Converts an STL or 3MF into a closed STEP solid, with optional smoothing that never risks the part's dimensions.
If you've ever imported an STL into CAD software like Fusion 360, SolidWorks, or Onshape, you've probably noticed it lands as a mesh body you can barely edit. STEP files describe exact solid geometry instead of a pile of triangles, so CAD tools can treat the result like a native part — with real faces and edges you can measure, fillet, or combine with other parts. This tool does that conversion for you, entirely on your own device: no upload, no account, and a 3D preview below so you can check the result before downloading.
STEP (.step / .stp) is the standard 3D file format used by CAD software like Fusion 360, SolidWorks, Onshape, and Inventor. Unlike an STL, which is just a mesh of triangles, a STEP file describes exact solid geometry, so professional CAD tools can open, edit, and measure it precisely.
Most CAD programs can import STL files, but only as a mesh body that's hard to edit — you can't easily change dimensions, add features, or combine it with other parts. Converting to STEP turns the mesh into a solid body that behaves like native CAD geometry. See the STL vs. STEP guide for the full explanation.
No. The entire conversion happens locally in your browser using WebAssembly — your file is never uploaded, stored, or sent anywhere. You can even disconnect from the internet after the page loads and the converter will still work.
Smoothing merges nearly-coplanar triangles into cleaner flat faces to reduce file complexity, but it never moves vertices beyond the tolerance you choose, so your part's outer dimensions stay accurate. You can disable it entirely if you need a 1:1 mesh-to-solid conversion. The smoothing guide goes through how to pick a tolerance.
You can upload .stl and .3mf files directly, or a .zip containing one of them, which is opened automatically. The output is always a standard .step file that opens in any CAD software.
There's no hard file size limit, but very dense meshes matter: up to a few hundred thousand triangles usually convert in well under a minute. Meshes in the millions of triangles (common with high-resolution 3D scans) can take a very long time and are likely to run out of the browser's memory during the "sewing" step and fail, since everything runs on your device rather than a server. If your mesh is extremely dense, simplify/decimate it first (most slicers, Meshmixer, or Blender's Decimate modifier can do this) before converting.
Yes, it runs on any modern browser, including Safari on iPad and Chrome on Android, since it's a normal web page rather than a native app.
Yes, the converter is completely free with no sign-up. If you find it useful, there's an optional way to support the project below.
About this project — why it exists and how the OpenCascade + WebAssembly conversion actually works.
STL vs. STEP: what's actually different, and when you need each
How to prepare a downloaded or scanned model for CAD editing
What "safe smoothing" does, and why it won't ruin your dimensions