How Short Selling Works

Buying a stock has a floor: the price can fall to zero and you lose your investment, but no further. Selling a stock short has no ceiling: you borrow shares you don't own, sell them at today's price, and must eventually buy them back to return them — and a stock can, in principle, keep rising forever. That asymmetry is the first thing to understand about shorting, before any of the mechanics.

The three steps

1. Locate and borrow. Your broker finds shares to borrow, usually from another client's margin account or an institutional lender, and posts you as a temporary borrower. You cannot sell short without a confirmed locate — selling shares you have no ability to deliver is a separate, prohibited practice called a "naked" short.
2. Sell. The borrowed shares are sold in the market at the current price, and the cash proceeds sit in your account, typically held partly as collateral against the position rather than paid out freely.
3. Buy to cover. At some point you buy the same number of shares back in the market and return them to the lender. If the buy-back price is lower than the sale price, the difference (minus borrow fees) is your profit; if higher, it's your loss.

Worked example

You short 1,000 shares at $50, for proceeds of $50,000. The borrow fee is 2% annualized, and you hold the position for one month (1/12 of a year). The stock falls to $44 and you cover.

• Gross gain: $(50 - 44) \times 1{,}000 = 6{,}000$, i.e. $6,000.
• Borrow cost: $50{,}000 \times 0.02 \times (1/12) \approx 83$, i.e. about $83.
• Net gain: roughly $5,917 before commissions.

Now suppose instead the stock rises to $65. Your loss is $(65 - 50) \times 1{,}000 = 15{,}000$, i.e. $15,000 — already three times the maximum you'd have made on the same size move in your favor, and there's no floor stopping the stock from rising further.

What this means in practice

Because losses are theoretically unlimited, brokers require margin on short positions and will issue a margin call — or forcibly close the position — if it moves against you enough to threaten the collateral. Short positions are also vulnerable to a short squeeze: if many shorts try to cover at once, the buying pressure itself pushes the price up, forcing even more covering. And the position can be recalled by the lender at any time, forcing you to either find another lender or close out, regardless of whether you wanted to.