Short Sale Calculator

Shorting a stock means borrowing shares and selling them now, hoping to buy them back cheaper later and pocket the difference. Unlike a long position, a short comes with ongoing costs (the stock borrow fee, charged for every day you hold the position) and is measured against margin required, not the full position value, since you never actually paid for the shares.

This calculator puts all of that together: gross profit, borrow costs, commission, and the resulting return on the margin you actually had to post.

How does this calculator work?

Enter the number of shares, your entry (short) price, and your exit (cover) price.

Enter your broker's margin requirement (commonly 50% under Reg T initial margin rules), the annualized stock borrow fee, how many days you held the position, and any commission.

The calculator computes gross profit, the borrow cost accrued over the holding period, net profit after costs, and your return on the margin actually required, both for the holding period and annualized.

Measuring return against the margin posted rather than the full position value is what magnifies both gains and losses, which is why a short is best entered with a deliberate risk budget. Traders who want defined downside instead often express the same bearish view with put options, whose fixed cost caps the loss a short leaves open. Comparing the result against simply holding for total return keeps the bet honest, and it always helps to weigh the probability the move actually happens.

Worked example

Short 100 shares at $50, cover at $40, with 50% margin, a 3% annual borrow fee, held 60 days, $10 commission.

Gross profit

$1,000

Borrow cost

$24.66

Net profit

$965.34

Margin required

$2,500

Return on margin

38.61% (234.9% annualized)

How the numbers work

The $10 price drop on 100 shares is a $1,000 gross gain, but shorting isn't free: the $5,000 position accrues a 3% annual borrow fee for 60 days, about $24.66, plus $10 in commission, leaving $965.34 net.

Because you only had to post $2,500 in margin (50% of the $5,000 position), not the full $5,000, that $965.34 profit represents a 38.61% return on the capital actually at risk, a much bigger number than the raw 20% price move might suggest.

Measuring a short's return against margin required, not the full position value, is what makes shorting capital-efficient, but it cuts both ways: losses are also magnified relative to the margin posted, and an adverse move can trigger a margin call well before the position would be a total loss.