What is a bill of materials (BOM)?

A BOM is the recipe: to build one unit of the finished SKU, you need these components in these quantities. A simple single-level BOM lists direct components. A multi-level BOM nests sub-assemblies, where a component is itself built from parts with its own BOM beneath it. Operationally, the BOM is what connects production to inventory. When a work order builds 100 units, the BOM tells the system to decrement 100 of each component (times quantity per unit) and increment 100 finished goods. It also powers the answer to "how many can we build right now?": divide on-hand stock of each component by its BOM quantity and the smallest result is your constraint. That calculation, run against live counts, is how a shop spots that the $0.40 gasket is about to halt a $400 product line. Where teams trip: BOM drift. Engineering substitutes a component, the floor adopts it, and the documented BOM keeps decrementing the old part. Within months, the system shows phantom stock of the old component and silent consumption of the new one, and the reorder report lies in both directions. Treat BOM changes like record changes anywhere else: versioned, dated, and owned by someone, with the effective date matched to when the floor actually switches.