Net Weight vs Gross Weight in a BOM: Why Scrap Changes Your PCF

A part that weighs 1 kg on the drawing may require more than 1 kg of material to make. Some of that material may be cut away, removed during processing, or rejected during production. Even though it does not remain in the finished part, the material used to produce it can contribute to the product's carbon footprint.

This is where net weight vs gross weight in a BOM matters. Net weight is the amount of material in the finished part. Gross weight, also called gross material input, is the total material put into production, including material later lost as scrap. The difference can come from scrap, yield loss, and other material losses during manufacturing. If a PCF calculation uses only the finished part's net weight, it may miss emissions linked to material that was consumed during production.

This blog explains the difference between net weight and gross weight, shows how scrap can affect PCF calculations, walks through a simple example, and explains the data needed to account for material losses accurately.

Net weight is the mass of a component or material that remains in the finished product. Gross weight, also called gross material input, is the total amount of material required or consumed to make the part, including material later lost as scrap, trimming, or rejected parts.

The relationship can be expressed simply when scrap is the only production loss being considered:

Gross weight = Net weight + Scrap

For example, if a finished component weighs 1 kg and 0.5 kg of material is scrapped during production, the gross weight is 1.5 kg.

Here is how they compare:

Neither measure is inherently right or wrong. They describe different quantities. Net weight tells you how much material remains in the finished component, while gross weight tells you how much material was required or consumed to produce it.

For PCF calculations, the relevant quantity depends on the defined system boundary, manufacturing process, available data, and chosen methodology. Using only the net weight may exclude material losses that are relevant to the product system, while using gross weight requires consistent treatment of scrap and other production losses.

An engineering BOM describes the product as designed. It typically lists the parts, quantities, materials, and specifications needed to define the finished product. The weight of a part generally represents the mass of the finished component rather than all the material that may be consumed during manufacturing.

A PCF has a different purpose. It quantifies the greenhouse gas emissions associated with a product within a defined system boundary. This can include emissions from raw materials, manufacturing processes, transportation, and other life cycle stages, depending on the scope of the assessment. The GHG Protocol Product Life Cycle Accounting and Reporting Standard and ISO 14067 provide frameworks for defining and quantifying these emissions.

This means the material quantity used in a PCF may differ from the net weight shown in an engineering BOM. If manufacturing requires additional material that is later removed as scrap or lost during processing, that material input may need to be included in the product inventory, depending on the defined system boundary and accounting methodology.

For example, a supplier may start with 1.5 kg of metal to produce a 1 kg finished component. The engineering BOM may show the finished component as 1 kg, while manufacturing or production records may show 1.5 kg of material input. The additional 0.5 kg represents material that does not remain in the finished component and needs to be addressed in the PCF calculation according to the selected methodology.

For PCF data collection, an engineering BOM and manufacturing or process data can therefore serve different purposes. The engineering BOM can provide the composition, quantities, and net mass of the finished product, while manufacturing records, process data, purchasing records, or supplier data can provide information about material inputs, yields, and production losses.

Neither source should automatically replace the other. The appropriate data depends on the manufacturing process, system boundary, calculation method, and level of data available.

Scrap can arise at different stages of manufacturing, and the amount and type of loss depend on the production process. Common sources include:

Some production scrap may be reused within the same process, recycled, sold for recovery, or sent for disposal. The appropriate treatment in a PCF depends on the defined system boundary and the selected accounting methodology. The way scrap and recycling are handled can therefore affect the calculated PCF result.

Scrap can affect a PCF in several ways, depending on the manufacturing process, system boundary, and accounting methodology used.

When more material is required to produce a finished component, the emissions associated with that additional material input may need to be included in the product inventory. Using only the net weight of the finished component can therefore exclude emissions associated with material that was consumed and subsequently became scrap.

Material that becomes scrap may still have gone through manufacturing processes before it was removed or rejected. The energy and other process inputs associated with those activities can contribute to the product footprint when they fall within the defined product system.

For example, material that is heated, machined, formed, or otherwise processed before being discarded can have process emissions associated with it, even though it does not remain in the finished product.

What happens to production scrap can also affect the calculation. Scrap may be reused within the process, recycled, sent for recovery, or disposed of. The appropriate treatment depends on the system boundary and the selected accounting methodology, including how recycling and allocation are handled.

The key point is that scrap rate can be an important input to a PCF calculation. It helps show how much material is required to produce the finished product and provides information that can be used to account for production losses consistently.

The numbers below are illustrative only. They are not real emission factors or industry benchmarks. They are used only to show how material input can affect a PCF calculation.

Suppose a machined metal bracket has these characteristics:

In this simplified calculation, using only the finished part's net weight results in 1.0 kg CO2e less material-related emissions than using the 1.5 kg gross weight. The difference comes from the 0.5 kg of material lost during machining.

The same principle can apply across a BOM with hundreds of components. Differences in material input, yield, and production losses can affect the overall PCF result when those inputs are relevant to the defined system boundary and accounting methodology.

This calculation excludes process energy, transport, and the treatment or recovery of production scrap to keep the calculation simple. A complete PCF would account for the relevant life cycle stages and flows according to the selected scope and methodology.

Scrap rate and yield describe related but different measures of material use. The calculation depends on the basis used for the percentage, so that basis should always be stated clearly.

Yield is the share of material input that becomes the finished product.

Scrap rate based on gross weight is the share of material input that becomes scrap.

Using the bracket above:

A 33.3% scrap rate based on gross weight does not mean that gross weight is 33.3% higher than net weight.

If a supplier reports a 33.3% scrap rate based on gross weight, the corresponding gross weight is:

Gross weight = Net weight ÷ (1 − Scrap rate)

For a 1.0 kg finished part:

1.0 ÷ (1 − 0.333) ≈ 1.5 kg

A scrap percentage should therefore always be recorded together with its calculation basis before it is used in a PCF calculation.

Once material losses are identified, the calculation also needs to account for what happens to the scrap. Production scrap may be reused in the process, recycled, recovered, or disposed of. The appropriate treatment depends on the defined system boundary and the accounting methodology being applied.

Recycling requires particular attention because different approaches can assign the environmental burdens and benefits of recycling differently. The selected approach should be documented and applied consistently.

Recycled content in purchased material also needs to be considered. The emission factor should be appropriate for the material and the data source used. A recycled-content material should not automatically be represented by the same factor as a primary material.

Clear boundaries and consistent accounting help avoid double counting. The same material flow should not be counted twice or receive overlapping treatment through both the material input and scrap accounting.

A few data issues can lead to inaccurate PCF calculations:

A practical PCF workflow can start with the BOM and then add the manufacturing information needed to represent material use accurately.

Net weight shows how much material remains in the finished product. Material input and production-loss data show how much material was required or consumed during manufacturing.

For a reliable PCF, distinguish between these quantities, record scrap using a clearly defined basis, and document how relevant production losses are treated. The right approach depends on the defined product system, system boundary, available data, and accounting methodology.

If your BOM currently contains only net weights, that is a useful starting point. Adding material input, yield, or scrap data where relevant can provide a stronger basis for PCF calculations and make the results easier to review and update.