A bill of materials (BOM) is a comprehensive list of parts, raw materials, assemblies, and instructions needed to manufacture, refurbish, or repair a product. It serves as a blueprint for production, detailing each step needed to build the finished product.

Manufacturing vs. engineering vs. sales BOMs

• Engineering BOM: Engineering BOMs focus on the design and technical details of the article. They define the parts and materials used, along with detailed technical specifications, such as CAD models, defined by the engineering team. This BOM is essential for creating prototypes and validating design concepts.

• Sales BOM: A sales BOM reflects the structure of the final product before assembly to allow sales teams and customers to visualize the product without having to assemble it. It is designed to help sales teams generate tailored quotes based on customer specifications. It lists the components and options available to customers, including product configurations and variants, and is brief compared to engineering and manufacturing BOMs.

• Manufacturing BOM: Manufacturing BOMs are used to guide the production process, listing all assemblies and parts required to make the final product. It translates the engineering design into a format that guides the manufacturing team. It lists the components and any additional items needed, such as packaging materials and assembly instructions. It is often more detailed and organized than engineering and sales BOMs.

Required fields

• Part number and name: Each component is assigned a specific name and number and listed in the BOM. For example, a component named “Screw” may be numbered as “SCR-102” with an identifier being part of the number.

• Description: A description adds more details to differentiate between parts. For example, if you manufacture faucets, you might have different models that vary by spout height or handle type. Detailed descriptions help clearly identify each variant and its features.

• Quantity: This field reflects the required quantity of a specific part, such as six or more cores for assembling a processor.

• Unit measure: If a part must be measured, this field indicates how much of it to use. For example, in lamp manufacturing, you need to specify the diameter of the lampshade and the square feet of fabric required to cover it.

• Procurement type: This field indicates whether the components will be sourced or manufactured. Some components are produced internally, while others are purchased.

• Reference designators: This specifies the unique identifier for each component to assist in tracking and identifying its exact position in a product design.

• BOM level: The BOM level indicates the position of parts and assemblies within the hierarchical structure of the product, helping to manage and organize the overall assembly process. For instance, in the manufacturing of a car, Level 0 is the complete car, Level 1 includes major assemblies like the engine and body, Level 2 includes sub-assemblies such as the engine block and car doors, and so on.

BOM hierarchy

Single-level

A single-level BOM includes only the primary components and parts required for assembling a product, without mentioning sub-assemblies. It is used for simple products with few components, such as a table, where each part is listed once along with basic instructions.

Multi-level

Multi-level BOMs show assemblies, sub-assemblies, and their associated components in hierarchical relationships. They are ideal for complex products where understanding the assembly structure is crucial. The finished product is listed at the top while the individual components and sub-assemblies are listed below. If the assembly is changed, it affects the subassemblies and its parts.

Indented

Indented BOMs are a specification of multi-level BOMs where the assembly and subassemblies are displayed in a hierarchy by indentation to show the parent-child relationships.

Steps to create a BOM

Step 1: Identify the finished product

Identify the end product you need to document with the BOM, such as a wooden table.

Step 2: Break down the product structure

Understand and lay out the product design. Break down the product into its major assemblies and sub-assemblies. For a table, that would be:

• Tabletop

• Four legs

• Support beams

• Apron

However, depending on the complexity of product, this is often much more detailed. 

Step 3: List and describe the components

List the components for each assembly and the materials needed. Mention which parts would be sourced and which would be manufactured in-house. Assign a unique part number and name to each item and provide a clear, detailed description.

Step 4: Specify quantities and units

For each component listed, determine the exact quantity and measurement for one unit. For manufacturing multiple units, list the total quantity required.

Step 5: Organize and compile the BOM

Arrange the components into a logical hierarchy that mirrors the product’s structure. Start with the top-level assemblies and work your way down to individual parts. This organization simplifies how components come together to form the final product.

Step 6: Review and update

Conduct a thorough review to ensure the BOM’s accuracy. Update it to maintain an accurate reference, reflecting any changes in design, parts, or suppliers.

Importance of a BOM

A BOM is an important document that shows the relationship between the final product and its parts. It helps estimate the overall cost of production by providing precise information on the quantities and types of materials needed. It ensures that all necessary components are procured in the right quantities and at the right time, minimizing the risk of shortages or overstocking. Accurate BOMs streamline the manufacturing process, reducing production errors and waste, preventing delays, and ensuring a smooth workflow.