What Does BOM Stand For? Bill of Materials Guide

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In manufacturing and ecommerce product development, BOM stands for “bill of materials.” This document lists the raw materials, sub-assemblies, intermediate parts, components, and quantities required to manufacture a finished product. It serves as the master blueprint that tells your team what they need to produce an item.

Operating with an incomplete or inaccurate bill of materials introduces serious financial waste. According to data from Siemens’ True Cost of Downtime 2024 Study, unplanned production downtime costs the world’s largest industrial organizations approximately $1.4 trillion every year, which translates to roughly 11% of their annual revenue. In practice, relying on an outdated BOM triggers production delays, builds up excess inventory, and distorts your unit costs.

This guide explains what a bill of materials is, how it integrates with your daily business operations, and the columns to include in your document. You’ll also learn about the difference between engineering and manufacturing BOMs and practical guidelines to structure your records for production.

What is a bill of materials (BOM)?

A bill of materials is the complete recipe for producing a product. It serves as an assembly component list that details all the components and materials required to build both the finished product and any sub-assemblies that go into it. Whether you manufacture complex electronics like a circuit board assembly, build custom bicycles, or bottle artisanal hot sauces, the BOM functions as the central reference point for your entire production process.

A well-structured bill of materials supports four areas of your manufacturing operations:

• Manufacturing. Your assembly floor relies on the BOM to know how to execute the assembly process. It acts as the step-by-step production bill, showing the hierarchical structure of how individual parts fit together to create finished goods.

• Purchasing.Procurement teams use the BOM to calculate what raw materials and child components to buy from suppliers and contract manufacturers. This ensures you purchase only what is needed, preventing costly excess inventory or critical material shortages.

• Inventory management. By linking the BOM to your inventory control systems, you can automate your inventory planning. When you run a production order, your inventory management software automatically deducts the required parts from your stock records, ensuring your inventory is accurate.

• Production planning. Production planning relies on the BOM to schedule machinery and labor. Knowing the exact materials BOMs require allows planners to schedule production runs based on lead times and material availability.

Engineering vs. manufacturing BOMs

As a product moves through the product development process, it relies on different types of BOMs. Understanding the distinction between these documents prevents translation errors between design and production.

The engineering BOM (EBOM) is created during the design phase, often generated automatically using computer-aided design (CAD) software. The engineering bill focuses on the product’s design structure as conceived by the design team. It includes the technical specifications, dimensions, and materials required for the design to function. 

For example, the design team for an ecommerce brand selling smart insulated mugs may use the EBOM to outline the precise placement of the circuit board assembly, heating element, and ceramic lining.

The manufacturing BOM (MBOM), by contrast, is the operational document used on the factory floor. The manufacturing bill translates the design into a practical, step-by-step assembly guide. It includes not only the physical parts of the final product but also the packaging materials, consumables (such as adhesive, solder, or grease), and the tooling required to perform the assembly. 

Returning to the example: Where the smart mug’s EBOM lists the heating coil, the MBOM also includes the specific epoxy adhesive used to seal the base, the protective peel-off film applied to the display screen, and the custom cardboard shipping carton.

Other types of BOMs

Businesses use other specialized bills of materials depending on their production process and sales strategy:

• Sales BOM. Use a sales bill when selling products as kits or bundles. Instead of listing assembly steps, the sales BOM lists all the finished goods grouped together under a single parent SKU (e.g., a skin care travel kit containing a separate cleanser, toner, and moisturizer).

• Service BOM. Create a service bill to list the specific replacement parts, brackets, and installation tools that field service technicians require to repair a product at a customer’s location.

• Modular BOM. Implement a modular BOM structure for highly customizable products. A modular BOM breaks down complex products into self-contained units. For example, a custom sofa brand may use a modular structure to let customers mix and match different frames, fabric coverings, and leg styles, making it easier to manage inventory requirements for hundreds of configurations.

What to include in a BOM

• BOM level
• Part number
• Part name
• Description
• Quantity
• Unit of measure
• Life cycle stage
• Procurement type
• BOM notes

To create an accurate BOM that keeps your supply chain running smoothly, you need to standardize your document structure. Whether you manage your product data in a spreadsheet or use specialized materials software and BOM software, ensure every bill includes these nine essential columns: 

BOM level

The BOM level is a unique number that establishes the hierarchical structure of the document. For simple products, a single level BOM is sufficient because it lists all parts of one flat level. For complex products, use a multi-level BOM structure to show how parts fit into sub-assemblies. 

This type of document nests child components beneath the parent sub-assembly, showing the relationship between individual parts. For example, a bicycle company may have a complete bicycle at level 0, the frame assembly tabulated at level 1, and the parts needed for assembling the pedal detailed at level 2.

Part number

Assign a unique part number to every item on the list. Never reuse a part number for different items, and avoid using vague descriptions as identifiers. 

Use a consistent, standardized numbering system across all your product lines to prevent purchasing mistakes. Keep the format simple, such as a basic alphanumeric string, to make scanning quick for warehouse staff.

Part name

Provide a clear, standardized name for each component, such as “M5 hex screw” or “left brake lever.” This lets manufacturing teams and suppliers identify parts without needing to look up part numbers in a database. Establish a naming convention, such as “Category_Subcategory_Dimension,” to keep your catalog organized.

Description

Include a brief but specific description of the item. Add material details, colors, dimensions, and tolerances. For a circuit board assembly, specify the exact voltage and capacity. For apparel, list the fabric blend and weight. Clear descriptions help sourcing teams verify that they are purchasing the correct parts from external vendors.

Quantity

List the quantity of each part required to complete one finished product unit. For bulk materials or raw materials like paint, wire, or glue, specify the volume, weight, or length needed. Factor in average scrap rates or waste on the production line so your quantity reflects the amount of raw material consumed during the assembly process.

Unit of measure

Define the unit of measure (UOM) for each quantity, such as “each,” “inches,” “grams,” or “ounces.” Standardizing your UOM prevents assembly teams from using the wrong proportions and ensures your purchasing department buys the correct volume. Avoid generic terms like “box” or “roll” as pack sizes vary between suppliers.

Life cycle stage

Record the current life cycle stage of each component. Label parts as “in design,” “in prototype,” “production-ready,” or “obsolete.” Tracking this data helps you manage product development smoothly and prevents your factory from using outdated component versions. Incorporating life cycle stages into your product lifecycle management (PLM) system ensures everyone uses the latest approved design.

Procurement type

Indicate how your business obtains each component. Use designations like “make” (manufactured in-house) or “buy” (purchased from an external vendor). This field informs your production scheduling system whether to generate an internal production order or a purchase order. For small brands working with contract manufacturers, this column distinguishes which parts you supply versus which parts the factory sources.

BOM notes

Provide a dedicated section for special assembly callouts, critical tolerances, or vendor references. Although full technical manuals and step-by-step assembly guides live in separate operating documents, BOM notes capture quick, vital details like torque specifications or required adhesive cure times. 

This section also lists manufacturer part numbers (MPNs) and pre-approved alternative vendors, ensuring your procurement team can source secondary parts quickly if primary supply chains face delays.