BOM
The BOM does not match the model.
The BOM and the 3D model come from the same design, but not always from the same moment. Quantities drift apart, there are lines that are not in the model, and parts in the model are missing from the list.
Published 21 September 2026
01 What it is
BOM against model, briefly explained.
A BOM should follow the product structure of the model: per part the part number, the description and the quantity. If that deviates, something is ordered that is not in the model, or the other way round.
There are four outcomes per line: the quantity matches, the quantity differs, the line is not in the model, or the part is in the model but not on the list.
How you notice it
- The quantity on the drawing and in the Excel differ
- A position number on the drawing belongs to a line that is missing from the model
- The workshop has parts left over or comes up short
- With an indented BOM the subtotals do not match
In a check
This shows up as error in the check of the BOM. You judge what has to be done about it.
02 Example
What it looks like.
Example BOM GRP-2210 (demo)
- Pos. 4 bracket
- BOM 8, model 8, matches
- Pos. 7 support profile
- BOM 2, model 4, differs
- Pos. 12 gasket
- not in the model
- BRK-114 cover
- in the model, not on the list
Pos. 7 is a classic indented BOM: two per frame, and there are two frames in the design. Without multiplying in the quantity of the parent assembly the BOM seems to be right.
How it comes about
- Model and BOM were exported at different moments
- A quantity adjusted by hand in the Excel that was never reverted
- An indented BOM where the quantity per parent assembly was not multiplied in
- Parts added to or removed from the model after the last export
- A placeholder as part number that belongs to several parts
03 Approach
Find, fix, prevent.
How to find it
You need two things: the product structure from the model, with the total quantity per part, and your BOM. You put those side by side line by line.
- 1 Get the BOM from the model with the free STEP to Excel: per part the total quantity.
- 2 Put your own BOM next to it, with the part numbers in the same spelling.
- 3 Compare first on part number, only then on description.
- 4 With indented lists, watch the quantity per parent assembly.
Fix
- Export the model and the BOM again, at the same moment.
- Undo manual adjustments in the Excel and fix the cause in the CAD.
- With indented lists, check whether the quantities are multiplied by the parent assembly.
- Give placeholders as part number a real number, otherwise the line cannot be matched.
Prevent
- Let the BOM come from the model instead of maintaining it separately.
- Always export STEP and BOM in one action at release.
- Check both against each other before release, not at purchasing.
With BOMproof
Every release, without having to think about it.
BOMproof puts your STEP model, your PDF drawings and your Excel BOM side by side and points out every deviation: with a pin in the 3D model and a balloon on the drawing. You tick off what is correct.
04 Frequently asked questions
BOM against model.
01 Why is there a part in the model that does not belong on the BOM?
That happens: auxiliary geometry, a welding or assembly aid, or a customer part that you only have in the model for reference. Such a line is not an error, but you do want to tick it off deliberately.
02 How do you compare when the descriptions differ?
Match on part number or file name and use the description only if there is no number. Matching on text alone almost always gives wrong pairs.
03 Can BOMproof do this automatically?
Yes. BOMproof puts your Excel BOM and the BOM on the drawing next to the product structure of the model, line by line, and indicates per line whether the quantity matches, differs, is missing or is extra.
05 Often seen together
Other errors from the same corner.
You never want to hunt for this error by hand again.
Upload your model, your drawings and your BOM and let BOMproof check them against each other. Book a demo with your own design.