquidation.

A long stock position bought outright cannot lose more than its purchase price, because the share price stops at zero. A short position has no equivalent arithmetic ceiling, since the repurchase cost rises with the price. Borrowed money changes the size of every move in the same way, as leverage changes the size of a move sets out.

Rule 204 and threshold securities set the delivery clock

The Regulation SHO page states that under Rule 204, adopted in final form in 2009, a participant must close out a failure to deliver for a short sale transaction no later than the beginning of regular trading hours on the settlement day following the settlement date. It also defines a threshold security: an equity security with an aggregate fail to deliver position at a registered clearing agency for five consecutive settlement days, totaling 10,000 shares or more, and equal to at least 0.5 percent of the issuer's total shares outstanding. A fail to deliver in a threshold security that persists for 13 consecutive settlement days must be closed out immediately by purchasing shares.

The dividend is paid by the short seller

If the borrowed stock pays a dividend while the loan is open, the long and short page states that the short seller is responsible for paying that dividend to the person or firm making the loan. Interest on the stock loan and other fees apply on top.

Those charges are cash out while the share obligation is still open, so the sale proceeds are never a finished result. The order type used to buy the shares back sets the price of closing the loop, which is the mechanism in what a stock quote tells you.

Source references

Primary sources used for this guide. Follow the links for the original scope, definitions and conditions.