BOM · 6 min read

Comparing a BOM with the 3D model.

Comparing a BOM with the model means: match every line of the list to a part in the product structure and then put the quantities side by side. Match on part number or file name, use the description only as a last resort, and watch out for indented lists where the quantity has to be multiplied by the parent assembly.

Published 21 September 2026

In short

Matching goes on part number or file name, not on description
Export model and BOM at the same moment
With an indented list the real quantity is the quantity times the parent assembly
Four outcomes per line: matches, differs, not in the model, only in the model

01 On this page

What you need

Two things: the product structure from the model, with the total number of instances per part, and your BOM. Both from the same version of the design, otherwise you compare two moments instead of two sources.

You get the product structure from your CAD package or straight from the STEP. A STEP with assembly structure contains for each part how often it occurs and under which assembly it hangs.

If you do not have a BOM at hand, you can make one from the model. The free STEP to Excel on this site gives the total quantity, the material and the dimensions per part.

Matching: what to use and what not

Matching is the hardest part. A BOM line and a part in the model have to be tied together, and there is not always an unambiguous key for that.

In order of reliability

File name of the part, as the PDM system knows it. It is often in the BOM and matches the name in the model exactly.
Part number, provided every part has its own number.
A type code from the description, for example a supplier type number.
The description itself, only if there is nothing else, and then confirmed by hand.

Where it goes wrong

A few situations come back so often that it is worth looking at them beforehand.

Placeholders as part number: one number that appears on ten different parts. Matching on that number gives random pairs.
Indented BOMs: two per frame and two frames means four. If the list says 2, that is not necessarily wrong.
Extensions in the name: the model knows the part as BRK-114, the BOM as BRK-114.ipt.
Different spellings: spaces, hyphens, leading zeros.
Parts that deliberately are not on the list, such as auxiliary parts or customer material.

Four outcomes per line

When you are done, every line falls into one of four categories. That is your work list straight away.

Matches: quantity in the model equals the quantity on the list.
Differs: matched, but the quantities differ. The most common case, and usually an indented list or a change after the export.
Not in the model: the line is on the list but nowhere in the design. Often rightly so, for example a gasket or paint, but tick it off deliberately.
Only in the model: the part is in the design but not on the list. This is the most dangerous one, because that part is not ordered.

How often do you do this

At every release, and after every change that affects the number of parts. In practice that is more often than it happens, because by hand it takes an hour or more for a list of a hundred lines.

It is work that lends itself well to automation: it is regular, it is dull and the rules are fixed. What a person has to do is judge what the deviations mean.

With BOMproof

This work automatically, at every release.

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.

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Comparing a BOM.

01 Can I get the BOM from the model if I do not have one?

Yes. From a STEP with assembly structure you can read the total quantity per part. The free STEP to Excel on this site turns that into a flat or structured list, with material and dimensions added.

02 What do I do with parts that are deliberately not on the BOM?

Leave them in the comparison and tick them off as reviewed. If you remove them from the list, the same question comes back next time and nobody remembers that it was a deliberate choice.

03 How does this work with several drawings?

Every drawing with a BOM on it is a comparison of its own. The list on the drawing, the Excel and the model should all three say the same thing, and contradictions between two drawings count too.

03 Knowledge base

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