What Is a Bill of Materials? Examples, Formats, and BOM Types

If you design, build, source, manufacture, or service products, the bill of materials is one of the most important documents in the entire business. It sits at the center of engineering, procurement, inventory, production planning, and service. Without a clear BOM, teams struggle to align on what the product is, which raw materials and parts must be purchased, how the assembly process works, and what the organization must do to deliver the final product on time.

A bill of materials is often described as a recipe, and that comparison works well. A recipe tells you the ingredients, quantities, sequence, and outcome. In the same way, a bill of materials BOM tells you what parts, materials needed, sub assemblies, and quantities are required to make a finished product. It helps teams identify all the components, all the raw materials, and any other components needed to move from design to the shop floor and then into service.

But many teams still confuse two different ideas. Some people use terms like single level BOM, multi level BOM, indented BOM, and flattened BOM as if they are BOM types. In practice, those are better understood as ways to represent or visualize the BOM structure. The real lifecycle BOM types are things like engineering BOM, manufacturing BOM, service BOM, and maintenance BOM. That distinction matters because product teams need both: the right format for the job and the right BOM type for each stage of the product lifecycle.

In this guide, you will learn what a BOM is, what it includes, how a bill of materials example looks in practice, how BOM formats differ, and how BOM types support engineering, manufacturing, service, and operations. You will also see how better BOM data improves supply chain management, inventory management, production planning, and efficient production processes across the manufacturing industry.

What Is a Bill of Materials?

A bill of materials is a structured list of all parts, materials, components, and sub assemblies required to make a product. More than a simple list, it defines the relationship between assemblies and parts, the quantities needed, and often key details such as revisions, units of measure, procurement sources, and references to the production process.

A good BOM becomes the single source of truth across the organization. The engineering team uses it to define the product as designed. Procurement uses it to buy the components required and raw materials. Manufacturing uses it to guide the assembly process and shop-floor execution. Service teams use it to identify spares and replacement items. Finance and operations use it for accurate cost estimation, unit costs, and visibility into associated costs.

In other words, a BOM supports much more than design documentation. It supports materials management, inventory control, production scheduling, and broader supply chain coordination. If the BOM is incomplete or wrong, teams face production delays, expensive rework, sourcing issues, and confusion on the production line. If the BOM is current and structured well, it becomes the foundation for better collaboration, better planning, and more reliable delivery of both the finished product and service parts over time.

For a manufacturing company, the BOM is also essential to efficient resource planning. It helps answer practical questions such as:

• What components needed must be purchased now?
• Which packaging materials are needed before shipping?
• What is the expected lead time for long-lead parts?
• How do changes in design affect manufacturing operations?
• How do we avoid excess inventory while still meeting customer demand?

These are not abstract questions. They are daily operational decisions, and they depend on a well-managed BOM.

What Does a Bill of Materials Include?

A modern BOM should provide enough detail for different teams to work from the same product definition. At a minimum, a bill of materials often includes:

• Part number or item number
• Part name and description
• Quantity
• Unit of measure
• Revision level
• Procurement type, such as make or buy
• Supplier or vendor reference
• Reference designators for electronics
• Notes or instructions
• Parent-child relationships for assemblies
• Cost fields or rollup values when needed

Depending on the industry and product, a BOM may also include electrical components, approved manufacturers, alternatives, compliance data, routing references, and links to drawings created in computer aided design systems.

Here is a simple materials template style example of what a BOM table might contain:

Part Number	Description	Qty	UOM	Buy/Make	Revision	Notes
ST-001	Seat panel	1	pcs	Make	A	Birch plywood
ST-002	Stool leg	4	pcs	Make	A	Hardwood
ST-003	Screw	12	pcs	Buy	A	Zinc plated
ST-004	Wood glue	1	tube	Buy	A	Assembly use
ST-005	Protective finish	1	can	Buy	A	Final coat

This kind of structure makes it easier to see all the materials, materials needed, and the components required to build the product. Even a small materials sample like this shows why a BOM is more than a note in a spreadsheet. The same structure can scale from a simple stool to a complex machine with multiple components, variants, and deeper parent-child relationships.

Bill of Materials Example for a Simple Product

A good bill of materials example makes the concept immediately clear. Let’s use a simple wooden stool. This is not a highly engineered product, but it still shows how BOM thinking works.

Sample BOM for a Wooden Stool

Part Number	Description	Qty	UOM	Type
WS-001	Seat panel	1	pcs	Make
WS-002	Stool leg	4	pcs	Make
WS-003	Cross brace	4	pcs	Make
WS-004	Wood screw	16	pcs	Buy
WS-005	Wood glue	1	tube	Buy
WS-006	Sandpaper	2	sheets	Buy
WS-007	Protective finish	1	can	Buy

This BOM example shows all the components needed to produce the stool. The wood parts are made internally. The screws, glue, and finish are purchased. From a planning perspective, this also becomes a materials example for cost rollups, purchasing, and work instructions.

Why does this matter? Because even a simple product requires coordination. The manufacturing team needs wood stock and purchased supplies. Purchasing must plan around lead time and supplier availability. Operations must align the BOM to the production routing and determine the sequence of the assembly process. Finance wants to track unit costs and associated costs. Quality wants to verify the correct parts are used. Service may later want a simpler service list if the stool is sold in kits.

This is why a bill of materials example is so useful. It connects concept to execution. It shows how all the raw materials and components flow into the final product, and how a BOM supports the broader materials management process.

BOM Visualization and Formats

Before discussing BOM lifecycle types, it is important to separate how a BOM is visualized from why it exists.

A BOM can be displayed as a flat list, a hierarchy, an indented BOM, a rollup, or another structured view. These formats help different teams understand the product from different angles. But they are not the same as lifecycle BOM types.

Single-Level BOM

A single level BOM, sometimes called a single level bill, is the simplest representation. It is a flat list of all parts and materials required to build a product, without showing parent-child relationships.

This format works best for simple products with few or no nested assemblies. It is easy to read, easy to export, and useful for quick purchasing decisions. A single-level view is often enough when there are no complex sub assemblies or when the organization simply needs a straightforward list of components needed for procurement.

The limitation is visibility. A single level BOM does not show how parts belong together, how subassemblies are structured, or what depends on what. As soon as products become more sophisticated, teams usually need more than a flat list.

Multi-Level BOM

A multi level BOM is a hierarchical structure that shows parent-child relationships between assemblies, parts, and lower-level items. It is the standard representation for complex products, especially where there are multiple levels of assemblies and dependencies.

In a multi-level structure, the top item is the finished product, often called Level 0. Below that are major assemblies, then lower-level components and raw materials. Each BOM level represents a different place in the hierarchy.

A simple multi level bill for a stool might look like this:

• Level 0: Wooden Stool
  • Level 1: Seat Assembly
    • Level 2: Seat Panel
    • Level 2: Protective Finish
  • Level 1: Leg Assembly
    • Level 2: Stool Leg x4
    • Level 2: Cross Brace x4
    • Level 2: Wood Screw x16
    • Level 2: Wood Glue

Indented (Multi-level) BOM and Modular BOM Structure

An indented BOM is a visual version of the hierarchy that displays the relationships with indentation. It is often the most readable way to review a BOM structure because you can immediately see parent-child relationships.

A modular BOM or modular BOM structure is another useful concept, especially for configurable or platform-based products. Instead of treating the entire product as one monolithic structure, a modular approach groups related parts into modules that can be reused across products or options. This is especially useful when organizations manage product families, variants, or platforms.

Flattened BOM or Rollup View

A flattened BOM removes the hierarchy and consolidates quantities across all levels into one complete list. It is often used for purchasing, cost rollups, and planning because it shows the total quantity of each part needed to build one unit or a batch.

For procurement and operations, this is often the easiest way to see everything needed. A flattened list supports inventory control, purchase planning, and material demand calculations. It can also help teams control inventory and reduce excess inventory by making totals easier to understand.

BOM Types Across the Product Lifecycle

Now we get to the BOM types that are defined by business purpose and lifecycle role.

A single product may need multiple BOM types. One view supports design, another supports shop-floor execution, another supports service, and another supports long-life maintenance. These are not duplicates for the sake of duplication. They exist because engineering, manufacturing, service, and operations use product data differently.

Engineering BOM (eBOM)

An engineering BOM, sometimes informally called an engineering bill, defines the product as designed. It is usually created from CAD or PLM environments and reflects design intent. This is the BOM the engineering team uses to capture structure, specifications, geometry-linked items, and relationships derived from computer aided design models.

The engineering BOM answers the question: what should the product be?

For a mechanical product, the eBOM may include fasteners, housings, structural parts, electrical components, and documentation references. For electronics, it may include PCB-related items, reference designators, and approved parts. In many companies, the eBOM is central to product lifecycle management, because it connects design revisions, approvals, and change control.

However, the engineering BOM is not always the same as the BOM needed in manufacturing. Design intent does not automatically equal shop-floor execution.

Manufacturing BOM (mBOM)

A manufacturing BOM, or manufacturing bill, defines how the product is actually built. This is the BOM used in the manufacturing process and on the shop floor. It supports production planning, production order creation, production scheduling, and execution across the production line.

The manufacturing BOM answers the question: how will we build the product?

Unlike the engineering view, the manufacturing BOM may include consumables, operation-specific grouping, packaging items, bulk materials, and packaging materials not always present in the design structure. It may also align parts to work centers, routing steps, and production routing. In some businesses, it includes kitting logic for workstations or external suppliers. It is often the BOM consumed by ERP and MRP systems to drive manufacturing operations, material allocations, and procurement requirements.

Consider a simple table assembly. The eBOM might show table top, legs, brackets, and hardware. The manufacturing BOM may add glue, labels, corner protectors, carton packaging, and grouping by operation sequence. It may also reflect how a contract manufacturers partner receives or builds the product.

This is why the difference between engineering BOM and manufacturing BOM is so important. The eBOM defines the product as designed. The mBOM defines the product as built. Together, they connect design to execution and help organizations avoid misalignment, scrap, and production delays.

eBOM vs. mBOM

The gap between design and manufacturing is where many companies struggle.

The eBOM is optimized for engineers. The mBOM is optimized for production. The eBOM may reflect the logical design breakdown. The mBOM may group items according to operations, sourcing, kits, or manufacturing cells. The eBOM may start in CAD. The mBOM may live downstream in ERP or MES-linked workflows.

When the two are disconnected, organizations lose trust in the BOM. When they are connected, teams can better manage BOMs, keep BOM data synchronized, and maintain an up to date BOM that supports both design and production.

Service / Support BOM

A service or support BOM is designed for maintenance, repair, and post-sale operations. This BOM helps service teams quickly identify spare parts, replacement kits, and service-relevant assemblies.

For example, an industrial pump may have a service BOM that includes seals, bearings, gaskets, and lubricants. A customer support team does not need the full design definition in every situation. They need the correct serviceable items, the common replacement parts, and the right documentation to support uptime.

This type of BOM improves service response and reduces downtime. It also helps organizations align service readiness to customer demand, warranty planning, and spare inventory strategy.

Maintenance BOM

A maintenance BOM is especially useful for long-life industrial equipment, facilities, or assets that require scheduled upkeep. It supports preventive maintenance, repair planning, lifecycle tracking, and ongoing operational readiness.

A maintenance BOM may include filters, belts, consumables, bearings, lubricants, and service intervals. It is commonly used in environments where products remain in operation for years and where uptime is critical. In these cases, BOM discipline is not only about building the product once. It is about sustaining it across its service life.

Other BOM Terms You May Encounter

Beyond the four core lifecycle BOMs, there are several related terms that often appear in practice.

A configurable BOM or configurable bill is used when products are offered with selectable options. Instead of maintaining a separate BOM for every possible variant, a configurable structure can define all options in one model and generate resolved BOMs per configuration.

A sales BOM or sales bill is typically used for quoting, selling, or commercial packaging of products. It can group items in a way that makes sense for sales, service offerings, or bundled products, even if the underlying engineering structure is different.

A modular BOM supports products built around reusable modules. This is useful in platform strategies and complex configurable products where common modules can be shared across variants.

These related BOM concepts become increasingly important as product complexity grows and as organizations try to meet diverse customer demand without losing control of cost or product data.

Why Accurate BOMs Matter for Production, Inventory, and Supply Chain

The business value of BOM accuracy is hard to overstate.

An accurate BOM improves production planning because teams know what materials and parts are needed, in what quantity, and at what time. It improves inventory management because item demand can be tied to real product requirements instead of guesswork. It improves supply chain management because purchasing teams can coordinate suppliers, manage lead time, and react faster to shortages.

A good BOM also supports inventory control and helps organizations control inventory with more discipline. If part quantities are unclear, companies often overbuy to protect themselves, which drives excess inventory. If the BOM is incomplete, companies risk shortages, late changes, and avoidable production delays.

For finance and operations, a reliable BOM supports accurate cost estimation. Material quantities, unit costs, and rollups can be traced back to product structure. This is essential for quoting, margin planning, and profitability analysis.

For quality and compliance, the BOM supports quality control by ensuring the right parts are used consistently. For procurement, it supports materials management and the broader materials management process. For the factory, it supports the production process, routing alignment, and better execution of efficient production processes.

In short, a good BOM helps organizations move from reactive coordination to repeatable, scalable execution.

How to Choose the Right BOM Types for Your Organization

There is no single BOM setup that works for every company, every product, or every workflow. The right setup depends on product complexity, lifecycle stage, audience, and manufacturing model.

If you produce simple products with short supply chains, you may only need a basic eBOM and mBOM structure. If you manage service-heavy products, you will likely need a service BOM. If you support industrial assets with long operating lives, a maintenance BOM becomes essential. If your products are highly configurable, a configurable BOM or modular BOM structure may be necessary.

A useful decision framework looks like this:

• Engineering BOM for design definition and change control
• Manufacturing BOM for production execution and material consumption
• Service BOM for repair, support, and spare parts
• Maintenance BOM for long-term asset operation
• Sales BOM for customer-facing bundles or commercial structure
• Configurable BOM for product options and variants

For organizations with complex products, the challenge is not whether to use multiple BOMs. The challenge is how to keep them aligned. That is where strong BOM management, digital workflows, and connected systems matter.

BOM Management Best Practices

To manage BOMs effectively, organizations need more than a spreadsheet. They need disciplined structure, clear ownership, and systems that can keep product data aligned across teams.

Here are the most important practices:

Use unique part numbers. Every item should have a stable identifier. This prevents confusion when descriptions change or when similar parts exist.

Standardize naming and units. Consistent descriptions and unit-of-measure rules make BOMs easier to maintain and easier to use in ERP, purchasing, and planning.

Apply revision control. BOMs change. New parts are introduced. Approved manufacturers change. Packaging changes. If teams do not control revisions, the wrong material may be ordered or built.

Keep an up to date BOM. A BOM that was correct last quarter but wrong today is still dangerous. Teams need a shared, current version of the truth.

Connect BOMs to process data. A BOM is strongest when connected to routing, planning, sourcing, and cost rollups. That includes links to the production bill, assembly bill, or operation-level planning data.

Support collaboration across functions. Engineering, procurement, planning, manufacturing, quality, and service should not be working from separate, disconnected files.

Use modern BOM software. Dedicated BOM software improves visibility, revision control, traceability, rollups, reuse, and collaboration. It also reduces the limitations of spreadsheet-based BOM tracking.

For companies that want to scale, better BOM management is not optional. It is a core capability that supports engineering speed, sourcing resilience, and execution quality across the product lifecycle.

Conclusion

A bill of materials is far more than a parts list. It is the product definition that connects design, sourcing, manufacturing, service, and lifecycle operations. It defines the materials needed, the relationships between assemblies, the quantities of raw materials, and the product structure required to deliver the finished product.

It is also important to separate BOM formats from BOM types. A single level BOM, multi level BOM, indented BOM, or flattened structure describes how BOM data is represented. An engineering BOM, manufacturing BOM, service BOM, or maintenance BOM describes how that same product data is used across the business.

The best BOM strategy depends on your products, your workflows, and your organizational needs. But the pattern is consistent: companies that maintain strong BOM discipline improve supply chain management, inventory management, production planning, cost visibility, and execution across the manufacturing process. They reduce risk, respond better to change, and support more efficient production processes over time.

If your team still relies on scattered spreadsheets and disconnected files, this is usually the point where BOM problems start to compound. Moving to better BOM software and stronger BOM management practices helps teams keep BOM data aligned, reduce production delays, and build a more reliable path from design to delivery.

OpenBOM helps teams manage bill of materials structures, revisions, rollups, and collaboration across engineering and manufacturing. Register for free to see how a modern BOM system improves product data management.

Explore BOM management software and item and BOM management. Learn more in BOM management best practices and avoid common issues like these 10 BOM mistakes that kill product launches.

Best, Oleg