Bill of Materials (BOM)

A bill of materials (BOM) is a structured, hierarchical list of every raw material, component, subassembly, and part required to manufacture a finished product, along with the quantity of each item needed. It is the authoritative reference document that connects engineering design to production planning, procurement, inventory management, and cost accounting. Without an accurate BOM, manufacturers risk production delays, excess inventory, and cost overruns.

Think of it as a recipe: every ingredient listed in the exact amount needed, in the exact sequence the process requires.

The Three Main Types of BOM

Different departments in a manufacturing organization rely on different types of BOM, each structured around the needs of that stage in the product lifecycle.

• Engineering BOM (EBOM). Created during product design, typically using computer-aided design tools. Lists all components as the engineering team defines them, from a functional perspective. The EBOM is the blueprint before production requirements are considered.
• Manufacturing BOM (MBOM). Translates the EBOM into what the production floor actually needs. Includes processing steps, tooling requirements, assembly sequences, lead times, and scrap factors. The MBOM feeds directly into materials resource planning (MRP) and enterprise resource planning (ERP) systems to trigger purchasing and scheduling.
• Sales BOM. Represents the product as it is configured and sold to the customer. Components appear as separate line items in the sales order rather than as a single finished-good SKU, which is common for configurable or modular products.

BOM Structure Is Hierarchical, Not Flat

A BOM is organized by level. Level 0 is the finished product at the top. Level 1 lists the major assemblies or components that go directly into the finished product. Level 2 lists the components that go into each Level 1 assembly, and so on. A multi-level BOM for a complex product like an automobile or circuit board can reach six or more levels deep.

This hierarchy matters because changing one component at a low level ripples through every parent assembly that uses it. Managing those relationships manually across hundreds of parts creates errors. That is why BOMs are maintained inside ERP and product lifecycle management (PLM) software rather than on spreadsheets.

BOMs Evolved From World War-Era Records

BOMs gained prominence in the 1940s as manufacturers needed precise records to manage wartime production at scale. By the 1960s, BOMs had become foundational to early computerized manufacturing systems. When MRP systems emerged in the 1970s and 1980s, the BOM became the central data structure that those systems depended on for calculating what to buy and when.

Today's digital BOMs are living documents updated continuously as designs change, supplier parts are substituted, and processes are optimized. A BOM error does not stay contained: it propagates into purchase orders, work orders, and cost estimates simultaneously.

Sources:
https://en.wikipedia.org/wiki/Bill_of_materials
https://www.netsuite.com/portal/resource/articles/erp/bill-of-materials-bom.shtml
https://www.techtarget.com/searcherp/definition/bill-of-materials-BoM
https://www.mrpeasy.com/blog/bill-of-materials/