Hard to borrow

A stock is hard to borrow when there are relatively few shares available for brokers to lend out to short sellers compared to the demand for shorting it. Every short sale requires locating and borrowing real shares first, and when a broker's inventory of lendable shares for a given stock runs low, that stock gets flagged as hard to borrow, and brokers place it on what is commonly called a hard-to-borrow list.

Being on this list has direct practical effects for anyone trying to short the stock. Borrow rates rise, sometimes sharply, since the fee to borrow scarce shares reflects how much demand there is relative to supply. Some brokers may also restrict or entirely block new short sales in a hard-to-borrow stock if they cannot locate enough shares to lend, regardless of what a client is willing to pay in borrow fees. A stock typically becomes hard to borrow because a large share of its available float is already tied up, whether through existing short positions, index fund holdings that do not lend shares, or a small total share count relative to trading interest.

Hard-to-borrow status often coincides with heavily shorted, high volatility stocks, and it feeds directly into the mechanics of a short squeeze. When a stock is both hard to borrow and moving sharply higher, existing short sellers face rising costs to stay in the position on top of mounting losses, which pushes some of them to buy to cover, adding further upward pressure on a stock that already had limited available shares to trade against.