What are Equivalent Components on my BOM?


This blog will start the series of intermediate blogs for Bill of Materials (BOM). The series will help users of Autodesk Inventor understand the capabilities and use of the inventor Bill of Materials, and how they relate to maintaining Parts Lists and interaction with Autodesk Vault, ERP, and PLM systems. The topics for BOM intermediate series include:

If you like to learn more about the foundational concepts related to the Inventor BOM capability, click here to read the Level 100 Series blogs on Bill of Materials starting with Part I: Intro to Bill of Materials.

Stay tuned for more blogs with advanced topics such as Virtual Component Use and how to make the most out of Part List capabilities.

What are Equivalent Components?

A common problem with CAD models is having duplicate files that are supposed to be the same component.  This can happen for a few reasons:

  1. You imported data, and the import creates multiple files for the same component or includes files that are equivalent to your own files such as screws and nuts.
  2. You accidentally created two parts that turn out to be the same, but replacing one with another could be problematic due to assembly constraints, data management headaches, etc.
  3. Furthermore, you created a part or assembly for a purchased component in your design (say a cylinder), and someone else creates the exact same component for their design.

So, how do you tell all drawings and Autodesk Vault that these two different files are actually the same component (i.e. they are equivalent)?

Part Numbers & Equivalent Components

Inventor BOM uses part numbers to determine if two or more parts are “equivalent”.  Part numbers tend to be unique; two different items usually have different part numbers.

So, if two or more files/components have the same Part Number, by default, BOM performs a “roll up” or “merge” on equivalent components, making them a single line item in the BOM.

Rules for Equivalence



The basic rules Inventor uses for equivalence are:

  1. When Inventor sees two or more components that have the same parent assembly and are the same part number, the BOM considers them to be the same component and treats them as the same component.
  2. Components with a blank Part Number, or a part number made up of “white space” characters such as spaces, are never equivalent
  3. If a Part and an Assembly are equivalent, then the resulting roll-up looks like the assembly (i.e. you get a line item with a quantity of 2, and that line items has the assembly’s child components in the BOM)

Cases Where Equivalence is Not Used

The Equivalence Rules are always on in Inventor’s BOM.  However, there are cases where more than one file has the “same” part number, but should not be treated as equivalent:

  • Cut Length components such as tubes, pipes, structural steel, wires, etc. often have a single part number for every component
  • Some common items such as fasteners tend not to have a part number, and use some “generic” entry such as “N/A”, “-“ etc. for their part number to indicate this.

Inventor can handle these cases in 2 different ways.



The first is in each Assembly document you can set a list of values to ignore when evaluating equivalence.  Open the BOM Editor, then select the Part Number Row Merge Settings button.  In the dialog you can add string values that the merge should exclude from acting on.  You can also turn off merging for the assembly document entirely.

In addition, Inventor has the concept of a Stock Number, which is discussed in BOM 202.


Inventor uses part numbers to solve a common problem with a Bill of Materials, how to tell that two or more files on disk are the same component in inventory.  This helps to reduce the number of duplicate items within data management systems, such as Autodesk Vault Professional.

Ready to learn more? Check out the next post on Part Numbers and Stock Numbers!

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