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A bill of materials (BOM) is a structured list of the components and materials needed to make a product or assembly, typically identifying each item and the quantity required. A BOM can show only the parts used directly in one assembly or expand into a hierarchy of subassemblies and their components. Its organization depends on whether it is being used for design, planning, manufacturing, assembly, or costing.
What does a BOM include?
At its core, a BOM connects a product or assembly to the items that make it up. SAP Help quotes DIN 199 Part 2 as defining a BOM as a formally structured list that records each component’s object number, quantity, and unit of measure. In practice, the exact fields depend on the organization and software. Microsoft Dynamics 365, for example, associates a BOM with an ID, name, component lines, and versions. SAP Help Microsoft Learn
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A BOM is more than an informal shopping list: it structures component identities and quantities in relation to a product definition. Version information can identify the product configuration and period for which a BOM applies, helping distinguish the right structure for a particular product or revision.
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How are BOMs structured?
Single-level BOM
A single-level BOM lists the components used directly in a parent item. For example, an illustrative bicycle BOM might list one frame and two wheels beneath the bicycle as the parent. This is a simple explanation, not a manufacturer’s specification.
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Multi-level BOM
A multi-level BOM shows nested assemblies: a component in the parent list can itself have a BOM listing its constituent parts. This creates a hierarchy that can be expanded to the level needed. ISO 10209:2022 describes a bill-of-material as a presentation of product-structure constituents whose decomposition can be adapted to the need. ISO 10209:2022
BOMs can cover materials as well as mechanical parts. A European Commission-hosted example for a soft-filled toy lists materials such as filling material and filling fibre. European Commission-hosted example
What is a BOM used for?
A BOM establishes which items and quantities belong to a product definition. Depending on its purpose and system context, it can support engineering work, material planning, purchasing, production execution, assembly, or product costing. In Microsoft Dynamics 365, planning BOMs help calculate component and ingredient demand from finished-product demand; BOMs combined with a route or production flow can support estimated cost calculation. Microsoft Learn
A BOM does not necessarily specify the order of assembly or the operations required to manufacture an item. Process details are often captured separately in a route, routing, or bill of process. Siemens describes the manufacturing BOM as the master definition of what to make, complemented by a bill of process describing how to make it. Siemens
Common BOM types and viewpoints
Organizations and software products use BOM labels differently, and not every company uses every type. These terms describe common purposes rather than a universal naming system.
| Type | What it emphasizes |
|---|---|
| Engineering BOM (EBOM) | The product as designed, organized around its design or functional structure. Siemens |
| Manufacturing BOM (MBOM) | The materials and components needed to manufacture the product, organized around how parts are used to build it. Siemens |
| Planning BOM | Component and ingredient demand derived from demand for finished products, in the Dynamics 365 context. Microsoft Learn |
| Production BOM | Products made from components and subassemblies in a production context; Business Central distinguishes it from an assembly BOM. Microsoft Learn |
| Assembly BOM | A parent item assembled from other items in a relatively simple assembly process, in Business Central. Microsoft Learn |
| Costing BOM | A structure used in costing contexts, such as standard-cost calculation or planned product-cost estimation. Microsoft Learn |
| Draft or sketch BOM | An early estimate of materials and attributes; Microsoft describes this type as not ordinarily used in ERP. Microsoft Learn |
How to compare or choose a BOM structure
When reviewing a BOM, focus on the product question it needs to answer rather than relying on its label alone:
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- Purpose: Is it for design, material planning, assembly, manufacturing, or costing?
- Level of detail: Does it list immediate components only, or show nested assemblies?
- Product and validity: Which product variant, revision, or period does the structure apply to?
- Process data: Are manufacturing operations documented separately in a route, routing, or bill of process?
These distinctions matter because one product can be represented from different viewpoints. An engineering structure may organize parts around design, while a manufacturing structure organizes materials around production needs; neither label alone guarantees identical fields or behavior across software.
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