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What is a bill of materials?

A bill of materials – known in English as a BOM – is a structured document that comprehensively describes all components of a product. It contains assemblies, sub-assemblies and individual parts, each with their associated quantity, unit and often additional attributes such as validity periods, spare parts or variant configurations. The bill of materials is therefore the central master data object accessed equally by planning, purchasing, manufacturing and warehouse logistics.

Several types of bills of materials are distinguished: the quantity BOM lists all parts without hierarchical structure, the structural BOM maps the multi-level assembly structure, and the variant BOM describes different product configurations with shared and variant-specific parts. Which type is appropriate depends on the depth of manufacturing and the range of variants – no universal recommendation can be made here.

How is the bill of materials used in practice?

In an ERP system, the bill of materials serves as the input for the MRP run (material requirements planning). Starting from the primary demand – i.e. planned sales or a production order – the system uses the bill of materials to derive the secondary demand for all required components. This automatically generates purchase proposals and production orders at the correct levels of the assembly structure.

In warehouse logistics, the bill of materials is the foundation for kitting and pre-assembly: before a product enters final assembly, all parts are picked and staged based on the bill of materials. Errors in the bill of materials – incorrect quantities, outdated part numbers or missing alternative line items – directly affect delivery capability and are a common, often underestimated source of errors.

In purchasing, the bill of materials supports procurement planning and supplier sourcing: which parts must be sourced externally, and which are produced in-house? Changes to products – for example due to engineering changes or new regulatory requirements – must be carefully maintained in the bill of materials so that all downstream processes receive accurate data.

What matters when maintaining and selecting a BOM?

Data quality in the bill of materials is critical. Outdated or inconsistent entries lead to shortages, unnecessary inventory or production stoppages. A clearly defined change management process is recommended to ensure that engineering, purchasing and logistics data are kept in sync.

• Ensure that bill of materials maintenance and the item master are organisationally aligned – loosely defined processes create data drift.
• When selecting an ERP system, assess how well variant BOMs and validity periods can be represented.
• For kitting processes, consider whether your WMS can read the BOM structure directly or must obtain it via an interface.

The distinction from the routing is important: the bill of materials describes what is built in, while the routing describes how and in what sequence manufacturing takes place. Both documents complement each other but are separate master data objects.