How units of measure work

Tooling items are rarely bought and stored in the same unit. A vendor sells drill inserts by the box; your warehouse stocks them by the piece. A coolant supplier invoices by the drum; your stock rules count in liters. Tooling Metrics handles this with a unit of measure system that converts quantities automatically, so order quantities and inventory quantities always agree — in their respective units.

The reference unit

Every organization has exactly one reference unit. It is the baseline through which all other units are expressed. All conversions between units go through the reference unit as the common denominator.

The choice of reference unit is significant because it anchors the entire unit system. Most manufacturing organizations use Piece (or Each) as their reference, since individual parts and tools are the most granular meaningful quantity. You can only have one reference unit; changing it after the system is in use would invalidate all existing ratios.

The three unit types

All other units are defined in relation to the reference unit:

Smaller units subdivide the reference unit. The ratio says how many of these units make one reference unit. For example, if the reference unit is Piece and you create a unit called Half with ratio 2, then 2 Halves = 1 Piece. Smaller units are uncommon in tooling but appear in contexts like fractional cutting inserts or partial packages.

Bigger units contain multiple reference units. The ratio says how many reference units are in one of these. For example, if the reference unit is Piece and you create Box with ratio 10, then 1 Box = 10 Pieces. This is the common case in tool purchasing — vendors sell in boxes, packs, or rolls while inventory is tracked by piece.

Unit type	Ratio meaning	Example
Reference	Always 1	1 Piece = 1 Piece
Bigger	1 unit = ratio reference units	1 Box = 10 Pieces (ratio: 10)
Smaller	ratio units = 1 reference unit	2 Halves = 1 Piece (ratio: 2)

Conversion between any two units

When two non-reference units need to be compared or converted, Tooling Metrics always routes through the reference unit:

conversion factor = (ratio of source unit in reference) ÷ (ratio of target unit in reference)

For example, converting Box (ratio 10) to Pack (ratio 5):

• Box in reference: 10 Pieces
• Pack in reference: 5 Pieces
• Conversion: 1 Box = 10 ÷ 5 = 2 Packs

This means you can freely mix unit types in your catalog without pre-calculating every pair of unit conversions.

Purchase unit vs. inventory unit

Each item in Tooling Metrics has two unit settings:

• Inventory UOM (uom) — the unit used to count stock (usually Piece).
• Purchase UOM (po_uom) — the unit used on purchase orders to that vendor (often Box or Pack).

These can be the same or different. When they differ, Tooling Metrics converts automatically at restocking time.

How conversion flows through a purchase

Consider an item with inventory UOM = Piece and purchase UOM = Box (ratio 12):

1. Purchase Order — You order 3 Boxes at €5/Box. The PO line shows 3 Box × €5 = €15.
2. Purchase Receipt — The vendor delivers 3 Boxes. You receive 3 Box on the receipt.
3. Purchase Restock — As items are placed in storage, Tooling Metrics converts: 3 Box × 12 = 36 Pieces added to on-hand inventory.
4. Inventory view — The item shows 36 Pieces in stock at the per-piece unit cost of €15 ÷ 36 = €0.42/Piece.

The operator works in Boxes throughout the purchasing flow. Inventory is tracked in Pieces throughout the storage and picking flow. No manual conversion is needed at any step.

Why two fields instead of one

The two settings on each item — UOM for inventory counting and Purchase UOM for vendor ordering — exist because the warehouse and the vendor rarely think in the same unit. Collapsing them into a single field would force a choice: either purchasing has to think in pieces when placing an order for a box vendor, or inventory has to think in boxes when picking a single insert off a shelf. Keeping them separate lets each side of the business use its natural unit, with the conversion happening automatically in between. When a vendor only sells individual pieces, both fields simply end up pointing at the same unit.

Reference unit choice matters for the same reason: whichever unit is Reference is the one every ratio in the system is expressed relative to, so it’s worth being deliberate about it rather than treating it as an arbitrary starting unit. Most manufacturing organizations settle on Piece precisely because it’s the most granular quantity anyone in the business actually thinks in — bigger and smaller units are then easy to add later without re-deriving anything.

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