Glossary term
Short Interest Ratio (SIR)
The short interest ratio, or days to cover, compares short interest with average daily trading volume to estimate how many trading days it could take short sellers to cover.
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What Is the Short Interest Ratio?
The short interest ratio, or days to cover, compares short interest with average daily trading volume to estimate how many trading days it could take short sellers to cover. It is often used to judge how crowded or difficult an exit could be for short sellers.
The ratio is not a countdown clock. It assumes average volume and does not know whether short sellers actually will cover, whether buyers will appear, or whether liquidity will change.
Key Takeaways
- The short interest ratio is often called days to cover.
- It compares shares sold short with average daily trading volume.
- A higher ratio can suggest more potential pressure if short sellers rush to exit.
- The ratio can be stale because short interest is reported periodically.
- It should be interpreted with liquidity, float, news, options activity, and fundamentals.
Short Interest Ratio Formula
A common version of the formula is:
If a company has 10 million shares sold short and trades 2 million shares per day on average, the short interest ratio is 5 days to cover. That means it would take about five average-volume trading days for all reported shorts to cover, assuming all volume were available for that purpose.
How to Interpret Days to Cover
Ratio | What it may suggest |
|---|---|
Lower days to cover | Short sellers may be able to exit more easily in normal volume |
Higher days to cover | Covering could be harder if many short sellers rush to buy |
Rising ratio | Short positioning may be growing faster than trading liquidity |
Why the Ratio Can Mislead
The short interest ratio can look precise, but it rests on old data and average volume. A major news event can change volume quickly. A stock's float can be constrained by insiders or long-term holders. Options activity can also change trading pressure in ways the ratio does not capture.
For that reason, a high SIR may indicate squeeze potential, but it is not proof that a short squeeze will happen.
The Bottom Line
The short interest ratio estimates how many average trading days it could take short sellers to cover. It is useful for understanding crowded short positioning, but it should not be treated as a standalone buy or sell signal.