If you’ve ever asked yourself, “what type of bill of materials do I actually need?”, you’re not alone. In my experience working with engineering and manufacturing teams, confusion around BOM types slows down product development, procurement, and production. This guide cuts through the noise.
A bill of materials (BOM) is like a recipe. It tells you what materials are needed, in what quantities, and how everything comes together to produce a finished product. More specifically, a BOM is a detailed structured list of all the components, materials needed, sub assemblies, and instructions required to manufacture a finished product.
In this guide, you’ll see practical BOM examples, understand how BOMs are structured, and learn how different BOM types support real-world workflows.
For a deeper understanding of BOM visualization formats, see: bill of materials types and formats.
What is a Bill of Materials (BOM)?
A bill of materials (BOM) is a detailed structured of all the components, materials needed, sub assemblies, and instructions required to manufacture a finished product.
A typical bill of materials BOM includes:
- Part number (unique identifier)
- Reference designator (or configuration)
- Name and description
- Quantity
- Unit of measure
- Procurement type (make or buy)
BOMs are essential for materials management and serve as the starting point for material requirements planning.
An accurate BOM acts as a single source of truth across engineering, manufacturing, and supply chain teams. Without an accurate BOM, production delays, cost overruns, and inventory issues are almost guaranteed.
BOM Structures vs BOM Types
Before diving deeper, it’s important to clarify a common misconception.
Terms like single-level, multi-level, and flattened BOMs are not different BOM types. They are ways to visualize or represent product structure.
- Single-level BOM: flat list of components
- Multi-level BOM: hierarchical structure with subassemblies
- Flattened BOM: consolidated list derived from a hierarchy
These describe how the BOM is structured or displayed, not why it exists.
BOM types, on the other hand, are defined by their role in the product lifecycle (engineering, manufacturing, service, etc.).
Single-Level BOM (Simple BOM)
A single level BOM is the simplest BOM structure. It shows a flat list of components required to build a finished product, without any parent child relationships. This type of BOM is suitable for products with limited complexity.
When to use a single-level BOM
- Simple products
- No sub assemblies
- Limited manufacturing complexity
Example: Single-Level BOM for a Bookshelf
This BOM lists all raw materials directly. There are no multiple levels or sub assemblies.
| Part Number | Description | Quantity | UOM | Procurement Type |
|---|---|---|---|---|
| BS-001 | Wooden panel | 6 | pcs | Buy |
| BS-002 | Screws | 24 | pcs | Buy |
| BS-003 | Wood glue | 1 | tube | Buy |
| BS-004 | Back panel | 1 | pcs | Buy |
This BOM example lists all the components needed to assemble the bookshelf.
Multi-Level BOM (Hierarchical BOM)
A multi level BOM introduces hierarchy. It shows parent child relationships between assemblies and sub assemblies across multiple levels, allowing for flexibility in updating or swapping other components.
A multi-level BOM is used for more complex constructions and includes subassemblies, often broken down into further levels.
Key concept
- Level 0: Finished product
- Level 1: Major assemblies
- Level 2+: Components and raw materials
Example: Multi-Level BOM Structure
- Level 0: Bookshelf (Finished Product)
- Level 1: Frame Assembly
- Level 2: Side Panels (2)
- Level 2: Top Panel (1)
- Level 1: Fastener Kit
- Level 2: Screws (24)
- Level 2: Dowels (12)
- Level 1: Frame Assembly
This multi level structure reflects how products are actually built. It also supports explosion (top-down) and implosion (bottom-up) views.
Why multi-level BOM matters
- Enables production planning
- Supports supply chain management
- Improves inventory planning and demand forecasts
A multi-level BOM also ensures consistency and accuracy throughout the production process, making it easier to manage updates and maintain quality.
For more detail, see revision control in multi-level BOMs.
Flattened BOM (Consolidated View)
A flattened BOM is a representation of a product structure where all components from a multi-level BOM are consolidated into a single list. It removes hierarchy and shows the total quantities of each part required to build the complete product.
A flattened BOM is typically derived from a multi-level BOM and is used to simplify analysis, procurement, and planning.
Key concept
- All levels are collapsed into one list
- Quantities are aggregated across the entire structure
Example: Flattened BOM for a Bookshelf
| Part Number | Description | Total Quantity | UOM |
|---|---|---|---|
| BS-001 | Side Panels | 2 | pcs |
| BS-002 | Top Panel | 1 | pcs |
| BS-003 | Screws | 24 | pcs |
| BS-004 | Dowels | 12 | pcs |
This view does not show how parts are grouped into assemblies, it only shows what is required in total.
Why flattened BOM matters
- Simplifies procurement and purchasing
- Supports material requirements planning (MRP)
- Helps with cost rollups and total quantity calculations
BOM Types Across the Product Lifecycle
While BOM structures (single-level, multi-level, flattened) describe how product data is organized, BOM types describe how that data is used by different teams throughout the product lifecycle.
In practice, the same product can have multiple BOM types, each tailored to a specific function:
- Engineering BOM (eBOM): defines the product as designed
- Manufacturing BOM (mBOM): defines how the product is built
- Service / Support BOM: defines how the product is maintained and repaired
- Maintenance BOM: supports long-term asset operation and lifecycle management
Each BOM type serves a different purpose but is typically derived from a shared product definition.
Engineering BOM (eBOM)
An engineering BOM comes from CAD or PLM systems. It reflects design intent and includes all components as designed.
What makes eBOM unique:
- Organized by product structure
- Includes technical specifications
- Focused on engineering teams
Example: An electronics eBOM might include printed circuit board (PCB), resistors, capacitors, and integrated circuits.
The engineering bill defines what should be built, not how it is built.
Manufacturing BOM (mBOM)
A manufacturing bill of materials (mBOM) is a comprehensive list of all components, assemblies, and materials needed to produce a finished product. The mBOM shows all parts and assemblies required to manufacture a product, including the assembly steps and processes involved.
The mBOM is essential for the manufacturing process and production process, ensuring that all parts are assembled correctly and supporting efficient, consistent workflows.
Manufacturing companies and manufacturing businesses rely on the mBOM for production planning, inventory management, and cost management. It helps coordinate procurement, scheduling, and automation processes on the factory floor.
For example, a manufacturing BOM for a wooden table specifies parts like four table legs and a table top.
Example: Manufacturing BOM for a Table
| Part Number | Description | Qty | UOM | Operation |
|---|---|---|---|---|
| TB-001 | Table Top | 1 | pcs | Assembly Step 1 |
| TB-002 | Table Legs | 4 | pcs | Assembly Step 2 |
| TB-003 | Screws | 16 | pcs | Assembly Step 2 |
| TB-004 | Packaging Box | 1 | pcs | Final Step |
Why mBOM is critical:
- Drives production orders
- Supports ERP system integration: Enterprise resource planning systems use the mBOM for production planning, cost accounting, and maintenance operations
- Enables efficient production and cost estimation
- Supports cost management by tracking cost per unit, lead time, and supplier information
Materials Requirements Planning (MRP) systems use the BOM to determine what parts need to be ordered and when, ensuring all components are available before production starts.
Connecting OpenBOM and ERP ensures seamless manufacturing operations. Learn more about connecting OpenBOM and ERP.
Service and Support BOM
A service BOM (or support BOM) includes information needed to maintain, repair, or service a product after delivery.
Includes:
- Spare parts
- Replacement components
- Service instructions
Example: Service BOM for an Industrial Pump
| Part Number | Description | Qty | UOM | Notes |
|---|---|---|---|---|
| SP-001 | Seal Kit | 1 | set | Replace every 12 months |
| SP-002 | Bearing Assembly | 2 | pcs | Critical spare part |
| SP-003 | Gasket Set | 1 | set | Required for servicing |
| SP-004 | Lubricant Oil | 2 | L | Maintenance use |
This BOM helps service teams quickly identify required spare parts and perform repairs efficiently.
Why it matters:
- Supports field service teams
- Improves maintenance efficiency
- Reduces downtime
Maintenance BOM
A maintenance BOM is typically used for long lifecycle assets such as industrial equipment or machinery.
Focus:
- Preventive maintenance
- Repair planning
- Lifecycle tracking
Example: Maintenance BOM for a CNC Machine
| Part Number | Description | Qty | UOM | Maintenance Interval |
|---|---|---|---|---|
| MT-001 | Air Filter | 1 | pcs | Replace every 3 months |
| MT-002 | Coolant Fluid | 5 | L | Monthly check |
| MT-003 | Drive Belt | 2 | pcs | Replace yearly |
| MT-004 | Spindle Bearings | 1 | set | Inspect every 6 months |
This BOM supports scheduled maintenance and ensures critical components are available when needed.
It is often derived from engineering and manufacturing BOMs but tailored for operational use.
Other BOM Terms You May Encounter
In addition to the main BOM types, you may come across a variety of BOM-related terms used in industry. These are not fundamentally different BOM types, but rather variations that describe configuration complexity, traceability requirements, or domain-specific views of the same product data.
Configuration and Variant-Oriented BOMs
These terms are typically related to managing product variability, options, and specific instances:
Configurable BOM (sometimes called 150% BOM) – A structure that defines all possible options and variants of a product in a single model. It allows you to generate different product configurations without creating separate BOMs for each variant.
Configuration BOM – A BOM that represents a specific configuration selected from a configurable BOM. It reflects a chosen set of options but may still retain some parametric or rule-based elements.
Resolved BOM – A fully defined BOM generated after all configurations, rules, and options have been applied. It represents an exact, build-ready structure with no ambiguity.
Serialized BOM – A BOM tied to a specific product instance identified by a serial number (S/N). It captures the exact components used in that individual unit, supporting traceability, compliance, and service history.
These BOMs are commonly used in environments where product variability, traceability, and customization are critical.
Discipline-Specific BOM Types
You may also encounter BOMs named based on engineering or functional disciplines. These are not separate BOM types, but rather filtered or specialized views of the same product data:
Mechanical BOM (MBOM in some contexts) – Focuses on mechanical components such as structures, enclosures, and moving parts.
Electronic BOM (EBOM in electronics context) – Lists electronic components such as PCBs, resistors, capacitors, and integrated circuits.
Packaging BOM – Includes materials required for packaging, shipping, and handling (boxes, foam inserts, labels).
Kit BOM – Groups components into kits for assembly, sales, or service purposes. Often used in logistics and field operations.
These views help different teams (mechanical, electrical, logistics, service) work with the same product structure in a way that is relevant to their domain.
Why BOM Types Matter
Different BOM types support different stages of the product lifecycle.
A well-structured BOM:
- Improves supply chain management
- Supports inventory management and inventory replenishment
- Enables demand forecasts
- Ensures efficient procurement
BOMs support quality assurance by ensuring consistency and accuracy throughout the manufacturing process. They also play a key role in cost management by tracking costs, lead times, and supplier information at every stage.
Without the right BOM structure, manufacturing companies face delays, errors, and increased costs.
How to Choose the Right BOM Type
There is no single “right” BOM type.
The appropriate BOM, or combination of BOMs, depends on product complexity, company processes, and organizational structure. Most companies use multiple BOMs across the lifecycle.
| BOM Type | Best For | Audience | Complexity |
|---|---|---|---|
| Engineering BOM | Product design | Engineering teams | High |
| Manufacturing BOM | Production execution | Manufacturing teams | High |
| Service / Support BOM | Maintenance & service | Service teams | Medium |
| Maintenance BOM | Asset lifecycle | Operations teams | Medium |
BOM Management Best Practices
To manage BOMs effectively:
- Use unique identifiers for every part
- Maintain version control
- Standardize units of measure
- Review BOMs regularly
- Integrate with ERP and product lifecycle management systems
Having a central point of control for BOM management ensures consistency and accuracy across departments, especially when updates are made or during the production process.
Avoid spreadsheets. BOM management software ensures accurate data, better collaboration, and efficient production.
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.
Video Demo
Watch the video of all these BOM Type variations and learn how OpenBOM can be used to manage a diverse set of information about the product across multiple systems and lifecycle stages.
There are many BOM types that you might need to manage throughout the product lifecycle. OpenBOM gives you a flexible and robust infrastructure to manage these different BOMs effectively, ensuring that all your product data remains consistent and accessible.
Frequently Asked Questions
What is the difference between a single-level BOM and a multi-level BOM?
A single-level BOM is a flat list of components. A multi-level BOM shows hierarchical parent child relationships and sub assemblies.
What is the difference between an eBOM and an mBOM?
eBOM reflects design intent from CAD. mBOM is adapted for manufacturing with routing, tools, and assembly instructions.
How many BOM types does a product need?
Simple products may only need eBOM and mBOM. Complex products often require multiple BOM types across the product lifecycle.
What should a BOM include?
Part number, reference designator (for electronics), description, quantity, unit of measure, procurement type, and revision level.
What is a configurable BOM?
A flexible BOM that supports product variants without duplicating structures.
When should I stop using Excel for BOMs?
When you need version control, structure, where used, collaboration, or ERP integration.
What is a flattened BOM?
A flattened BOM consolidates all components from a multi-level BOM into a single list.
How does a BOM connect to ERP systems?
BOMs feed ERP (or MRP) systems for production planning, procurement, and inventory management.
Conclusion
Understanding BOM types is essential for building products efficiently. BOM structures (single-level, multi-level) help represent product data, while BOM types (engineering, manufacturing, service, maintenance) define how that data is used across the product lifecycle.
Using the right combination of BOMs ensures accurate production, better cost control, and improved collaboration.
If you want to manage BOMs effectively and move beyond spreadsheets, using a modern BOM management system is the next step.
REGISTER FOR FREE and explore how OpenBOM helps you manage BOMs across the entire product lifecycle.
Best, Oleg
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