Bill of Materials (BOM): Definition, Types and Examples

How a Bill of Materials Works

A BOM starts with the finished product or top-level asset and breaks it down into every input required to produce or service it. Each item in the list includes a part number, description, quantity, and unit of measure. In hierarchical BOMs, items are organized by level: the finished product sits at Level 0, major assemblies at Level 1, their components at Level 2, and so on down to individual purchased parts or raw materials.

When a production order or maintenance work order is created, the BOM determines what materials need to be pulled from inventory, ordered from suppliers, or prepared in advance. Without an accurate BOM, procurement teams overbuy, technicians show up without the right parts, and production lines stop waiting for components that should have been stocked.

In manufacturing, BOMs are typically managed in a Product Lifecycle Management (PLM) or Enterprise Resource Planning (ERP) system. In maintenance and reliability operations, the same logic applies: a maintenance BOM lives inside a CMMS and is attached to each asset, so every planned work order already knows what materials it needs.

Components of a Bill of Materials

Every BOM entry contains a standard set of fields. The exact fields vary by industry and system, but the core data is consistent across most applications.

Some BOMs also include cost per unit, lead time, make-or-buy designation, and preferred supplier. These fields matter most when BOMs are tied to procurement and cost management workflows.

Types of Bill of Materials

The type of BOM used depends on where in the product or asset lifecycle it applies. Each type serves a different function and is used by a different team.

Engineering BOM (EBOM)

Created by the design or engineering team. The EBOM reflects the product as it was designed, organized by engineering function rather than how it is manufactured. It typically lives in a PLM system and is the starting point before production planning begins.

Manufacturing BOM (MBOM)

Derived from the EBOM, the MBOM reflects how the product is actually built on the shop floor. It accounts for manufacturing processes, sub-assemblies produced in-house, and the sequence of operations. This is the BOM that drives production orders and materials requirements planning (MRP).

Sales BOM (SBOM)

Used when a product is sold as a configurable bundle. The SBOM lists the components that the customer receives as a finished set. It is relevant for configure-to-order and assemble-to-order environments.

Service / Maintenance BOM

Lists the parts and materials associated with maintaining or repairing an asset. Instead of reflecting how something is built, it reflects what a technician needs to service it. Maintenance BOMs are linked to assets inside a CMMS and are used when generating work orders for planned or corrective maintenance tasks.

Configurable BOM (CBOM)

Used for products with variants. A CBOM defines rules for which components apply depending on the product configuration selected. Common in industries like automotive, electronics, and industrial equipment where a single product family has dozens of variants.

Single-Level vs. Multi-Level BOM

The most important structural distinction in BOM design is between single-level and multi-level formats.

A single-level BOM is faster to build and easier to maintain. A multi-level BOM gives procurement and production teams the full picture of how components roll up into assemblies, which is critical for accurate cost roll-ups and lead time planning.

Bill of Materials in Manufacturing vs. Maintenance

BOMs originated in manufacturing, but maintenance teams depend on them just as much, and sometimes more.

In manufacturing

The BOM drives the materials requirements planning (MRP) process. When a production order is released, the system explodes the BOM to determine what raw materials and components must be available on the shop floor. Accurate BOMs prevent production shortages and overstock. Inaccurate BOMs create line stoppages and waste.

In maintenance

A maintenance BOM defines the spare parts, lubricants, filters, seals, and consumables associated with a specific asset. Before a technician starts a job, the CMMS checks the BOM against current inventory levels. If a critical part is not in stock, a purchase request is triggered automatically.

This connection between BOMs and maintenance planning is one of the most direct ways that data quality reduces unplanned downtime. A pump that fails because a technician could not find the correct bearing is a data problem as much as a reliability problem.

How to Create a Bill of Materials

Building an accurate BOM requires input from engineering, operations, procurement, and maintenance. The steps below apply to both manufacturing BOMs and maintenance BOMs.

1. Define the top-level item. Start with the finished product, system, or asset the BOM will describe. Assign it a unique identifier.
2. Break it down into assemblies. Identify the major sub-systems or assemblies that make up the top-level item. These become Level 1 items.
3. List components for each assembly. For each Level 1 assembly, identify the individual parts, materials, and purchased items it requires.
4. Add quantities and units. For every item, specify the exact quantity required and the unit of measure (each, kg, liter, etc.).
5. Assign part numbers. Every item needs a unique identifier. Use an existing numbering convention or establish one before populating the BOM.
6. Validate against drawings and specs. Cross-check the BOM against engineering drawings, OEM manuals, and technical specifications to catch omissions and errors.
7. Load into your system. Enter the BOM into your ERP, CMMS, or PLM system. Link it to the relevant asset, work order template, or product record.
8. Review and maintain. BOMs are not static. Update them when parts are substituted, assemblies change, or new OEM recommendations are issued.

BOM vs. Work Order

A BOM and a work order are related but serve different purposes.

In a well-configured CMMS, every work order template is linked to a BOM. When the work order is created, the system automatically suggests the parts from the BOM, checks inventory availability, and creates purchase requests for anything that needs to be ordered.

Benefits of an Accurate Bill of Materials

• Reduced downtime. Maintenance teams can confirm parts availability before a job starts, not after the asset is already taken offline.
• Accurate cost estimation. Material costs are calculated from the BOM, making labor and parts budgets more reliable.
• Better inventory control. Stock levels for maintenance parts can be tied to actual consumption patterns derived from BOM usage on closed work orders.
• Faster onboarding. New technicians can read the BOM to understand what parts an asset uses, without relying on tribal knowledge from experienced colleagues.
• Fewer errors. A centralized BOM eliminates the risk of technicians ordering the wrong part because they recalled a spec from memory.
• Regulatory traceability. In regulated industries, BOMs provide the documentation trail that demonstrates which materials and parts were used in production or maintenance activities.

Frequently Asked Questions

The Bottom Line

A bill of materials is one of the most fundamental documents in manufacturing and maintenance operations. It defines exactly what is needed to build or service an asset, and it provides the data that procurement, planning, and maintenance teams rely on to do their jobs without unnecessary delays or errors.

For maintenance organizations specifically, the BOM is the link between an asset and its spare parts. When that link exists and is accurate, technicians arrive at jobs prepared. When it does not exist or is outdated, parts are missing, downtime extends, and costs climb.

Building and maintaining accurate BOMs is not a one-time project. It is an ongoing data discipline that pays dividends every time a planned maintenance job runs without a parts delay.