STL to STEP Converter
Drop an STL and get a .step file with the same geometry as a faceted solid, ready to open in CAD.
How to use
- Step 1Drop an .stl file, binary or ASCII. Meshes under about 50,000 triangles give STEP files CAD opens comfortably; above that the page warns, and it stops at 200,000.
- Step 2Choose the unit the STL was modelled in: millimetres for nearly everything from a slicer or a model site, inches for some US CAD exports. Press Convert.
- Step 3Open the .step in Fusion 360 (Upload, or File > Open), SolidWorks, Onshape (Import), FreeCAD or Inventor. It arrives as a solid made of flat triangular faces: measure it, mate it, cut it, or remodel over it.
What kind of STEP this makes
STL knows only triangles, so the STEP written here is a faceted solid: an AP214 file whose FACETED_BREP holds one planar face per triangle, with the corners welded into shared points and the faces closed into a shell. A box exported from CAD as STL has twelve triangles; it comes back as a solid with twelve flat faces, not six. No curves are recovered, no fillets or holes are recognised as features, and there is no history tree. This is the same thing Fusion 360's Convert Mesh (Faceted), SolidWorks' mesh-to-solid conversion and FreeCAD's Part > Create shape from mesh produce, written to a file that opens anywhere STEP opens.
The file is in millimetres. STL carries no unit, so the page asks which one the mesh was drawn in and scales inch meshes by 25.4 before writing; the size shown next to the download is what CAD will measure.
| Faceted STEP from STL | Native CAD part | |
|---|---|---|
| Faces | One flat face per triangle | A few exact surfaces |
| Measure, mate, Boolean cut | Yes | Yes |
| Push/pull, fillet, edit sketches | Barely: every edge is a triangle edge | Yes |
| Opens fast | Up to a few tens of thousands of faces | Always |
| File size | About 300 bytes per triangle | Small |
Why CAD is slow with it, and what helps
A CAD kernel keeps topology for every face, edge and vertex, so a hundred thousand faces is a giant model even though the same triangles are trivial for a slicer. Fusion 360's own mesh conversion warns above 10,000 facets and refuses above 50,000, and a STEP with that many faces behaves the same way once opened; the page warns at 50,000 for the same reason.
Reduce the mesh first when you can: "Simplify model" in PrusaSlicer, Bambu Studio and OrcaSlicer, the Decimate modifier in Blender, or Quadric Edge Collapse Decimation in MeshLab bring a 300,000-triangle scan down to 30,000 with little visible change. Keep only the part you need. If the goal is real surfaces rather than triangles, use a tool that fits them: Fusion 360's Convert Mesh has a Prismatic option (paid subscription) that merges coplanar facets into planes and recognises cylinders; recent SolidWorks versions can import the STL itself as a mesh body and segment it into planar and cylindrical faces; FreeCAD's Part > Refine shape merges coplanar triangles, which turns a boxy print back into proper flat faces.
Watertight meshes and units
- For CAD to see a solid, the mesh must be closed: every edge shared by exactly two triangles, all facing outwards. The page counts open edges and warns; repair the STL first in the slicer, in Meshmixer, in Windows 3D Builder, or with Blender's 3D-Print Toolbox (Make Manifold).
- Only corners with identical coordinates are welded. Meshes exported with tiny gaps between triangles show up as open edges; a repair pass fixes those too.
- Zero-area triangles are dropped, since a face without area is not valid geometry; the count is reported.
- If the part measures 1/25 of what you expect in CAD, the STL was in inches: convert again with Inches selected.
Questions and answers
Can Fusion 360 open a faceted STEP?
Yes. Upload the .step to the Data Panel or use File > Open; it appears as a body with one planar face per triangle. Keep the triangle count in the tens of thousands or opening and every later operation crawl; Fusion's own Insert Mesh plus Convert Mesh gives the same faceted result and refuses above 50,000 facets.
Does the STEP keep colours?
STL has no colours, so there is nothing to keep: the STEP contains geometry and a part name taken from the file name. The rare colour hacks some binary STL writers use are ignored.
Why is the STEP so much bigger than the STL?
Binary STL spends 50 bytes on a triangle. The STEP is text and spends about 300 bytes on each face (a loop, a plane, two directions, a bound and the face) plus the shared points, so a 2.5 MB STL with 50,000 triangles becomes a STEP of roughly 15 MB.
Will I be able to edit it like a normal CAD part?
You can measure it, mate it in assemblies, cut it with Booleans and sketch on its flat faces. Features are not there to edit: no fillet to change, no hole to resize. For an editable part, trace the important sketches over the faceted body and remodel, or use a mesh-to-surface tool such as Fusion's Prismatic conversion or FreeCAD's Refine shape.
Why is the part 25 times too small in CAD?
The STL was drawn in inches but converted as millimetres. STL files do not say which unit they use, so the page has to be told: convert again with Inches selected and the coordinates are multiplied by 25.4.