STEP to GLB under a size limit: what we built and what we measured

Measured on 6 October 2026 with 3D CAD Converter 1.0.31

A folder of STEP assemblies has to become small GLB files for a web product viewer, AR or a game engine. From version 1.0.31 you enter the size you can afford in MB, and 3D CAD Converter writes a GLB that fits, with every part still named and coloured. Everything runs offline on your PC and nothing is uploaded. Online converters have upload size limits; here there is no upload.

STEP file

19.7 MB65-part assembly

GLB, no limit

60.18 MB2,100,994 triangles

GLB, 5 MB limit

4.85 MB129,368 triangles

What we measured

Input	No limit	15 MB limit	5 MB limit	1 MB limit
65-part assembly, 19.7 MB STEP	60.18 MB2,100,994 triangles	14.57 MB456,292 triangles	4.85 MB129,368 triangles	Not reachable. Smallest file 4.65 MB, reported as over the limit
23-part assembly, 8.7 MB STEP	20.33 MB697,220 triangles	14.77 MB495,428 triangles	4.87 MB146,400 triangles	Not reachable. Smallest file 1.69 MB, reported as over the limit
Single part, 2.6 MB IGES	6.54 MB237,938 triangles	6.54 MB237,938 trianglesalready fits, left untouched	4.57 MB157,174 triangles	0.96 MB26,056 triangles
Single part, 2.3 MB STEP	3.00 MB90,740 triangles	3.00 MB90,740 trianglesalready fits, left untouched	3.00 MB90,740 trianglesalready fits, left untouched	0.93 MB22,604 triangles

Fine quality. Sizes in MB, with 1 MB counted as 1,000,000 bytes. A file that is already under the limit is left untouched. Conversions took from about 3 seconds for the small parts to about 2 minutes for the 65-part assembly; the test machine was busy, so we give no exact times.

At Standard quality the 65-part assembly is 14.64 MB with no limit and 4.92 MB with a 5 MB limit. The 23-part assembly, which cannot reach 1 MB at Fine, fits at Standard: 0.95 MB and 22,860 triangles.

Every size-limited GLB kept the same part names, nesting and colours as the unlimited one.

How the limit is reached

Meshed onceThe STEP file is turned into triangles one time, at the quality you chose.
Re-exported until it fitsThe mesh is written again with fewer triangles until the file is under your limit.
The most detail that fitsYou get the highest triangle count that stays under the limit. It never goes below 5% of the triangles at the chosen quality.
A miss is called a missIf the limit cannot be reached, the smallest file is still written and clearly marked as over the limit. It is never reported as a success.

The exact commands

# One assembly, 5 MB at most
cadconvert convert -i assembly.step -o assembly.glb --quality fine --max-size 5

# A whole folder, with a CSV report
cadconvert batch -i .\step -o .\glb -f glb --max-size 5 --report report.csv

# A watched folder: every new STEP file becomes a GLB
cadconvert watch -i .\incoming -o .\glb -f glb --max-size 5 --report report.csv

--max-size takes a value in MB and works on convert, batch and watch, for glTF and GLB output. When a file ends up over the limit, the CLI exits with code 6, the JSON result says withinBudget:false and the report says OverBudget.

In the app

Options step, glTF card, field Maximum file size (MB).

For AI agents

The MCP tool convert_cad takes max_size_mb.

Fixed: a STEP assembly is no longer one grey mesh

Before version 1.0.31, a STEP assembly exported to GLB came out as a single grey mesh. That was a defect, and the same release fixes it. A STEP assembly now arrives in the GLB like this:

One named node per part

Nested the same way as in the STEP assembly.

Part and face colours

A pen model with 5 colours keeps all 5.

Surface normals

Now written into the GLB.

One report for the whole batch

Add --report report.csv to batch or watch, or click Export CSV report on the results screen of the app. You get one row per file: input and output file and format, triangles before and after, input and output size, the size limit, whether the file is within it, duration, result and message.

What it does not do

Part positions are baked into the geometry. Node transforms are identity.

Repeated parts are not instanced.

IGES and BREP files do not carry colours or structure through.

Colour names and transparency from STEP are not carried.

Using mesh repair can drop face colours.

GLB stays Z-up unless you add --up-axis y.

Try a size limit on your own STEP assemblyFree trial: 10 conversions, no credit card. Windows 10/11, 64-bit.

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