What Is Bill of Materials (BOM)?

A bill of materials, or BOM, is the build recipe for a product. It lists every part, sub-assembly and raw material you need, with the quantities, laid out so purchasing can buy it and the shop floor can assemble it. People mix it up with a bill of quantities. And its two main forms, the engineering BOM and the manufacturing BOM, describe the same product at two different stages.

Why is a bill of materials important ?

A bill of materials keeps engineering, purchasing and manufacturing working from the same product definition. Without an accurate BOM, production delays, material shortages and assembly errors become far more likely. That is why the BOM sits at the center of material planning, procurement and manufacturing planning, and why many teams manage it in dedicated BOM software rather than in spreadsheets.

What does a bill of materials include ?

A BOM is more than a shopping list. It sets out the product structure as a hierarchy, and each line defines one component along with everything the plant needs to source it, stage it and build it. A single line usually carries:

Part number, the ID that ties the line to engineering and to stock.
A description a person can read, not just a code.
Quantity: how many the parent assembly needs.
Unit of measure, so pieces, meters and liters never get mixed up.
BOM level: where the item sits, and what nests inside it.
Make or buy, which routes the item to production or to purchasing.
In electronics, a reference designator marking exactly where it goes.

What are the main types of bill of materials ?

Most BOM confusion comes from mixing up different views of the same product. The most common types are:

Engineering BOM (EBOM): the design view from CAD, grouped by function.
Manufacturing BOM (MBOM): the production view, with packaging, consumables and the build order added.
Sales BOM (SBOM): what the customer sees, a finished product and what’s in the box.
A single-level BOM is one flat list of direct components, nothing nested.
A multi-level BOM expands the full tree, every sub-assembly down to raw material.
A configurable BOM runs rules to generate a variant per customer option.

How does an EBOM differ from an MBOM?

An EBOM answers one question: what is the product? It comes straight from design, ordered the way engineers think. An MBOM answers a different one: how do we build it? It takes the same parts and rearranges them into the sequence the line actually uses, then adds the things design never lists: glue, labels, the box it ships in.

The MBOM is used by ERP systems for material planning and by MES software to run production. It forms the basis for a production order; process plants use a recipe instead of a discrete BOM. The EBOM usually lives in a PLM system as part of the product lifecycle, while the MBOM lives in the ERP. If the two fall out of sync, the result is scrap and rework, which is what happens when a BOM is managed in spreadsheets instead of proper BOM management.

EBOM vs MBOM at a glance

Use this table to quickly distinguish between EBOM, MBOM and SBOM.

Aspect	Engineering BOM	Manufacturing BOM	Sales BOM
The question it answers	What is the product?	How do we build it?	What does the customer get?
Who owns it	Design / R&D	Manufacturing	Sales / order management
Where it lives	CAD / PLM	ERP / MES	ERP / CRM
What it adds	Function and geometry	Packaging, glue, build order	Finished items and kits
Used for	Design intent	Running the line	Quoting and shipping

Free EBOM to MBOM sync checklist

It walks the ten spots where an EBOM and MBOM typically drift apart, so you can find those differences before they cause missing parts on the line. Download the checklist (PDF). You can also complete the checklist digitally in flowdit on your phone, tablet or PC.

Related Terms

Work in Progress (WIP) – tracks assemblies part-way through a build.
Manufacturing Operations Management (MOM) – the layer that runs BOM-driven production.
Manufacturing Asset Management – looks after the equipment that consumes the BOM.
Lead Time – set largely by how the BOM sources and builds.