What are Bill of Materials (BOM)?

Bill of Materials (BOM) Definition

A bill of materials (BOM) is a comprehensive list of parts, items, assemblies, subassemblies, intermediate assemblies, and other materials required to create a product, as well as instructions required for gathering and using the required materials. A BOM can be thought of as the recipe and shopping list for the creation of a product, presented in a hierarchical format. The bill of materials includes instructions for how to assemble the product from the various parts that need to be sourced and built.

Bill of Materials illustration


BOMs enable companies to produce high-quality products while avoiding cost overruns and compliance issues. Creating and sharing accurate and up-to-date BOMs with all team members, including supply chain partners, is critical to avoiding production errors that lead to costly scrap and rework and ultimately product delays.


Because BOMs contain a wide assortment of product information, they are accessed by several different disciplines across an organization. This includes engineering, document control, operations, manufacturing, purchasing, suppliers, and contract manufacturers. These teams will frequently collaborate on the information contained in the BOM to ensure that production runs smoothly, and the right product gets built.


The following data should be included in a BOM to ensure that a product is accurately built:

  • BOM Level
    Displays a number for each part or assembly to indicate where it fits within the hierarchy of the BOM
  • Part Number
    Unique numerical code that is assigned to each individual part or assembly to facilitate quick identification
  • Part Name
    Unique name that is assigned to each part or assembly to enable easy identification
  • Phase
    Indicates the stage each part is at in its lifecycle
  • Description
    Provides a detailed description of each part so that teams can easily distinguish between similar items and identify them quickly
  • Quantity
    Indicates number of parts to be used in each assembly or subassembly
  • Unit of Measure
    Measurement in which a part is used or purchased (e.g., each, inches, feet, ounces, drops)
  • Procurement Type
    Describes how each part is purchased or made (e.g., off-the-shelf or made-to-specification)
  • Reference Designators
    If the product includes printed circuit board assemblies (PCBAs), the reference designator indicates where a particular part fits on the board within the BOM
  • BOM Notes
    Captures additional details to keep all team members on the same page


Over the past several decades, BOM management solutions have evolved from manual spreadsheets to more structured and intelligent digital systems which facilitate cross-team collaboration and allow for greater control and traceability of the entire product design.


Spreadsheets are useful if you are starting off with a basic single-level parts list; however, they are nearly impossible to maintain as products increase in complexity and require a multilevel BOM structure. Because spreadsheets are disconnected from the product record and engineering change orders, it is difficult for teams to control changes and keep track of the latest design revision.

Computer-Aided Design (CAD)

BOMs are typically exported from mechanical and electrical CAD (MCAD/ECAD) solutions for use in other downstream systems. Because these systems are disconnected and maintained by engineering, remaining team members (e.g., quality, procurement, manufacturing, suppliers) are unable to easily identify the latest BOM revision.

Product Data Management (PDM)

PDM solutions are used primarily by mechanical engineers to collaborate and manage design iterations. As with CAD solutions, they prohibit remaining product team members and supply chain partners from collaborating on designs and keeping track of the latest BOM revision.

Product Lifecycle Management (PLM)

Enterprise Cloud PLM solutions centralize the entire product record, including BOMs, engineering change processes, quality documentation, and design files (electrical, mechanical, and software) along with associated drawings in a single system. Everything is connected, providing product teams and supply chain partners real-time access from any location. Automated change processes and revision controls are also applied to BOMs to ensure that everyone is always accessing the most current and accurate product information.


How are BOMs structured?

A BOM can be structured as either a single-level or multilevel BOM. A multilevel BOM includes parent-child relationships between each level of the assembly and shows the hierarchical structure for each subassembly that makes up the finished product. A single-level BOM shows all of the parts that make up a single lower-level assembly.

How do I create a bill of materials?

Depending on where you create the BOM, the process may be slightly different. If you use a PLM, quality management system (QMS), or enterprise resource planning (ERP) software solution for instance, you typically create the item master (part/component) record first. Then you create the BOM (assembly) by including all the components and/or subassemblies that comprise the BOM. You will also show quantities, reference designators, and other detailed BOM information required to design and build the assembly. .

If any of the items happen to have their own assemblies (with existing BOM structures), then the software system will automatically leverage the subassembly BOM to create a multilevel BOM.

If you are creating a BOM using a manual approach like spreadsheets or other types of documents, you will need to identify and add all of the components and subassemblies that comprise the BOM structure.

Whether you create the BOM using software or manually in a spreadsheet, you will need to make sure the following information is created:

  • Assign each part or assembly a number to indicate the level at which it resides in the BOM (software will help automate population of lower level subassembly BOMs).
  • Create, then assign a part number for every component that comprises the BOM.
  • Record the unique name of each part or assembly (this is done on the part record or item master for software and automatically reflected in the BOM when you add a component).
  • Record what status/stage each part is at in its lifecycle (e.g., Unreleased, In Design, Prototype, Production). Note: This is done in a more automated and controlled fashion in PLM or QMS software and managed by the engineering change orders (ECOs) that formally release each part or assembly.
  • Provide a detailed description of each part (software systems manage description in the item master/part record).
  • Record the quantity of each component needed to create the BOM (each assembly). For example, a bicycle BOM would need to list 2 tires, 1 seat, 1 chain, 2 rims, 1 frame, and 1 handlebar.
  • Classify the unit of measure (UoM) that will be used to purchase a part. UoMs can be each (EA), inch (IN), feet (FT), and others for liquids or other consumables.
  • Document how each part is purchased or made (i.e., purchased, make, off-the-shelf, or made-to-specification).
  • If the product contains printed circuit board assemblies (PCBAs), include reference designators that detail where the part fits on the board.
  • Include any relevant notes for each part.
  • In ERP systems, routing information may also be added to show the process used to make a given item/component/subassembly on the production floor.
What are the types of bill of materials?

There are three main types of BOMs:

  1. Manufacturing bill of materials (MBOM) – An MBOM consists of a structured list of subassemblies that are needed to produce a shippable product. It includes information on the parts that need processing prior to assembly and shows how the different parts are interrelated within a product.
  2. Engineering bill of materials (EBOM) – An EBOM specifies parts or assemblies that are designed by the engineering team. It shows the component structure from a functional perspective and typically includes a technical or mechanical drawing of the product.
  3. Sales bill of materials (SBOM) – An SBOM provides details of a finished product prior to its assembly during the sales phase. Both the finished product and components appear as separate items in the sales order document.

*Source: https://cadtalk.com/2020/02/27/bom-101-types-of-bills-of-material/

What's the difference between BOQ and BOM?

A bill of quantities (BOQ) is a document which itemizes materials, parts, labor, and their associated costs. A BOQ is used primarily in the construction industry for bidding projects. A bill of materials (BOM) is more hierarchical in nature—providing a detailed list of raw materials, parts, subassemblies, and quantities that are needed to build a finished product. A BOM is used primarily for product manufacturing.

*Source: https://www.omniaccounts.co.za/the-difference-between-bom-and-boq/


  • Bills of materials serve as the recipe for building a finished product. They include a comprehensive list of parts, items, assemblies, subassemblies, and intermediate assemblies, as well as instructions required for gathering and using the required materials.
  • The three main types of bills of materials are manufacturing BOMs (MBOMs), engineering BOMs (EBOMs), and sales BOMs (SBOMs).
  • Creating and sharing accurate and up-to-date BOMs with all team members, including supply chain partners, is critical to ensuring the right product gets built on time and within budget.
  • Cloud-based PLM and QMS solutions aggregate BOMs and other key elements of the product record into a single system—enabling product teams and suppliers to easily create, share, and approve BOMs. Because automated change processes and revision controls are applied to BOMs, everyone can be confident that they are always accessing the most current product information.

Best Practices for Bill of Materials Management

Good bill of materials (BOM) management is essential for accurate and effective manufacturing processes.