A single product can have more than one bill of materials, and the most important pair is the engineering BOM and the manufacturing BOM. They describe the same product but they are written for different readers and different purposes. This piece explains the difference and why both exist.
Two BOMs, one product
It surprises people that a product can have two bills of materials. Surely a product is made of what it is made of? It is, but a BOM is not just a parts list, it is a parts list organised a particular way for a particular use. The engineering BOM and the manufacturing BOM organise the same product differently because design and production look at the product differently.
The engineering BOM
The engineering BOM, often called the EBOM, describes the product as designed. It is created by the design engineers, and it is organised the way the design is conceived: by the functional structure of the product, by how the design is broken into systems and parts. The engineering BOM is the authoritative statement of what the product is, in design terms. It is closely tied to the drawings and the design specifications, and it changes through engineering change processes.
The engineering BOM answers the question "what is this product, by design". It is not concerned with the sequence in which the factory assembles it, or with the consumables and packaging that production needs but design does not think about.
The manufacturing BOM
The manufacturing BOM, often called the MBOM, describes the product as built. It is organised the way the factory actually assembles the product: by the real sequence of assembly, by the sub-assemblies that physically come together on the floor. And it includes things the engineering BOM typically leaves out, because the factory needs them and the designer does not:
- Consumables used in production, adhesives, lubricants, fasteners not on the design drawing.
- Packaging materials.
- The grouping of parts into the actual assembly steps and intermediate sub-assemblies the floor builds.
The manufacturing BOM answers the question "what does the factory need, and how is it built". It is the BOM that production, planning, and purchasing actually work from.
Why the two differ
The two BOMs differ because design and manufacturing are genuinely different activities. A designer organises the product by function and by the logic of the design. A factory organises it by the logic of assembly, what gets built first, what is joined to what, in what order, with what consumables. The same set of parts, organised by design intent, is the engineering BOM; organised by build sequence and supplemented with what production consumes, it is the manufacturing BOM. Neither is wrong; they serve different work.
Why a manufacturer needs both
A manufacturer needs the engineering BOM because it is the authoritative design definition, the controlled record of what the product is, kept current through engineering changes. It needs the manufacturing BOM because that is what production, MRP, and purchasing run on, the realistic, complete, build-oriented list. Using only the engineering BOM on the floor means the factory is missing consumables and packaging and is not organised by how it actually builds. Using only an informal manufacturing BOM with no controlled engineering BOM means there is no authoritative design record.
The transition from EBOM to MBOM
The practical work is the transition: turning the engineering BOM into the manufacturing BOM. That means reorganising the structure into assembly sequence, adding consumables and packaging, and grouping parts into the sub-assemblies the floor builds. A good manufacturing system supports this, keeping the EBOM and MBOM linked so that an engineering change to the design flows into the manufacturing BOM in a controlled way, rather than the two drifting apart. When they drift apart, the factory builds to a definition that no longer matches the design, which is exactly the failure both BOMs exist to prevent.
The takeaway
The engineering BOM is the product as designed, organised by design intent; the manufacturing BOM is the product as built, organised by assembly sequence and including the consumables and packaging production needs. A manufacturer needs both, and needs them linked, so the design definition and the build definition stay consistent. For how we approach manufacturing systems, see our manufacturing work.