A bill of materials (BOM) is a structured list of every part, component, and quantity needed to build, assemble, or repair a product.
A bill of materials (BOM) is a structured list of every raw material, part, component, and quantity needed to build, assemble, or repair a product - the recipe, with the product as the dish. It is the document that lets purchasing order the right parts ahead of supplier lead times, lets production build without guessing, and lets finance work out what the product actually costs to make.
What a BOM includes
A usable BOM line carries more than a part name:
- Part number and name - the unique identifier, plus a human-readable description.
- Quantity and unit of measure - 4 pieces, 0.5 metres, 20 grams. Mixed-up units are a classic source of over- and under-ordering.
- Procurement type - made in-house or bought in, which decides whether the line triggers a work order or a purchase order.
- Reference designators - in electronics, where each instance of the part sits on the board.
- Version and notes - which revision of the product this BOM describes, and anything an assembler needs to know.
Single-level vs multi-level BOMs
A single-level BOM is a flat list one level deep: the product, then every part directly beneath it. It is quick to create and fine for simple products. A multi-level BOM (also called an indented BOM) nests sub-assemblies inside the parent: the finished product contains a leg assembly, which in turn contains its own parts. The multi-level form mirrors how complex products are really built, supports reusing a sub-assembly across several products, and makes it possible to cost or stock the sub-assembly as a unit - at the price of more structure to keep current.
A simple example
A single-level BOM for a workshop desk might read:
| Part no. | Part | Qty | Unit |
|---|---|---|---|
| DSK-TOP-01 | Desktop panel | 1 | pc |
| DSK-LEG-02 | Steel leg | 4 | pc |
| DSK-BRK-03 | Corner bracket | 4 | pc |
| FAS-M6-16 | M6 x 16 bolt | 16 | pc |
| DSK-TRY-04 | Cable tray | 1 | pc |
In a multi-level version, “leg assembly” would appear once as a sub-assembly line, with the leg, bracket, and four bolts listed beneath it - and the same assembly could then be reused in the matching workbench’s BOM.
EBOM, MBOM and service BOMs
The same product often has more than one BOM. The engineering BOM (EBOM) describes the product as designed; the manufacturing BOM (MBOM) describes it as actually built, adding things engineering does not draw - adhesive, packaging, labels, consumables. A service BOM lists the spares and consumables needed for repair rather than assembly: a bike-hire workshop, for instance, keeps a per-model parts list so brake pads and inner tubes for its rental bikes are on the shelf before the season starts.
Common mistakes
The classic failures are organisational rather than technical. No version control, so the shop floor builds revision A while purchasing orders for revision B. Missing consumables - fasteners, glue, packaging - that someone discovers at the bench. Single-sourcing a long-lead part with no noted alternative. And treating the BOM as separate from stock: a parts list is only actionable if inventory management can say whether those parts are actually on hand, which in turn depends on records accurate enough to trust.
Related terms
- Lead Time - the supplier delay purchasing must plan around for every bought-in BOM line
- Inventory Management - the discipline that connects the parts list to real stock
- Inventory Accuracy - why the on-hand quantities behind a BOM check must be right
- Just-in-Time Inventory - ordering BOM components to arrive as production needs them
- Dead Stock - what over-ordered or superseded components become