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Short answer
Days sales of inventory (DSI) is the average number of days a business takes to turn its inventory into sales. It is commonly written as average inventory divided by cost of goods sold (COGS), multiplied by the number of days in the period. A lower number means stock moves faster, but companies define the inputs differently, and for laboratories and clinics the number only means something when read next to expiry dates.
Definition and scope
DSI is an accounting ratio built from two figures on a company’s financial statements: inventory and cost of goods sold. It describes how many days of cost-of-sales the inventory on hand represents. The same idea goes by several names: days inventory outstanding (DIO), days of inventory on hand, days in inventory, and days on hand.
It is a seller’s metric. It belongs to distributors, manufacturers and retailers that carry inventory as an asset and report COGS. A research lab or clinic that buys consumables for its own use has no sales, so it cannot calculate DSI in the strict sense. What such a site usually tracks is a related but different quantity, days of supply: stock on hand divided by average daily usage. The two are easy to confuse, and the distinction is covered under common misunderstandings below.
How DSI is calculated in practice
The commonly written formula
Most explainers give the calculation in three steps:
- Average inventory = (beginning inventory + ending inventory) / 2.
- Inventory turnover = COGS / average inventory.
- DSI = days in the period / inventory turnover (365 for a year).
Worked example, with illustrative numbers rather than any real company: a distributor reports annual COGS of $120,000 and an average inventory of $10,000. Turnover is 120,000 / 10,000 = 12. DSI is 365 / 12, about 30.4 days.
How one real filer defines it
The formula above is a convention, not a single legal standard. Public companies state their own definitions. PSS World Medical, a distributor of medical supplies, explained in its fiscal 2012 Form 10-K filed with the U.S. Securities and Exchange Commission that it measures “days on hand” (DOH):
- “Days on hand (‘DOH’) is average inventory divided by average daily cost of goods sold (‘COGS’).”
- “Average inventory is the sum of inventory at the beginning and end of the most recent four quarters divided by five.”
- “Average daily COGS is COGS for the most recent four quarters divided by 360.”
- “Inventory turnover is 360 divided by DOH.”
The filing reported fiscal 2012 days on hand of 53.7 for its physician business and 66.8 for its extended care business. These figures describe one company in one year and are shown only to illustrate its own definition; they are not a benchmark for any other business.
Related concepts and how the definitions differ
| Item | Textbook version | PSS World Medical 10-K version |
|---|---|---|
| Name used | Days sales of inventory (also DIO) | Days on hand (DOH) |
| Inventory figure | Beginning plus ending inventory, divided by 2 | Beginning and end of the last four quarters, divided by 5 |
| Days in the year | 365 | 360 |
| Turnover | COGS / average inventory | 360 / DOH |
Because the averaging method and the day count both change the result, two published “DSI” figures can differ for the same business without either being wrong. When comparing companies, compare the definitions first. For background on how supply organizations talk about turnover, see hospital supply chain.
Where it matters for labs, clinics and supply decisions
Three points from the filing and the regulation cited in the sources below change how the number should be read in a laboratory or clinical setting.
Expiry limits what stock can be used
Under the U.S. CLIA regulations, 42 CFR 493.1252(d), reagents, solutions, culture media, control materials, calibration materials and other supplies must not be used when they have exceeded their expiration date. Stock that sits long enough to outlast its expiry cannot be used at all, so a high number of days on hand is not just money tied up in inventory; for dated items it can mean material that has to be discarded.
Distributors reserve for slow-moving stock
PSS reported that it adjusts for excess or slow-moving inventory on the expectation that certain items will become obsolete, with adjustments between 1.3% and 2.1% of gross inventory across its two businesses over fiscal 2009 to 2011. It also valued inventory at the lower of cost, determined on a first-in, first-out (FIFO) basis, or net realizable value. A reported days figure therefore sits on top of accounting choices about valuation and write-downs.
A checklist to read beside DSI
- Which definition is in use: the day count (360 or 365) and the averaging method.
- Expiry dates of dated items against how long the stock is expected to last (42 CFR 493.1252(d)).
- The inventory valuation method, such as FIFO, and any reserve for slow-moving or obsolete stock.
- Whether the figure is a seller’s DSI (based on COGS) or a user’s days of supply (based on usage).
For related supply-side topics see vendor-managed inventory, the comparison of consignment and vendor-managed inventory, central stores, par level inventory and medical laboratory supply wholesale pricing.
Common misunderstandings
“Lower DSI is always better”
A low figure means inventory is converting to sales quickly, but it can also reflect very lean stock. The ratio says nothing on its own about whether the right items are on the shelf, so it is read together with other measures of availability.
“DSI and days of supply are the same thing”
DSI is derived from COGS and describes how fast a seller turns inventory into sales. Days of supply divides stock on hand by usage and describes how long a user’s stock will last. The first needs financial statements; the second needs consumption records.
“There is one standard formula”
There is not. The PSS filing above uses a five-point average and a 360-day year, while textbook versions use a two-point average and 365 days.
“A DSI figure is comparable across industries”
The figure varies with industry and product mix, so a number from one business is not a target for another.
FAQ
What does days sales of inventory measure?
The average number of days it takes a company to sell its inventory, based on inventory held and cost of goods sold.
Is DSI the same as days inventory outstanding?
Yes. DIO, days of inventory on hand, days in inventory and days on hand are used for the same idea, although filers set their own formulas.
How is DSI related to inventory turnover?
DSI is the number of days in the period divided by inventory turnover, so a higher turnover gives a lower DSI.
Can a laboratory calculate DSI?
A lab that only consumes supplies has no sales or COGS in the usual sense. It can calculate days of supply from stock and usage instead.
Sources
- PSS World Medical, Inc., Form 10-K for fiscal 2012, U.S. Securities and Exchange Commission EDGAR filing: sec.gov/Archives/edgar/data/0000920527/000119312512249598/d337497d10k.htm (management’s discussion and analysis; checked 2026-10-29).
- 42 CFR 493.1252(d), Clinical Laboratory Improvement Amendments, Electronic Code of Federal Regulations: ecfr.gov/current/title-42/part-493/section-493.1252 (checked 2026-10-29).
