Drop an STL. The converter welds the triangles, repairs what the exporter got wrong, merges every flat area into one face and builds a solid your CAD tool can open — in your browser, without uploading the file. Then it tells you what came out: how many faces, solid or open surface, and whether the volume still matches the STL.
An STL is triangles. The cylinders, fillets and sketch the part was made from are not in it, and no converter can put them back. What you get is a faceted solid: flat faces where the mesh was flat, narrow flat strips where it was curved. Fusion, SolidWorks, FreeCAD and Onshape open it as a solid you can cut, pocket and measure between flat faces — but a hole is still a ring of strips, not a hole whose diameter you can change.
Need a STEP with true cylinders? Export it from the model's source: OpenSCAD to STEP → Parametric generators (STEP export) →
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No sign-up. The first visit downloads about 8 MB of WebAssembly, which your browser keeps.
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Binary or ASCII, up to 2,000,000 triangles.
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Faces before and after merging, solid or open surface, the STL's and the STEP's volume, and what was repaired — item by item. The merged faces show in the view.
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AP242, which Fusion, SolidWorks, FreeCAD, Onshape and other current CAD tools import.
The set: 135 STLs from our generators' print verification kit, 45 printed parts of the Prusa i3 MK3 (exported from OpenSCAD), 10 OpenCascade sample meshes and 120 STLs from public CAD projects on GitHub. Every STEP up to 30 MB was read back to check it.
STEP files are big: each face is written out in full, with its edges. The 137 KB test bar (2,812 triangles → 719 faces) makes a 1.1 MB STEP; a 117,694-triangle scan makes 84 MB in 15 s. That is already without the 2D copy of every edge on every face (40–60% of a plain export).
Triangles wound the wrong way are turned around, inside-out bodies flipped, corners a hair apart joined, zero-area slivers taken out, edges shared by two touching bodies split — each one counted in the report.
Coplanar neighbouring triangles merge, and holes in a face stay holes. Only true planes (within a millionth of the part's size): nothing curved is flattened.
A mesh with holes is written as an open surface and the report says so; it is never labelled a solid.
An inner shell becomes a cavity cut from the solid around it, and the volume still matches.
Every conversion shows the STL's volume next to the STEP's.
It runs in your browser; the page uploads nothing. No sign-up, nothing stored.
The same part, two kinds of STEP:
| STEP made from an STL | STEP exported from the CAD source | |
|---|---|---|
| A hole | A ring of flat strips (as many as the STL had) | One cylindrical face with its exact diameter |
| Flat faces | One planar face each | One planar face each |
| Make a hole bigger | Not possible — redraw it | Change the feature or parameter |
| Fillets | Rows of narrow facets | True rounded faces |
| File size | Large (MB even for small parts) | Small (tens of KB) |
| Good for | Adding holes, cutting, designing mating parts around it, CAM roughing | Everything |
Open the converter, drop the STL, read the report and click Download STEP. In FreeCAD the same takes the Part workbench's Create shape from mesh, then Convert to solid, then a STEP export; in Fusion, Insert Mesh and Convert Mesh. Here nothing has to be installed, and you see straight away whether you got a solid.
Because an STL holds only triangles: the cylinders have already become rings of flat faces, the fillets rows of narrow ones, and the dimensions and features were never stored. A conversion can only tidy the triangles into a clean solid; it cannot guess the original design. For an editable model you need the CAD file or source code the part was made from.
Yes. It is a standard AP242 file, which Fusion, SolidWorks, FreeCAD, Onshape and others import; we read every one up to 30 MB back with OpenCascade and checked its bodies and volume. Files with tens of thousands of faces are sluggish in any CAD tool.
An STL takes 50 bytes per triangle; a STEP writes every face as a plane, an edge loop, edges and vertices — about 1.2 to 1.5 KB per face in our tests. We leave out the 2D edge copies importers can derive (40–60% smaller than a plain export), and a big mesh still makes a big STEP.
No: the hole is a ring of flat faces, not a cylinder feature. In CAD you can delete those faces and drill again; it is easier at the source — if the part comes from OpenSCAD or one of our generators, change the parameter and export the STEP again.
When the STL is closed (every edge shared by exactly two triangles), yes, and it passes OpenCascade's validity check. When the STL has holes, it is written as an open surface and the report gives the number of open edges; repair the mesh in your slicer or Meshmixer first.
2,000,000 triangles. Organic meshes — scans, sculptures — have almost no flat faces to merge, so even a few tens of thousands of triangles make a STEP of tens of megabytes; decimate first.
No. The conversion runs in WebAssembly in your browser, on an origin of its own; the page has no upload. All that reaches us are anonymous counts (faces, time taken), never the file or its name.
Yes. .stp and .step are two extensions of one format (ISO 10303-21); you can rename the download to .stp.
Going the other way? STEP to STL →
A real STEP from source code: cylinders and holes as true surfaces. If you have the .scad, don't convert the STL.
The other direction: view a STEP, mesh it at the accuracy you choose, check the mesh is closed.
Gears, boxes, brackets and more: set the parameters online and export STEP directly.
