What is Short Interest? Definition, Formula, and Example
Short interest is the total number of shares of a stock that have been sold short but not yet covered, reported as a raw count and as a percentage of public float.
Short Interest: Plain-English Definition
Short interest is the total number of shares of a stock that have been sold short and not yet bought back to close the position. It is reported both as a raw share count and as a percentage of the stock's public float. Short interest is a snapshot of bearish positioning — a count of open bets that the stock will decline — and is one of the core data points used to identify short squeeze candidates, gauge investor sentiment, and assess the cost and availability of borrow.
How It Is Calculated
FINRA requires all broker-dealers to report short positions held in customer and proprietary accounts twice per month — as of the 15th and the last business day of each month. FINRA and the exchanges aggregate these reports and publish consolidated figures on T+8 business days.
Key ratios derived from the raw number:
Short % of float = Short Interest / Public Float × 100
Short interest ratio = Short Interest / Average Daily Volume (also called "days to cover")
Short % of shares outstanding = Short Interest / Shares Outstanding × 100Days to cover estimates how many trading days of average volume it would take for shorts to fully buy back their positions — a proxy for squeeze vulnerability.
Worked Example
GME reported short interest of roughly 61.8 million shares in mid-January 2021 against a public float of approximately 46.9 million — ~140% of float short. This was possible due to rehypothecation and re-borrowing of the same shares, producing the most extreme short interest ratio on a major U.S. equity in recent history. Average daily volume at the time was roughly 10 million shares, implying days to cover above 6. The subsequent squeeze drove GME from $20 to $483 in two weeks. For comparison, large-cap names like AAPL consistently run at 0.6–1.2% of float short, TSLA at 2–4%, and AMC peaked near 20% during the 2021 meme-stock cycle.
When Traders Use Short Interest
Short squeeze hunters screen for stocks with short interest above 20% of float combined with positive catalysts — earnings beats, regulatory approvals, activist filings, or rising retail options flow. Fundamental investors use aggregate short interest as a contrarian sentiment gauge — extreme bearishness on the overall market has historically marked bottoms. Risk managers watch short interest on holdings to anticipate forced-buy-in risk when shares recall. Borrow-fee-focused traders use short interest alongside cost-to-borrow data to identify crowded shorts nearing capacity.
Limitations and Common Misconceptions
Short interest data is stale — the ~2-week settlement lag means positioning reflects the market from mid-month ago, not the current tape. Synthetic short exposure via options, total return swaps, and CFDs does not show up in the FINRA number, meaning true bearish positioning is understated. Conversely, rehypothecation can push reported short interest above 100% of float without representing 100% directional conviction. High short interest alone does not cause a squeeze — a catalyst and trapped dealer gamma usually drive the actual move. Short interest can also mark a stock as legitimately troubled; high shorts on companies like bankrupt retailers or fraudulent microcaps reflect informed bearish positioning rather than squeeze opportunity. Finally, short interest published by bulk providers like Bloomberg, S3, and Ortex can differ because S3 and Ortex build proprietary estimates using lending data rather than waiting for the official FINRA release.