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UOM (unit of measure) is the unit in which an item is counted on a purchase line, such as each, box or case. On a supply invoice, the UOM and the quantity per UOM together decide what one price actually buys. Unit of issue and purchase unit of measure are procurement terms for the issuing and buying units of the same item. This guide is general information, not medical advice.
Short answer
A unit of measure tells you how many individual items one ordering line stands for. “1 CS” and “1 EA” can be the same product at very different quantities. Always read the UOM together with the quantity per UOM, and convert prices to one common unit before comparing them.
Definition and scope
In purchasing, inventory and invoicing systems, the unit of measure is the label attached to a quantity: each, box, case, pack, pallet. The same item can exist at several levels, and each level has its own UOM. A buyer’s system usually stores one base unit and a conversion factor for every larger unit.
Two terms you will see beside it:
- Purchase unit of measure: the unit in which the item is ordered from the supplier, for example a case.
- Unit of issue: the unit in which stock is handed out from the storeroom to a requesting department, for example a single box or each.
These two are common procurement and inventory-system usage rather than regulatory definitions, and organisations set them differently. That is why the same product can be bought by the case and issued by the box.
How UOM is used on a purchase line
A purchase line normally carries four things that must agree: the item, the UOM, the quantity per UOM, and the price per UOM. When any one of them is unclear, a “same” product can look cheaper or dearer than it is. Our guide to medical and laboratory supply wholesale pricing covers the each-versus-box-versus-case ordering traps; the PO number guide shows where the unit of measure appears on an invoice, and the three-way match guide explains why order, receipt and invoice quantities must use the same unit.
The identifier on the package follows the same logic. UNICEF Supply Division’s packaging and labelling guidance (P. Adler, 1 November 2023) states that the GTIN used on UNICEF logistics labels must be the same unit of measure as the UNICEF material number, typically the sales unit. Its example separates a pack of 10 blister packs of 10 tablets from a single blister pack of 10 tablets: different levels, different units. The labelling requirement applies to cases and pallets and was enforced for purchase orders placed after 2 October 2023.
Unit of issue vs unit of measure
Short answer: unit of measure is the general label for any counting unit; unit of issue is the one specific unit chosen for issuing stock to users. Every unit of issue is expressed in a unit of measure, but not every unit of measure is a unit of issue.
Healthcare adds a further unit that catches buyers out: the unit of use. The AHRMM talk track “LUC UOU Explained” quotes FDA’s definition: a virtual identifier assigned to an individual medical device when a Unique Device Identifier (UDI) is not labelled on the individual device at the level of its unit of use, so that use of a device can be associated with a patient when a base package contains more than one device. AHRMM’s example is a lowest package level that is a sealed tray holding twenty-five individual syringes; the individual syringes get an unmarked unit-of-use identifier. AHRMM stresses that unit of use is not the same as the lowest packaged level, because in most supply chains product cannot be distributed in any form lower than the lowest package level. The related UDI and production identifier guide explains the identifier itself.
FDA’s rule in 21 CFR 830.3 defines a device package as a package containing a fixed quantity of a particular version or model of a device. That is the regulatory anchor for why a package, not a loose item, is what carries a device identifier.
Comparison of the units
| Unit | What it describes | Basis |
|---|---|---|
| Purchase unit of measure | The unit the item is ordered in from the supplier | Common procurement usage |
| Unit of issue | The unit stock is issued in to a user or department | Common procurement usage |
| Sales unit | The unit the supplier sells; UNICEF requires the label GTIN to match the material number’s unit, typically this one | UNICEF Supply Division, 2023 |
| Device package | A package with a fixed quantity of one device version or model | 21 CFR 830.3 |
| Unit of use | The individual device that is used on a patient, where it differs from the lowest packaged level | FDA definition as quoted by AHRMM |
How do I convert case price to each price?
Short answer: divide the case price by the number of eaches in the case. Per-each price = case price / eaches per case. If the line is priced per box, first work out eaches per box, then eaches per case.
Worked example (illustrative figures, not a market price): a case holds 4 boxes of 50 gloves, so 4 x 50 = 200 eaches. If the case line shows 120.00, the per-each price is 120.00 / 200 = 0.60 and the per-box price is 120.00 / 4 = 30.00. A competing line quoting 28.00 per box of 50 is 0.56 per each, so it is the lower price per unit even though the case line looks like the bigger bargain at first glance.
Checklist before comparing two lines:
- Read the UOM on each line (each, box, case).
- Find the quantity per UOM on the product listing or invoice, and confirm it states the net content.
- Convert every price to the same base unit.
- Check that the unit you will actually use or issue (unit of issue or unit of use) matches the unit you convert to.
- Confirm that order, receipt and invoice all use the same UOM.
AHRMM’s best-practice points support steps 2 and 4: clearly indicate the net content for all products, and clearly indicate the unit of use whenever it differs from the lowest packaged level.
Where it matters for labs and clinics
Staff who reorder, receive and issue supplies meet UOM in every step. The spend side is covered in spend analysis, where unit-price comparisons only work after quantities are normalised, and in the medical supply wholesaler guide for how supplier catalogues present pack levels. On the clinical side, AHRMM lists the problems an unclear unit of use causes: an unmarked unit of use with nothing to scan, inconsistently loaded GUDID data, and unit of use being confused with the lowest packaged level.
Common misunderstandings
- “Each” always means one item. It means one of whatever the system treats as the base unit; check the quantity per UOM.
- A larger UOM is always cheaper per unit. Only the arithmetic above can tell you.
- Unit of use equals the lowest packaged level. AHRMM says they differ: a tray of 25 syringes is the lowest package, each syringe is the unit of use.
- The identifier follows the item automatically. Identifiers attach to a packaging level, which is why UNICEF requires the label GTIN’s unit to match the ordering unit.
FAQ
Unit of issue vs unit of measure?
Unit of measure is the general counting unit on a line; unit of issue is the unit in which stock is issued to users. They can be the same or differ, for example buying by the case and issuing by the box.
How do I convert case price to each price?
Divide the case price by eaches per case, after multiplying out any inner boxes. For 4 boxes of 50, a case holds 200 eaches.
What is a purchase unit of measure?
The unit used to order an item from a supplier, usually the case or box, as opposed to the unit used when it is issued or used.
What is UOM conversion?
Restating a quantity or price from one unit of measure to another using the quantity per unit, such as eaches per box and boxes per case.
Sources
All sources checked 2026-10-31.
- U.S. FDA, 21 CFR 830.3, definitions (device package, device identifier), eCFR: https://www.ecfr.gov/current/title-21/chapter-I/subchapter-H/part-830/subpart-A/section-830.3
- AHRMM, “LUC UOU Explained” talk track: https://www.ahrmm.org/sites/default/files/ahrmm/luc-uou-explained.pdf
- UNICEF Supply Division, “GS1 compliance with Packing, Packaging and Labelling”, P. Adler, 1 November 2023: https://www.unicef.org/supply/media/19866/file
