CAD · Boundary representation · Meshes
STEP is a boundary representation: exact surfaces and topology, no triangles anywhere in the file. Converting to STL means tessellating that real CAD geometry, which is why this page loads an actual CAD kernel rather than parsing text.
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step · stp · iges · igs · brep
Working
STL is what slicers expect. Every slicer reads it, it carries nothing but triangles, and that is usually all a printer needs. If you want units and materials to survive as well, convert to 3MF instead.
A STEP file describes a solid the way a CAD package thinks about it: trimmed NURBS surfaces, edges, faces and the topology joining them. There are no triangles in it. Producing a mesh means evaluating those surfaces and covering them with facets — real CAD work, which is why this page loads OpenCascade compiled to WebAssembly. It is about 7 MB, it downloads only when you drop a file in, and it runs on your machine like everything else here.
The quality setting is a tolerance, not a vague quality slider. It is the furthest the flat mesh is allowed to sit from the true curved surface, in millimetres. Measured on a 50 mm cylinder, comparing tessellated volume against the exact analytic volume:
| Setting | Tolerance | Triangles | Volume error |
|---|---|---|---|
| Draft | 0.5 mm | 100 | −0.97% |
| Standard | 0.1 mm | 248 | −0.17% |
| Fine | 0.05 mm | 500 | −0.04% |
| Maximum | 0.01 mm | 1,004 | −0.01% |
Flat faces are unaffected by any of this — a plain box comes out as twelve triangles at every setting, because a plane needs no approximation. Only curvature costs triangles.
A mesh always sits inside the solid. Every facet is a chord cutting across the true surface, so a tessellated volume reads slightly under the real one and a hole comes out fractionally undersized. The vertices themselves land exactly on the surface; it is the material between them that is missing. If a bore has to be right, tighten the tolerance or drill it after printing.
Features do not survive. The output is triangles. Fillets, holes and patterns stop being editable features and become geometry, and there is no route back — the parametric history was never in the STEP file to begin with.
Units are normalised to millimetres. STEP records its own length unit, and plenty of North American files are in inches. That is resolved during import, so the mesh is in millimetres whatever the source said. Use the unit converter to move it to something else.
Assemblies are flattened. A multi-part STEP file converts to one mesh containing every part. The part count is reported after conversion so you can tell whether that is what you expected.
No. The CAD kernel is downloaded to your browser and your file is read inside the page. The only thing that crosses the network is the engine itself, which is the same for everyone and contains nothing of yours.
The CAD engine is about 7 MB and loads once. After that it stays in memory, so a second file converts without downloading anything again.
STEP (.step, .stp), IGES (.iges, .igs) and OpenCascade BREP (.brep). Native formats such as SLDPRT, IPT and CATPart are not readable — export a STEP from your CAD package first.
The main converter exports STEP from a mesh, merging coplanar triangles into real planar faces. It is genuine CAD geometry but faceted, not a parametric model.
Most often it is not really a STEP file, or it was exported in a schema the reader does not accept. Re-exporting as AP203 or AP214 fixes almost every case.
If a file will not convert, or the result looks wrong, I would rather hear about it. Include the format and roughly what you were doing — that is usually enough to reproduce it.