Convert STEP to 3MF Online
This converter turns the exact surfaces in a STEP file into a triangle mesh and packages it as a 3MF labeled in millimeters, so it loads at the right size in a slicer. The 3MF we write contains geometry only: no colors, no material assignments and no part names, because STEP colors are never read in the first place.
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Why convert STEP to 3MF
STEP (.step, .stp) is the ISO 10303 exchange format that CAD programs use to hand solid models to each other. It stores boundary representation geometry: trimmed surfaces, exact circles, exact fillets. Most slicers and mesh tools cannot use that surface description directly, and the ones that can, such as recent PrusaSlicer or Bambu Studio builds, tessellate it with their own mesher at settings you do not control.
On our servers the file is opened by FreeCAD 1.1 on top of OpenCASCADE 7.9. The solid is read as a shape, then tessellated with a fixed chord tolerance of 0.1 mm and an angular tolerance of about 29 degrees, and the resulting triangles are written as a 3MF package. The 3MF is a zip containing the model part and its relationships, with the unit attribute set to millimeter, which matches the millimeters FreeCAD uses internally after import. A metre-tagged or inch-tagged STEP is converted to millimeters on read, so the numbers in the 3MF describe the part at real size.
What survives is the shape and its scale. What does not survive is everything STEP carries around the geometry: color, material, product names and the assembly tree are not read by our importer and are not written into the 3MF. The package has no thumbnail and no print settings, so a 3MF from here is a geometry container rather than a prepared print job. Curved faces become flat facets at the 0.1 mm tolerance, and the whole model arrives as a single 3MF object, so a multi-body assembly is one item in the slicer rather than several selectable parts.
How to convert a STEP file to 3MF
- Export one self-contained STEP from your CAD program. AP203, AP214 and AP242 all work, as long as it is text Part 21. An assembly that only references external part files converts to nothing, because the geometry is in files we never received.
- Check that the model contains solids or surfaces. A STEP holding only sketches, curves or wireframe has nothing to triangulate and will come back as an error.
- Pick the file on this page. Guests can upload up to 50 MB per file and 10 files per request; a free account raises the size to 100 MB, and paid plans go to 200 MB and 1 GB.
- Start the conversion and wait on the page. Over the last 90 days files on this pair finished within a few seconds of upload; very large or surface-heavy models take longer, and a single engine run is cut off at 10 minutes.
- Download the .3mf. The link stays valid for about four hours after the job finishes.
- Open it in your slicer and confirm the bounding box in millimeters, then assign colors, materials or per-object settings there.
What this converter does
- FreeCAD 1.1 with the OpenCASCADE 7.9 kernel reads the B-rep, so you do not need SolidWorks, Fusion 360, Inventor or FreeCAD installed to get a mesh out of a STEP file.
- Output is a 3MF with unit="millimeter" and millimeter coordinates, so the part lands at true size without a scale guess.
- Damaged or awkward STEP input is repaired before parsing: gzip-compressed files are unpacked, UTF-16 is recoded, a byte-order mark is stripped, and a missing END-ISO-10303-21 terminator is added back.
- A STEP drawn in a 2D context is lifted to z=0 instead of failing outright.
- If the STEP holds nothing that can be triangulated - only curves, sketches or surfaces the kernel cannot mesh - the job fails with an error message instead of returning a result.
- Converting is free with no sign-up, within the guest limits above.
Video Tutorial
Why use MiConvert for STEP to 3MF? It's free, fast, and works in your browser! 🔗 https://miconvert.com/en/step-to-3mf?utm_source=youtube&utm_medium=social&utm_campaign=step-to-3mf ✨ Why MiConvert?
STEP to 3MF questions
Does the 3MF keep colors and materials from the STEP file?
No. Our importer reads geometry only, so STEP colors, material names and part names are never loaded, and the 3MF we write contains no color or material data. If you need a colored print, assign colors per object or per region in your slicer after import.
Can I choose the mesh resolution or triangle count?
No, there is no quality setting on this page. Tessellation always runs at a 0.1 mm chord deviation and a 0.5 radian angular deviation, the same for a 5 mm screw and a 2 m frame. If you need a finer or coarser mesh than that, export the mesh from your CAD program where you can set the deviation yourself.
Why do I get "No valid 3D geometry found in this file"?
That message means the file opened but held nothing meshable. The usual reasons are a STEP that only contains curves or sketches, an assembly file that points at external part files, or surfaces the kernel could not triangulate. Re-export with geometry included in one file, and make sure solids are present.
I saw a FreeCAD error mentioning trimesh. What is that?
That message comes from our 3MF writer, not from your file. Your STEP is most likely fine - the 3MF writer could not start on our side. Report the job if you keep seeing it; in the meantime our STEP to STL page uses a different writer.
Is 3MF better than STL for this conversion?
3MF is a compressed zip that declares millimeters, so it is usually smaller than the equivalent binary STL and cannot be misread as inches. The geometry is the same triangle mesh at the same tolerance and neither output carries color, so use our STEP to STL page if your slicer prefers STL.
Can you convert 3MF back to STEP, like Fusion 360 does?
No. From a 3MF we only offer DXF and 3DM targets, and both of those are meshes too, so turning a triangle mesh back into a parametric solid stays a reverse-engineering job for CAD software such as Fusion 360's mesh workspace.
Can I convert a folder of STEP files at once?
You can select up to 10 files in one request as a guest or free user, 25 on Plus and 50 on Pro. Each file is converted separately and downloaded separately, as there is no combined ZIP download.
What happens to my file after the conversion?
Everything runs on our servers, not in your browser. Your uploaded STEP is deleted as soon as the job finishes its final attempt, and the 3MF can be downloaded for about four hours before it is removed automatically. File names, sizes and outcomes are kept in our logs, and signed-in users also get a history entry.