A bill of materials (BOM) is a structured list of the components and materials required to make a product or assembly, including each item’s identity and quantity. It may show only the parts used directly in a product or map a multi-level hierarchy of subassemblies and their components. Its purpose and detail depend on whether it is used for design, planning, production, manufacturing, or costing.
What does a bill of materials include?
At its core, a BOM identifies the items that make up a product or assembly and states how much of each is needed. SAP Help, quoting DIN 199 Part 2, describes a BOM as a formally structured component list containing each component’s object number, quantity, and unit of measure. In a software system, it may also have an ID, name, and version; the exact fields vary by organization and system. SAP Help and Microsoft Learn describe these system-specific details.
Depending on the product, listed items can include raw materials, ingredients, parts, or semi-finished goods. For example, a bicycle BOM might name a frame and wheels, with a required quantity for each. If a wheel is managed as an assembly, its own BOM can list its constituent parts. This is an illustration, not a specification for a particular bicycle.
How is a BOM structured?
A single-level BOM lists the components used directly in a parent product. A multi-level BOM allows subassemblies to be expanded into their own components, forming a hierarchy. ISO 10209:2022 describes a bill of material as a presentation of product-structure constituents whose decomposition level can be adapted to the need. In practice, the structure should show enough detail for its intended task without implying that every BOM must use the same number of levels. ISO 10209:2022
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What are BOMs used for?
A BOM gives people and business systems a shared definition of what belongs in a product and in what quantities. Depending on its type and context, it can support:
- Engineering: recording the product’s designed or functional structure.
- Material planning and purchasing: determining component demand from planned finished-product demand.
- Production: identifying components and subassemblies used to make a product.
- Costing: estimating or calculating product costs from required materials, often alongside process information.
Microsoft Dynamics 365 describes planning BOMs as a basis for material-requirements planning and explains that a BOM combined with a route or production flow can support estimated cost calculation. Microsoft Learn
Common BOM types and viewpoints
BOM labels reflect different purposes and software conventions; they are not used identically by every company or product. These are common distinctions:
| BOM type | What it emphasizes |
|---|---|
| Engineering BOM (EBOM) | The product as designed, organized around its design or functional structure. |
| Manufacturing BOM (MBOM) | The materials and components needed to manufacture the product, organized for how it is built. |
| Planning BOM | Components or ingredients used to calculate material demand from finished-product demand. |
| Production BOM | Products made from components and subassemblies in a production context. |
| Assembly BOM | A parent item assembled from other items in a relatively simple assembly process; Business Central distinguishes this from a production BOM. |
| Costing BOM | Materials used in a costing context, such as standard-cost calculation or planned product-cost estimation. |
| Draft or sketch BOM | An early estimate of materials and attributes; Microsoft describes this type as not ordinarily used in ERP. |
Siemens contrasts the EBOM’s design-oriented structure with the MBOM’s manufacturing-oriented structure. The relationship between them can depend on a company’s processes and systems. Siemens Microsoft’s descriptions of planning, costing, draft, and production BOMs apply to its Dynamics 365 concepts, while Business Central documents its own assembly-versus-production distinction. Business Central
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What a BOM does not describe
A BOM answers what components and quantities belong to a product structure. It does not necessarily specify the assembly sequence, work steps, or production operations. Those details are typically represented separately in a route, routing, or bill of process. Siemens describes the MBOM as the master definition of what to make and the bill of process as the complementary description of how to make it. Siemens
What to check when reading or comparing BOMs
- Purpose: Is the structure for design, planning, assembly, manufacturing, or costing?
- Decomposition: Does it list immediate components only, or expand subassemblies into multiple levels?
- Product and version: Does it apply to the product variant and revision you need? In Dynamics 365, versions define the product and period for which a BOM can be used. Microsoft Learn
- Process information: Are operations and sequence documented in a separate route, routing, or bill of process?
A BOM can list materials, not just machine parts
The same structured approach applies to products made from non-mechanical materials. A European Commission-hosted example titled “BOM: Bill of Materials” lists materials for a musical soft-filled toy, including filling material and filling fibre. European Commission example
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