# Days to cover

Also written short interest ratio.

*https://stockmarketstack.com/glossary/days-to-cover · next to Insider, 13F & Congressional Trade Trackers*

**Definition:** Short interest divided by average daily trading volume, expressed as a number of days. FINRA computes it from its own twice-monthly filing using the volume between the two settlement dates; vendors compute it over ten, thirty or ninety-day windows of their own choosing. The label is the same everywhere and the arithmetic is not, so two published figures for one stock rarely agree.

## How it works

Days to cover is a ratio, and FINRA defines it precisely for the file it publishes: the number of
days of average share volume it would take to buy all the shares sold short during the reporting
cycle. The formula is short interest divided by average daily share volume, rounded to hundredths.

The interesting part is the denominator, which FINRA also defines: total volume — or split-adjusted
volume where there has been a split — divided by the number of trading days from the day after the
previous settlement date through the current one. In other words, FINRA's average daily volume window
is the reporting cycle itself, roughly ten trading days, and it moves with the calendar.

Nobody else is obliged to use that window, and most vendors do not. [S3 Short Interest
Data](https://stockmarketstack.com/tools/s3-partners) publishes days to cover over ten, thirty and ninety-day windows, which is
the honest treatment: three numbers, each labelled. [ORTEX](https://stockmarketstack.com/tools/ortex) carries days to cover beside
its lending-derived estimate. [ChartExchange](https://stockmarketstack.com/tools/chartexchange) computes it against average daily
volume on top of the FINRA filing. Each is doing something defensible, and the results are not the
same number.

## Why two correct figures disagree

Take a stock with ten million shares reported short. Over the current FINRA reporting cycle it traded
two million shares a day, so FINRA's file prints 5.00. Over the last ninety days it averaged five
million a day, because the cycle just ended was unusually quiet, so a vendor using a ninety-day window
prints 2.00. Same filing, same stock, same day, and a figure two and a half times apart. Neither is
wrong. They are answers to different questions, and only one of them is labelled with its window.

Now compound that with the numerator. A reported short interest figure describes a settlement date
already a week or more in the past by the time it is published — the whole of
[short interest](https://stockmarketstack.com/glossary/short-interest)'s timing problem is inherited here, unchanged. A vendor's
daily estimate is not that number at all: it is modelled from securities-lending inventory, so the
ratio becomes shares on loan divided by a volume average. That is a third quantity with the same name
on the label.

Three smaller mechanisms make the same point:

**Split adjustment.** FINRA adjusts the volume series for splits explicitly. A pipeline that joins
raw historical volume to a short interest series produces a ratio that breaks at every split, in the
same way and for the same reason that any unadjusted series does — see
[corporate action](https://stockmarketstack.com/glossary/corporate-action).

**Window length changes what the ratio measures.** A short window tracks current liquidity and swings
violently around earnings and index events. A ninety-day window is stable and describes a market
condition that may have ended weeks ago. Neither is a better default; the choice is the answer.

**The denominator is not buying power.** Every share traded is counted, including the short sales
themselves and the market-maker turnover that never establishes a position. The ratio treats total
volume as if it were capacity available to close shorts, which is an upper bound and usually a loose
one.

## Why it matters here

The practical rule is short. Never compare a days-to-cover figure between two products in
[this category](https://stockmarketstack.com/categories/insider-13f-trackers), and never quote one without its window. If a vendor
does not publish the window and the source of the numerator, the number is not comparable to anything,
including its own history if the methodology changed.

If the figure matters, compute it. FINRA's short interest file is free and carries its own days to
cover with a documented window; a volume series you already have plus that file gets you a ratio whose
definition you know. [ChartExchange](https://stockmarketstack.com/tools/chartexchange) sells the filing with an API from $19.95 a
month if the download is not worth automating.

And for the question days to cover is usually a proxy for — how hard and expensive the short side of
this stock has become — the borrow market answers more directly and moves daily rather than twice a
month. [iBorrowDesk](https://stockmarketstack.com/tools/iborrowdesk) publishes one broker's borrow fee and availability for
nothing, and a fee moving from 2% to 40% has told you something no ratio over stale positions can.

## Where you will meet this

- [ChartExchange](https://stockmarketstack.com/tools/chartexchange.md)
- [ORTEX](https://stockmarketstack.com/tools/ortex.md)
- [S3 Short Interest Data](https://stockmarketstack.com/tools/s3-partners.md)
- [iBorrowDesk](https://stockmarketstack.com/tools/iborrowdesk.md)

## FAQ

### Can I compare two vendors' days to cover for the same stock?

Not usefully. The numerator may be a reported filing or a modelled estimate, and the denominator is an average over a window each vendor picks for itself — a reporting cycle, ten days, thirty, ninety. Both figures can be arithmetically correct and differ by a factor of two. Compare the inputs, or compute the ratio yourself from one volume series.

### Does a high days to cover mean a squeeze is coming?

It means the position is large relative to recent turnover, which is a statement about liquidity rather than a forecast. The ratio also assumes every share traded is available to close a short, which is not how volume works, and its numerator describes a settlement date already a week or more in the past. Borrow cost and availability move faster and say more.

## Sources

1. [Equity Short Interest Data Glossary](https://www.finra.org/finra-data/browse-catalog/equity-short-interest/glossary) — Financial Industry Regulatory Authority, read 2026-09-21
2. [Equity Short Interest Data](https://www.finra.org/finra-data/browse-catalog/equity-short-interest) — Financial Industry Regulatory Authority, read 2026-09-21
3. [Short Interest Reporting](https://www.finra.org/filing-reporting/regulatory-filing-systems/short-interest) — Financial Industry Regulatory Authority, read 2026-09-21

*Last updated 2026-09-21. A reference page, corrected in place — not a dated post.*