Bill of Materials (BOM): What It Is and How to Structure It
A Bill of Materials (BOM) is the document that details every component, raw material, and subassembly needed to manufacture a product, along with the exact quantity of each. It works as a company’s manufacturing recipe: without it, there’s no systematic way to know what needs to be bought, when, and in what quantity.
What a bill of materials is
It’s a structured document that lists, piece by piece, everything needed to assemble or manufacture a product: from basic raw materials to already-assembled subassemblies. Its main purpose is to prevent production from relying on knowledge scattered across different team members, centralizing that information into a single reference document.
What a BOM contains
A well-built BOM includes, at minimum:
- Number and name of each part, component, or subassembly.
- Quantity of each item needed to build one unit of the final product.
- Description and technical specifications that distinguish one part from a similar one.
- Unit cost and total cost per component.
- Sourcing information: supplier, lead time, and availability.
The structure is usually presented hierarchically: the finished product at the top level, and components and subcomponents at lower levels, down to basic raw materials.
Single-level versus multi-level structure
There are two main ways to represent this list depending on product complexity:
| Structure | What it shows | When to use it |
|---|---|---|
| Single-level | Direct components of the final product, without breaking down subassemblies | Simple products with few components |
| Multi-level | Full hierarchy: subassemblies and their own components | Complex products with assemblies within assemblies |
A simple example: the parts list for a table (single-level) can be limited to four legs, a tabletop, screws, and nuts. A bicycle (multi-level) needs to break down, beyond the wheels and frame, the internal components of each wheel or the gear system.
Types of BOM by function
Beyond structure, these lists are also classified according to their purpose within the company:
- Engineering BOM (EBOM): used during the design phase, shows components from the technical perspective of the product as originally conceived.
- Manufacturing BOM (MBOM): the most widely used, describes how components are actually assembled in production, including processes and timing.
- Sales BOM (SBOM): lists finished products and components as they’re sold, useful for orders that bundle several items together.
It shouldn’t be confused with an MTO (Material Take Off), a similar document but specific to the construction industry rather than product manufacturing.
Why keeping it up to date matters
An outdated or inaccurate bill of materials is one of the most common causes of costly production errors: wrong purchases, inventory mismatches, delays caused by a component nobody had recorded. The document isn’t built once and forgotten, it needs constant review whenever a design, material, or supplier changes.
The benefits of managing it well are concrete:
- Reduces excess inventory and prevents stockouts of critical components.
- Improves purchasing accuracy, avoiding unnecessary or insufficient orders.
- Makes it easier to calculate the real production cost, component by component.
- Allows production planning based on reliable timing and availability data.
How it connects to the rest of the operation
The bill of materials doesn’t work in isolation, it’s the starting point for other critical processes. Without an accurate BOM, an MRP system can’t correctly calculate which materials are needed or when to launch each purchase or production order. And every component listed needs, in turn, a consistent identification code, usually a SKU, so the ERP and the warehouse can locate, purchase, and control it without ambiguity.
When a company relies on external suppliers for several components in its bill of materials, keeping visibility into the status of each order, whether in transit or in production at the supplier’s plant, is what prevents a single delayed component from halting the entire production line. Automating this list with software connected to the ERP, instead of managing it in loose spreadsheets, is what keeps this data consistent across design, purchasing, and production.
Frequently asked questions
What does BOM stand for?
BOM stands for Bill of Materials. It’s the document that details every component needed to manufacture a product, along with its exact quantities.
How is it different from an MTO?
An MTO (Material Take Off) is a similar document but specific to the construction industry, while a BOM applies to product manufacturing processes. Both serve a similar purpose, listing required materials, but in different contexts.
Who is responsible for keeping it up to date?
It’s usually a shared responsibility between engineering (design changes), purchasing (supplier or cost changes), and production (assembly process changes). Without a clear document owner, it’s easy for it to become outdated.
Is it necessary for small companies with few products?
Yes. Even with a small catalog, not having this document structured usually leads to improvised purchasing, production errors, and knowledge scattered among team members instead of documented systematically.
How does it relate to inventory management?
It determines which components need to be in stock and in what quantity to meet planned production. It’s one of the key inputs for calculating replenishment needs and avoiding both excess and shortage of materials.