Mass calculations in Sustainability analysis

When you run a Sustainability analysis or report, it calculates mass values for items in the BoM. These are displayed in the analyzed BoM and the Excel report, and are used to calculate indicator values.

The key mass indicator values in a Sustainability analysis or report are Reported Mass and Mass Before Processing. These are shown as columns in the analyzed BoM and in the BoM Details sheet of the Excel report, and are used to calculate the Climate Change and Embodied Energy indicators for the Materials, Processes, and Transport phases.

Reported Mass

For each Part and Material in the BoM, the analysis calculates a 'Reported Mass' that represents the total mass of the part or material at that location in the BoM. For materials, this is the final mass after any subtractive processing (removal of material).

Note: The Reported Mass is for the quantity of the item at that location in the BoM, taking into account the quantities of parent assemblies.

For example: for a part in the BoM, the Reported Mass is for the number of parts specified in the Quantity column, multiplied by the Quantity of its parent assembly, and similarly by the Quantity of each of its ancestors in the BoM hierarchy.

The Reported Mass for the top-level assembly represents the total mass of the product, and is shown on the Summary, Materials, Processes, and Transport sheets of the Excel report as Product Mass. This total product mass is used to calculate the environmental impacts of the Transport phase, and is shown as the 'Reported Mass' for each Transport stage in the BoM.

Mass Before Processing

For each Material in the BoM, the analysis calculates a 'Mass Before Processing' that represents the initial mass of material before processing.

Note: For materials that have one or more secondary processes assigned, this may either be specified as an explicit mass value in the BoM (for example, where the initial mass of a billet of material is known), or calculated by the analysis, based on the mass of the part and the amount of material removed by each secondary process.

The Mass Before Processing is used to calculate the environmental impacts of primary production of the material (for the Materials phase of the analysis) and primary processing of the material (for the Processes phase of the analysis).

For parts and assemblies, the analysis may also calculate a Rolled-up Mass (per UoM) value, either as an interim stage towards calculating Reported Mass values across the BoM, or as a 'Mass Check' for the calculated values.

Mass values are displayed in the Excel reports and BoM Analyzer with a maximum of 3 significant figures. The full-precision value can be viewed in tooltips on cells, or in the Excel formula bar.

Terminology for mass data specified in the initial BoM

The following terms are used to refer to different types of mass data that may be specified in an initial BoM for Sustainability analysis.

Measured Mass

Mass data that has been specified for a BoM item as an absolute value (rather than as a percentage of the parent item).

A Measured Mass can be specified in units of mass in the Quantity and Unit of Measure columns of the initial BoM.
Alternatively, if the Quantity is specified using a non-mass unit, the Measured Mass can be calculated using additional information provided in the Mass per UoM and Mass unit columns of the initial BoM (see Quantities, measures, and masses for Sustainability).

Mass calculated as a percentage

A mass that has been specified for a BoM item as a percentage (by weight) of its parent item (see Quantities, measures, and masses for Sustainability), and then calculated from the mass of the parent item.

Note: A mass value can only be calculated if the parent has a ‘Measured Mass’, or if the mass of the parent item can itself be calculated as a % (by weight) of an item further up in the hierarchy that has a ‘Measured Mass’.

If the parent does not have a Measured Mass and there is insufficient data to calculate the parent’s mass as a percentage, then the BoM item will not have a ‘Mass calculated as a %’.

Mass of material removed

For a subtractive secondary process that has dimension 'Mass removed' (such as machining or grinding): The amount of material that is removed by that process. This must be specified in the initial BoM, either as an absolute mass value, or as a % (by weight) of the material's mass immediately before that process is applied.

Calculating Reported Mass values across the BoM

As the first stage of the mass calculation, any mass data that has been specified in the initial BoM (either as a Measured Mass, or as a percentage of the parent item) is used to calculate Reported Mass values for Parts and Materials across the BoM.

The analysis uses the following methods, in order of priority:

Measured Mass data is available: Wherever Measured Mass data is available for a Part (either in the Mass per UoM column, or in the Quantity column where the quantity is specified as a mass – see Quantities, measures, and masses for Sustainability), the Reported Mass for that Part is calculated using its Measured Mass.
Note: In Sustainability analysis, if the Quantity of a Material has been specified as an absolute mass value, this is assumed to represent the initial mass before secondary processing.
Part or Material as a percentage: If a Part or Material is specified as a percentage (by weight) of its parent item, its Reported Mass is calculated using the Measured Mass of the parent item (if this is available), or an ancestor item further up the BoM hierarchy (if enough data is available).
Note: In Sustainability analysis, if the Quantity of a Material has been specified as 100% of the parent Part, this is assumed to represent the final mass after processing.

Any Reported Mass values that have been obtained from Measured Mass data (Method 1 above) are then used to calculate a 'Rolled-up Mass' for each item in the BoM (wherever possible) by summing the Reported Mass values of its constituent items, from the lowest level of the BoM upwards.

Note: A Rolled-up Mass can only be calculated if all constituent items have a Reported Mass.

To avoid any risk of circularity in the mass calculations, if the Reported Mass of a constituent item has been calculated as a percentage of its parent (Method 2 above), then a Rolled-up Mass is not calculated for the parent item.

If a Rolled-up Mass can be calculated for a BoM item, it is shown in the Rolled-up mass (per UoM) column of the analyzed BoM, and in the Sustainability report.

If a Measured Mass and a Rolled-up Mass are both available for an item, a Mass Check comparison is carried out between the two values: Measured Mass per UoM and Rolled-up mass per UoM. The % Deviation column shows the result of this check, relative to the Measured Mass per UoM.
If an item has no Measured Mass, and is not specified as a percentage, then its Rolled-up Mass value (where available) can then be used to calculate the Reported Mass for the item.

If none of the above sources (in order of priority: Measured mass, Mass calculated as a percentage of the parent, or Rolled-up Mass) is available to calculate the Reported Mass for a BoM item, its Reported Mass column is shown as blank.

Calculating the Mass Before Processing for a material

If a Material does not have any subtractive Secondary Processes assigned, its Mass Before Processing is the same as its Reported Mass.

If a Material has one or more subtractive Secondary Processes assigned, the analysis calculates the mass of material at each stage of processing, in one of two ways.

If you specified the Quantity of the Material as an absolute mass value ('Measured Mass'), this is assumed to represent the initial mass of the billet, which is used to calculate the Mass Before Processing value for the BoM item.
The analysis then calculates the 'Mass of material removed' by each secondary process, and the mass of material remaining after each process, in the order specified in the BoM. The mass remaining after secondary processing is used to calculate the Reported Mass value for the BoM item.
If you specified the Quantity of the Material as 100% of the parent Part, this is assumed to represent the final mass of material, which is used to calculate the Reported Mass value for the BoM item.
The analysis then back-calculates the 'Mass of material removed' by each secondary process, and the mass of material immediately prior to each process, in the reverse of the order specified in the BoM. The initial mass of material before secondary processing is used to calculate the Mass Before Processing value for the BoM item.