Short answer: Export both BOM revisions as UTF-8 CSV, load them as Before and After, and map the Part number and Quantity columns (Description and Material are optional). Then select "Compare BOMs." The tool matches parts by exact, case-sensitive part number and marks each one Added, Removed, Changed or Unchanged. You can filter the result and export it as CSV.
Steps (per the tool's guide page)
1. Export each revision from CAD, PDM or a spreadsheet as CSV UTF-8. Excel workbooks aren't read directly, so save them as CSV first. TSV and pasted text also work. The limit is 4 MB per file.
2. Clean the data.
- Each part number must appear once per BOM. The tool names any duplicate.
- Quantities must be plain non-negative numbers with a dot decimal, no units and no thousands separators.
3. Load Before and After, or select "Try sample revisions."
4. Map the columns. Part number and Quantity are required. Item, Description and Material are optional. Map Description and Material on both sides if you want changes in those fields detected.
5. Select "Compare BOMs" and filter by status.
6. Read the statuses.
- Added: only in After.
- Removed: only in Before.
- Changed: in both, with a different quantity, description or material.
- Unchanged: all three fields match.
7. Export (bom-comparison.csv) and review it against the drawing change record before purchasing or assembly.
Worked example (generic)
Rev B and Rev C of a gas cabinet BOM are compared. The result shows:
- one filter Removed and a new filter Added;
- tube quantity Changed, with After minus Before shown;
- one screw that appears as both Removed and Added.
Checking the screw shows a trailing space in the Rev C part number. After the engineer fixes it, the screw is Unchanged. The remaining changes match the ECO.
Common mistakes
- Uploading .xlsx directly.
- Leaving units in quantities ("4 EA").
- Duplicate part numbers from flattened multi-level BOMs that weren't summed.
- Expecting hierarchy or geometry comparison. It compares flat tables only.
Preparing both files
1. Use the same columns in both files, or map them to the same fields. Part number and Quantity are required. Item, Description and Material are optional.
2. Save as UTF-8 CSV or TSV under 4 MB, or paste the text.
3. Normalize part numbers consistently in both files: same case, no trailing spaces, same separators. Matching is exact and case-sensitive.
4. Resolve duplicates. The tool rejects duplicate part numbers. If a part appears in several rows (for example, in a flattened multi-level BOM), sum the quantities into one row first, or compare at a level where each part appears once.
5. Clean quantities to plain non-negative numbers with dot decimals. Remove units, text like "AR," and comma decimals.
6. Flatten multi-level BOMs if needed. The tool compares flat tables, so export a single level or a summarized parts list.
Reading the result well
- Added: in the new file only. Check whether this is a real addition or a part number that was renamed.
- Removed: in the old file only. Look for a matching Added row with a similar description. That pair is often a part number change, not a real removal.
- Changed: same part number, different quantity or other mapped field. Check quantity changes against the ECO.
- Unchanged: no difference in mapped fields. Unmapped columns aren't compared, so a change in them won't show.
Edge cases
- Renamed part numbers show as one Removed and one Added. That is correct, because the matching key changed.
- Whitespace differences can make identical parts look different. Trim before comparing.
- Description-only edits show as Changed only if you mapped Description.
- Items moved between subassemblies don't show in a flat comparison if the total quantity is the same. Compare each subassembly level separately if location matters.
A second worked example (generic)
An engineer compares the as-designed BOM of a gas cabinet with the as-built list from production. The first upload is rejected because a gasket part number appears in three rows. They sum the gasket quantities into one row in both files and try again. The result shows two Removed and two Added rows with almost identical descriptions. These turn out to be the same valves, with part numbers typed in lowercase in the production list. After normalizing case, the pairs become Unchanged, and the real differences are left: one quantity change and one added bracket. The engineer exports bom-comparison.csv and attaches it to the ECO.
FAQ
Does it compare CAD models?
No, only flat BOM tables.
Why is a part both removed and added?
Matching is exact and case-sensitive. Look for spaces, capitalization, or O vs 0.
How do I get a BOM from a drawing first?
Use the free Drawing to BOM tool and export CSV.
Does it compare multi-level BOMs?
It compares flat tables. Compare one level at a time, or flatten and summarize first.
Why was my file rejected?
Common causes are duplicate part numbers, non-numeric quantities, comma decimals, or a file that isn't UTF-8.
Can I compare a BOM from the Drawing to BOM tool?
Yes. Export CSV from Drawing to BOM, check it, then use it as one side of the comparison.
Is my data kept?
Check the tool page for its current data handling statement before uploading sensitive BOMs.