Units of measure reference
This page is a lookup for the rules that govern a product's units of measure (UoMs): the flags that mark a row as stock, purchase, or sales; how the conversion factor auto-fills for same-family system units; when the stock UoM becomes locked; every error message you can see when saving units; how display precision behaves; and where the per-product UoM set connects to the global units-of-measure catalog.
For the task of adding, editing, and saving UoM rows, see Configure a product's units of measure. For the actual stock numbers a product carries in its stock UoM, see View a product's stock.
The UoM model at a glance
Every product has a set of UoM rows. Each row pairs the product with one global unit and carries a conversion factor plus three flags. Exactly one row is the stock UoM — the canonical unit inventory is tracked in — and every other row's conversion factor is expressed relative to it.
| Field | What you enter | Meaning |
|---|---|---|
| Unit | Pick from the global catalog | The unit this row measures in (see Global catalog). |
| Conversion factor | A number greater than 0 | How many stock units equal one of this row's units. Always 1 on the stock row. |
| Display precision | A whole number, 0–8 | Decimal places used when showing a quantity in this unit. Defaults to 2. |
| Stock UoM | Checkbox | Marks the canonical inventory unit. Exactly one row must be checked. |
| Purchase UoM | Checkbox | Default unit when purchasing. At most one row checked. |
| Sales UoM | Checkbox | Default unit when selling. At most one row checked. |
The conversion factor is the number of stock units contained in one of that row's units. If a product is stocked in Each and you add a Box row where one box holds 12 units, the Box row's factor is 12. Converting a quantity to stock is quantity × factor; converting between two non-stock rows goes through stock: multiply by the source row's factor, then divide by the target row's.
Rows are always listed stock-first, then the purchase row, then the sales row, then the rest.
Purchase and sales flags
Purchase UoM and Sales UoM let a product be bought in one unit and sold in another — for example, purchased by the Box but sold by the Each.
| Rule | Behavior |
|---|---|
| Default value | Both flags default to false. A product with only a stock UoM has no dedicated purchase or sales unit. |
| At most one purchase UoM | Marking a second row as purchase is rejected on save (see Validation errors). |
| At most one sales UoM | Marking a second row as sales is rejected on save. |
| Client-side uniqueness | Ticking Purchasing or Sales on one row automatically clears the same flag on every other row, so you normally never reach the back end error. |
| Stock row can also be purchase/sales | The three flags are independent — the stock row may also carry the purchase and/or sales flag. |
The purchase UoM is the default unit surfaced when this product is added to a purchase order or a supplier record; see Manage a product's suppliers.
Conversion-factor auto-fill (canonical factors)
When you pick a UoM whose family (weight or volume) matches the stock UoM's family, and both are built-in system units, the editor auto-computes the conversion factor for you. It does this from a canonical table of each unit's size relative to its family base (kilogram for weight, litre for volume):
| Unit | Family | Canonical factor (relative to family base) |
|---|---|---|
KG | Weight | 1 |
G | Weight | 0.001 |
MG | Weight | 0.000001 |
LB | Weight | 0.45359237 |
OZ | Weight | 0.028349523125 |
L | Volume | 1 |
ML | Volume | 0.001 |
GAL | Volume | 3.785411784 |
QT | Volume | 0.946352946 |
PT | Volume | 0.473176473 |
CUP | Volume | 0.2365882365 |
FLOZ | Volume | 0.0295735295625 |
TBSP | Volume | 0.01478676478125 |
TSP | Volume | 0.00492892159375 |
The factor written into the row is pickedCanonical ÷ stockCanonical. For a product stocked in KG, picking G fills 0.001; picking LB fills 0.45359237.
Auto-fill has three deliberate limits:
- Count units are excluded.
EACH,BOX, andPALLETaren't in the canonical table because their conversion is product-specific — a box of screws holds a different count than a box of labels. You must type those factors yourself. - It only fills a still-default row. Auto-fill runs only while the row's factor is still the default
1. Once you have typed a custom value, re-picking the unit won't overwrite your override. - It never touches the stock row. The stock row's factor is locked at
1and is skipped.
A cross-family combination (for example, a weight stock UoM with a volume alternate) converts through the product's density instead, once one is set. See Convert between volume and weight with density. Any custom unit is left at 1 for you to fill in manually.
The stock-UoM-change lock
Once a product has transactional history, its stock unit can no longer be changed — reinterpreting existing movements or BOM lines against a different canonical unit would silently corrupt every recorded quantity.
A product has transactional history when any of the following exist for it:
- an inventory movement,
- a BOM component line (the product is used as a component), or
- a BOM output line (the product is produced by an assembly).
When history exists, the editor locks the stock selector: the Stock radio is disabled and a hint reads "Stock unit cannot be changed — product already has inventory or BOM history." The lock only blocks changing which row is the stock UoM — you can still add or remove alternate purchase/sales rows and edit their conversion factors and display precision.
The editor knows about that history before you try to save, which is why the control is already locked. The same rule is re-checked when you save, so a locked stock unit can't be changed by any other route either — see the stock-change error below.
Validation errors
You save the whole unit-of-measure set at once, and it's checked in two passes.
First pass — the basics
| Condition | Message |
|---|---|
| No rows at all | At least one unit of measure is required. |
| A unit that isn't one of SKU.io's units | The selected unit of measure does not exist. |
| A conversion factor that's blank, non-numeric, or zero or less | Conversion factor must be greater than zero. |
| A display precision outside 0–8, or not a whole number | A range error on that field. |
Second pass — the configuration rules
Checked once the basics pass:
| Condition | Message |
|---|---|
| Empty set | A product must have at least one unit of measure. |
| The same unit appears on two rows | Each unit of measure can only appear once per product. |
| Any row's factor is zero or less | Conversion factor must be greater than zero. |
The stock row's factor isn't exactly 1 | The stock unit of measure must have a conversion factor of 1. |
| No row, or more than one row, marked as stock | Exactly one unit of measure must be marked as the stock unit. |
| More than one purchase row | At most one unit of measure can be marked as the purchase unit. |
| More than one sales row | At most one unit of measure can be marked as the sales unit. |
| Stock unit changed after history exists | The stock unit of measure cannot be changed after the product has been used in inventory movements or BOMs. |
The editor catches the most common of these — a missing unit on a row, exactly one stock unit, and a positive factor — and blocks the save with an inline message, so you usually get a friendly prompt before anything is sent.
Display precision
Display precision controls how many decimal places a quantity is shown with in that unit — it's a presentation setting only and doesn't affect stored quantities or conversion math.
| Aspect | Value |
|---|---|
| Default | 2 decimal places when unset. |
| Allowed range | 0–8. |
| Stored type | Integer. |
| Applies to | The quantity display for that row (for example, a PALLET row set to 0 shows whole pallets; a KG row set to 3 shows gram-level detail). |
Precision is set per row, so different units on the same product can display at different precisions.
The global units-of-measure catalog (seam)
A product's UoM rows don't define units themselves — each row picks its Unit from the tenant-wide global units-of-measure catalog. The catalog is a separate screen, Manufacturing → Units of Measure, with its own list and create, edit and delete actions. A product's UoM editor picks from it, and its picker can also create a missing unit with Create new unit of measure. To add, edit or delete catalog units, see Manage the units of measure catalog.
Every tenant is seeded with these built-in system units, which can't be deleted:
| Code | Name | Family |
|---|---|---|
MG | Milligram | Weight |
G | Gram | Weight |
KG | Kilogram | Weight |
OZ | Ounce (weight) | Weight |
LB | Pound | Weight |
ML | Millilitre | Volume |
L | Litre | Volume |
TSP | Teaspoon (US) | Volume |
TBSP | Tablespoon (US) | Volume |
FLOZ | Fluid ounce (US) | Volume |
CUP | Cup (US) | Volume |
PT | Pint (US) | Volume |
QT | Quart (US) | Volume |
GAL | Gallon (US) | Volume |
EACH | Each | Count |
BOX | Box | Count |
PALLET | Pallet | Count |
Only these system units — the weight and volume ones — participate in conversion-factor auto-fill. Custom units you add to the catalog are always available to pick, marked custom, but you must type their conversion factors by hand.
Next steps
- Configure a product's units of measure — the step-by-step task for editing and saving a product's UoM set.
- Product fields & validation reference — the rest of a product's fields and their rules.
- Set a product's price and Manage a product's suppliers — where the sales and purchase UoMs are used.
- View a product's stock — how stock is counted and reported in the product's stock UoM.