Most components are counted in whole pieces, and that is the default — a resistor, a capacitor, or an IC is always counted, and you never set a unit. Some materials are measured instead of counted: wire by length, solder paste by weight, conformal coating by volume, sheet material by area. For these, give the part a unit of measure, and PartsBox keeps stock, pricing, purchasing, and builds correct in that unit.
A measured part holds fractional quantities. You can have 2.5 meters of wire in stock, use 1.5 meters in a build, and set a low-stock warning at 0.5 meters. Quantities are stored and shown in the part's own unit, with the symbol next to the value. Units come in five categories, each with metric and imperial units:
Parts without a unit stay in whole pieces, and fractional piece counts are not allowed.
The hard part is that one part lives in three different units at once, and PartsBox handles all three:
The package quantity ties them together: it is how much of the part, in its own unit, one vendor package contains. Wire tracked in millimeters and sold on spools of 10 meters has a package quantity of 10000. Order three spools and PartsBox records the order in spools and your stock in millimeters, and pricing shows both the price per spool and the price per unit, such as $0.14/mm.
A minimum purchase amount is not a package quantity. Parts sold on reels of 5000 but priced per single piece keep a package quantity of 1; the reel size belongs in the offer's minimum order quantity.
Set the unit in the part's Settings page, in the 'Unit of measure' section. While a part is new and unused, the unit changes freely. Once the part has stock or sits in a BOM, PartsBox can still change the unit, but it reinterprets the stored quantities: the numbers stay and only their meaning changes, so a stock of 5 becomes "5 mm" when you assign millimeters. PartsBox warns you first, because this cannot be undone.
Once the part is referenced by an order, is a substitute for another part, or is a member of a meta-part, the unit is locked until those references are removed. A part with substitutes or a meta-part with members can only change unit within the same category — millimeters to centimeters, not millimeters to grams. The package quantity cannot change while the part has open orders.
Meta-parts and lots respect units too. A meta-part converts members measured in different units of the same category, so a wire tracked in meters and one tracked in feet combine into one stock figure. A lot tracks its remaining length or weight as you consume it — a spool that started at 10 meters shows what is left. Low-stock thresholds and attrition amounts are entered in the part's unit as well, so 2.5 m of feeder leader is a valid attrition floor. Sub-assembly parts are always counted in pieces.
Units of measure are in every plan, including the free Maker plan.