Engineering BOM vs Manufacturing BOM for PCF: What's the Difference and Which Should You Use?

Two teams in the same company can maintain different bills of materials for the same product, and both can be correct. An engineering BOM represents the product from a design perspective, while a manufacturing BOM organizes the product structure from a manufacturing perspective.

For a Product Carbon Footprint (PCF), the relevant BOM data should provide the materials, components, quantities, and other product-structure information needed to build the inventory within the defined system boundary. Using an incomplete or inappropriate BOM view can result in relevant inventory data being missed or represented incorrectly, affecting the calculated PCF and its traceability.

This blog explains how eBOM and mBOM differ, how each can contribute to PCF data collection, and how to determine which BOM data is appropriate for the calculation.

For a cradle-to-gate PCF, the engineering BOM (eBOM) can provide the design structure, including the components and materials specified for the product. The manufacturing BOM (mBOM) provides a manufacturing-oriented view of the product and can include the materials and components needed for production and assembly. Rather than treating either BOM as universally sufficient, use the BOM or combination of BOM data that best represents the product inventory within the defined PCF scope and system boundary.

In practice, using both can provide a broader basis for reviewing PCF inventory data. The eBOM can help confirm the product's designed components, materials, specifications, and quantities, while the mBOM can provide manufacturing-oriented information relevant to how the product is produced and assembled. The two can be reconciled to help make the PCF inventory consistent and traceable and to identify potentially missing inventory data.

A BOM is only one source of PCF inventory data. A cradle-to-gate assessment can also require information about manufacturing processes, energy use, material losses or waste, transportation within the defined boundary, and other relevant activities. The appropriate data depends on the product, study scope, system boundary, and applicable PCF methodology.

Quick definitions:

An Engineering Bill of Materials (eBOM) is developed from the product design and engineering perspective. It defines the product structure based on the components, materials, specifications, and requirements established during product development.

An eBOM commonly lists parts, components, and subassemblies and may include detailed engineering information such as product specifications, engineering standards, and tolerances. It is primarily concerned with how the product is designed and what components are specified as part of that design.

For carbon accounting, this distinction matters. An eBOM can help identify the materials, components, and quantities specified for a product, but it may not contain all the information needed for a PCF calculation. Supplier information, manufacturing processes, production energy use, material losses, packaging, transportation, and other inventory data may be maintained in separate systems or records.

What the eBOM is good for in a PCF:

Where the eBOM may have limitations:

A Manufacturing Bill of Materials (mBOM) is created and maintained from the manufacturing perspective. It represents the materials, components, and assemblies needed to manufacture and assemble a product and organizes them according to how production is carried out.

An mBOM is often derived from or linked to the eBOM, but it can reorganize the product structure to reflect manufacturing requirements. Depending on the company's processes and systems, an mBOM may also be connected to production operations, work centers, routings, tooling, procurement information, or supplier records. Packaging may also be included when it is part of the manufacturing or product-fulfillment structure, although this varies by company.

For PCF calculations, the mBOM can provide useful manufacturing-oriented information that may not be represented in the eBOM. However, an mBOM should not automatically be treated as a complete PCF inventory. Additional information may be needed for manufacturing energy, process inputs, material losses, waste, transportation, packaging, and other activities within the defined PCF scope.

What the mBOM is good for in a PCF:

Where the mBOM needs care:

The table below highlights differences that can matter when collecting and reviewing PCF data. Exact BOM structures and data fields vary between companies and systems.

A PCF depends on the completeness and quality of inventory data within the defined system boundary. The calculation should identify the life-cycle stages, processes, materials, and inputs included in the assessment and apply any relevant exclusions or cut-off criteria consistently.

Differences between eBOM and mBOM data can affect the calculated PCF when they result in different inventory inputs, quantities, or configurations being included.

Packaging may be maintained outside the eBOM and may be included in or linked to manufacturing or fulfillment data. If packaging falls within the defined PCF scope, the calculation should account for the relevant packaging materials regardless of where that data is maintained.

Supplier information can help identify relevant inventory or emissions data for specific components. An mBOM may be linked to supplier or procurement information, depending on the company's systems. Where suitable supplier-specific or other primary data is unavailable, appropriate secondary or other applicable data sources may be used.

The eBOM and mBOM may organize the same product differently. When both are used, components, quantities, units, revisions, and relationships should be reconciled. Differences in structure do not inherently cause errors, but poor mapping can result in omissions or double counting.

Manufacturing may involve materials or inputs that are not represented in the eBOM, such as adhesives or process materials. Whether these belong in the PCF depends on the defined system boundary, inventory requirements, and applicable cut-off criteria.

The appropriate BOM data depends on the purpose of the calculation, the defined PCF scope and system boundary, and the quality and availability of the underlying product and manufacturing data. In some cases, an eBOM may provide the starting point; in others, an mBOM may provide additional manufacturing information. Using both can help identify and reconcile differences between the designed product structure and the manufacturing structure.

A useful way to think about the distinction is that the eBOM describes the product from a design perspective, while the mBOM describes the product from a manufacturing perspective. For a PCF, neither structure should automatically be treated as the complete inventory. The relevant BOM data should be reconciled with other required information, such as manufacturing processes, energy use, material losses, packaging, transportation, supplier data, and emission factors, according to the defined PCF scope.

When both an eBOM and mBOM are available and relevant to the PCF, reconcile them as part of building the PCF inventory. This helps identify differences in components, materials, quantities, and manufacturing-related inputs.

The goal is to reconcile relevant information from both structures and connect it with the broader inventory data required for the PCF.

The engineering BOM represents the product from a design perspective, while the manufacturing BOM represents the product from a manufacturing perspective. For a PCF, the appropriate BOM data depends on the product, calculation stage, system boundary, and available inventory data.

The practical approach is not to assume that one BOM is universally sufficient. Reconcile the relevant eBOM and mBOM information, review differences against the defined PCF scope, and connect the BOM data with the other inventory information required for the calculation.

This provides a more consistent and traceable basis for the PCF while keeping the calculation aligned with the product and manufacturing configuration being assessed.