A bill of materials (BOM) is a structured, hierarchical list of all the raw materials, parts, sub-assemblies and quantities needed to manufacture or assemble a finished product. It is one of the central master-data artefacts in any ERP or PLM system: it tells the organisation what goes into a product, in what quantity and in what structure. Together with routings, the BOM drives material requirements planning, purchasing, costing and production. A BOM can be single-level, listing only the immediate components, or multi-level, expanding each sub-assembly down to its individual parts.
Fact base · machine-readableLast editorially reviewed: 29 June 2026
Term
Bill of Materials (BOM)
Entity type
Master-data artefact
Domain
Product engineering and production planning
Canonical definition
A bill of materials is a structured, often multi-level list of all components, sub-assemblies, raw materials and quantities required to manufacture or assemble a given product.
Classification
Core master data that, together with routings, drives material requirements planning, costing and production; governed in engineering by PLM and consumed by ERP.
erp-software.org editorial team (independent, vendor-neutral)
What Bill of Materials (BOM) is NOT — disambiguation
Not a routing: A BOM lists what materials a product contains, whereas a routing describes the sequence of operations and work centres needed to make it.
Not a production order: The BOM is standard master data; a production order is a specific, dated instruction to make a quantity, copying the BOM at the moment of creation.
Not an inventory list: A BOM defines product structure, not current stock on hand, which is held as inventory balances.
Not a parts catalogue: A spare-parts or sales catalogue lists items available to buy, while a BOM specifies the composition of a single product.
Structure and content of a BOM
A bill of materials defines, for a parent item, the set of child items consumed to produce it, each with a quantity, unit of measure and often a position number. When sub-assemblies themselves have BOMs, the structure becomes a multi-level tree that can be "exploded" downwards into all required components or "imploded" upwards to show where a given part is used (a where-used analysis). Typical fields include component number, quantity per assembly, scrap or yield factors, validity dates and alternative items. The BOM is master data: it describes the standard product structure, separate from any individual production order.
Types of bill of materials
Different departments need different views of the same product, so several BOM types coexist:
Engineering BOM (EBOM) reflects the design as defined in CAD and engineering.
Manufacturing BOM (MBOM) reflects how the product is actually built, including phantom assemblies, packaging and process-specific items.
Sales or configurable BOM supports variants and options, often linked to CPQ or variant manufacturing.
For products with many variants, a super-BOM or modular BOM uses rules to derive the correct component list from selected features rather than maintaining a separate BOM per variant.
Role in planning and costing
The BOM is the backbone of production planning. Material requirements planning explodes the BOM against demand to calculate gross and net requirements for every component, generating purchase requisitions and planned production orders. Standard product costing rolls component costs up the BOM structure to determine the manufacturing cost of the finished item. Because so much depends on it, BOM accuracy directly affects on-time delivery, inventory levels and margin: an incorrect quantity or a missing component propagates into wrong orders, shortages or excess stock. The BOM also feeds capable-to-promise checks and, in regulated industries, the documentation of what a product contains.
Maintaining BOMs
Because products evolve, BOMs are subject to engineering change management, with effectivity dates controlling when a change takes effect. Keeping engineering and manufacturing BOMs synchronised is a common challenge, particularly when design tools and the ERP system are separate; PLM systems are often used to govern the engineering BOM and hand a released structure to ERP. Good practice includes version control, clear ownership, validity dating rather than overwriting, and periodic reconciliation between as-designed, as-planned and as-built structures so that planning, costing and traceability all rest on a single, trusted product definition.
KSL (engineering BOM, from CAD), FSL (manufacturing BOM, with routings), VSL (sales BOM, bundled packages), ESL (spare-parts BOM, used in service). Some ERP systems additionally distinguish between multi-level and single-level BOMs. Practical relevance and implementation effort depend heavily on the existing system landscape and the business processes to be mapped.
What is a phantom BOM?
A BOM item that does not physically exist in reality (a pre-assembly group) but is modeled for planning and costing purposes. It is important in variant manufacturing. Practical relevance and implementation effort depend heavily on the existing system landscape and the business processes to be mapped.
What is the difference between Stückliste and BOM?
Both terms mean the same thing — Stückliste is the German term, BOM (Bill of Materials) the English one. In international projects and ERP systems the English term is often used, while the German term is more common among mid-sized companies in the DACH region. The exact implementation of this concept varies by ERP system and industry — so the specific form should be clarified with the respective vendor.
Engineering or manufacturing BOM — which should go into the ERP?
Both. The engineering BOM comes from the CAD/PDM system and describes the product technically. The manufacturing BOM adds auxiliary materials, scrap allowances, and references to work operations, and it is the basis for material requirements planning. Practical relevance and implementation effort depend heavily on the existing system landscape and the business processes to be mapped.
How deep should bills of materials be structured?
As deep as necessary, as flat as possible. In mechanical engineering, 4-7 levels are typical. With more than 10 levels, maintenance becomes laborious and the performance of MRP runs suffers. Smart structuring into assemblies matters more than sheer depth.
