What Is a Bill of Materials (BOM)? Definition, Types & Importance in Product Development
A Bill of Materials (BOM) is a structured list of all the components, parts, raw materials, sub-assemblies, and instructions needed to manufacture, assemble, or develop a product. It serves as a comprehensive recipe for building the product — specifying not just what components are needed, but how many of each, their specifications, and how they relate to each other within the product’s structure.
In hardware and physical product development, a BOM is a foundational engineering document. In software development, the concept has evolved into software BOMs (SBOMs) that catalog all software dependencies, libraries, and components that make up a software product.
Why BOMs Matter
Manufacturing and Supply Chain Management
For physical products, a BOM is the starting point for everything downstream in production: procurement knows what to order, manufacturing knows what to assemble, and supply chain planning knows what inventory to maintain. Without a complete and accurate BOM, production cannot proceed reliably.
Cost Estimation and Pricing
BOMs enable accurate cost calculations by providing a complete list of materials and components at specified quantities. This makes reliable cost estimation possible before a product enters production.
Quality and Compliance Management
A BOM identifies all materials and components, enabling quality verification that each meets required specifications. For regulated products, the BOM provides the documented basis for compliance verification.
Change Management
As products evolve, changes to components must be managed systematically. A versioned BOM tracks what changed and when — essential for managing the lifecycle of complex products.
Product Lifecycle Management
BOMs anchor product lifecycle management (PLM) systems — providing the product structure that quality management, change management, and production planning all reference.
Types of Bills of Materials
Engineering BOM (eBOM)
Created by the engineering team during product design. The eBOM represents the product as it is designed — organized around how the product’s components relate to each other functionally. It is the design-side view of the product structure.
Manufacturing BOM (mBOM)
Derived from the eBOM and organized around how the product is assembled and manufactured. The mBOM reflects the production sequence and may organize components differently from the eBOM to reflect manufacturing processes and sub-assembly sequences.
Sales BOM
Represents the product as it is sold to customers — potentially including configurations, options, and service components that aren’t present in the engineering or manufacturing view.
Software Bill of Materials (SBOM)
A list of all software components, libraries, open-source packages, and their versions that make up a software product. SBOMs have grown dramatically in importance as supply chain security concerns have increased — making it critical to know exactly what components are included in software products and whether any have known vulnerabilities.
BOM Structures
Single-Level BOM
Lists only the immediate components of the top-level product, without decomposing sub-assemblies into their component parts. Simple and easy to read, but insufficient for complex products.
Multi-Level BOM
Shows the complete product structure in a hierarchical tree — the product at the top, with assemblies below, sub-assemblies below those, and individual components at the lowest level. Multi-level BOMs provide complete product visibility but can be complex to manage.
Flattened BOM
A single-level view of all individual components across all levels of the product hierarchy — useful for procurement planning and cost rollups.
Key Takeaways
A Bill of Materials is the structural backbone of product development for physical products — and increasingly for software products as well. For hardware products, an accurate and well-maintained BOM is a prerequisite for manufacturing, quality management, and supply chain operations. For software products, the SBOM has become a critical artifact for security, compliance, and supply chain transparency. In both domains, the BOM transforms product design from a collection of knowledge in individual engineers’ heads into a shared, structured, and auditable specification.