UOM (Unit of Measure)

What it means

A unit of measure is the noun attached to a quantity. "12 cases" only means something if you know what a case is. UOMs make that explicit so the system, the buyer, and the warehouse all agree on what they're counting.

Most items have more than one UOM in their lifecycle. You might buy beer by the pallet, store it by the case, sell it by the six-pack, and count it by the bottle. Each of those is a valid UOM for the same physical product. The trick is that you need a single base UOM — the smallest, most granular unit — that everything else converts back to.

Example: a hardware shop sells screws. The base UOM is "each." Purchasing buys them in boxes of 500. Sales might sell them in packs of 10 at the counter. Three UOMs, one item, one consistent count.

Base vs purchase vs sales UOM

Three roles you'll see in any decent inventory system.

Base UOM is the canonical unit. It's what stock-on-hand is stored in. For a brewery's bottled beer, the base UOM is "bottle." Every other count rolls up to bottles.

Purchase UOM is how you buy from the supplier. The brewery buys empty bottles by the pallet (2,400 bottles) and full cases of bottle caps by the box (10,000 caps). The PO is in purchase UOM. The receipt converts to base UOM.

Sales UOM is how you sell to the customer. The brewery sells the finished beer in three sales UOMs — single bottle, six-pack, and case of 24. Each invoice line picks one. Each sale decrements base UOM in the background.

A real example end-to-end:

• Role | UOM | Quantity | Equivalent in base UOM

• Base | each | — | 1

• Purchase | case | 1 case | 24 eaches

• Sales (S) | six-pack | 1 six-pack | 6 eaches

• Sales (M) | case | 1 case | 24 eaches

Buy 50 cases (1,200 each), sell 80 six-packs (480 each), and your on-hand is 720 each. The system handles the math — you just need the conversion factors set right.

Conversion factors

A conversion factor is the multiplier that turns one UOM into another. The math is simple; the discipline is in keeping the factors current.

For the beer example:

• 1 case = 24 each (factor: 24)

• 1 six-pack = 6 each (factor: 6)

• 1 case = 4 six-packs (factor: 4)

When a PO arrives for "10 cases," the system multiplies 10 × 24 = 240 each, and adds 240 to base on-hand. When a sales order ships "3 six-packs," it deducts 3 × 6 = 18 each.

Where this goes wrong is when conversion factors change and nobody updates the master. The supplier switches from 24-bottle cases to 12-bottle cases for shipping efficiency. If your system still thinks "1 case = 24 each," every receipt is double-counted. Audit conversion factors on every vendor PDF change.

UOM mistakes that break counts

Four recurring failures.

Mixing units in one column. A spreadsheet inventory where one row says "5 cases" and another says "5 each" — but the column header is just "Qty." Within a week, nobody knows what's what.

Missing conversions. Adding a new purchase UOM (say, "pallet = 40 cases") without defining the conversion. Receipts go in as "1 pallet" and the system treats it as 1 each. You're suddenly 39 cases short on paper.

UOM in the description field. Putting "Sold per dozen" in the item description instead of setting the UOM properly. The system sees a quantity of 1; the picker pulls 12. Or the picker pulls 1 and ships a single muffin instead of a dozen.

Changing the base UOM after the fact. Switching base UOM from "case" to "each" after a year of history. Unless every prior transaction is converted, your reports will mix the two and give garbage totals.

Setting UOMs in StockZip

Each item in StockZip gets a base UOM at creation. You add purchase and sales UOMs with conversion factors, and the system handles the math at every receipt and shipment. Conversions are stored on the item, not the document, so changing one PO line doesn't quietly rewrite history on the rest.

Track it in StockZip

StockZip supports base, purchase, and sales UOMs on every item, with conversion factors stored on the item record. Buy in pallets, count in eaches, sell in six-packs — the math runs in the background. Start a free trial and set up your first multi-UOM item in under a minute.

Frequently asked questions

What does UOM stand for?

Unit of Measure. It's the noun attached to any inventory quantity — each, case, kilogram, liter, meter, pallet, and so on.

Can I have multiple UOMs for one item?

Yes, and most items will. You typically have one base UOM (the smallest unit), one or more purchase UOMs (how you buy), and one or more sales UOMs (how you sell). Each non-base UOM needs a conversion factor back to base.

How do conversion factors work?

A conversion factor is a multiplier. If 1 case = 24 each, the factor is 24. Receive 10 cases, the system adds 240 to base on-hand. Sell 18 each, the system deducts 18 from base. All math goes through base UOM.

What's the most common UOM mistake?

Treating UOM as a description instead of a setting. Writing "sold per dozen" in the item name doesn't tell the system anything — the system still counts a quantity of 1 as one unit. Set the UOM in the actual UOM field with the right conversion factor.