What changes when a STEP file becomes STL
A STEP part is a boundary representation: exact faces, edges and curves. An STL holds only triangles, so every curved face is approximated by flat facets. The table shows what survives the trip.
| Property | In the STEP | In the STL |
|---|---|---|
| Geometry | exact B-rep surfaces | triangles at the chosen precision |
| Units | declared in the file | none stored; numbers written as millimetres |
| Color | assembly colors, shown in the viewer | not stored |
| Bodies | separate bodies | one mesh; Info counts the separate parts |
Units: from the declared unit to millimetres
The viewer reads the unit written in the STEP header, shows it, and converts the geometry to millimetres. In our tests a 10 mm cube saved in mm reads 10 mm, and a 1 inch cube declared in inches reads 25.4 mm.
Some exporters write millimetre numbers but label them as inches. The CAX-IF AS1 assembly from Pro/E is one such file: its 200 mm length turns into 5080 mm. When a part comes out that large, the tool shows Check the units. Flip the switch Numbers are millimetres and AS1 reads 200 × 87 × 150 mm, the size its author intended. Do this before you press Download, because the STL carries whatever numbers are on screen.
Precision: Coarse, Medium or Fine
Precision decides how densely OpenCascade covers curved faces with triangles. On our rounded-cube test the three levels give 48, 84 and 172 triangles. Flat faces cost little at any level; fillets, holes and threads gain the most from Fine.
The page converts at Medium. To change it, press Other formats: the Convert panel has a Tessellation menu, and the model is re-tessellated when you pick a level. Choose it before you start editing.
Pick Coarse for a quick look or a model you will only measure. Pick Fine for parts you print, where visible facets on a cylinder would show in the plastic. Very dense meshes are heavy: about 1 million triangles open in 5 to 12 seconds, and above roughly 1 to 2 million the tab may run out of memory.
How to convert STP to STL
- Press Open File or drop the .step or .stp file on the tool above.
- The first STEP file downloads the OpenCascade engine once: 7.6 MB, or 3.1 MB compressed. Later files start at once.
- Open Info and compare X × Y × Z with the drawing. If you see Check the units, use the switch.
- Press Download STL. Other formats gives 3MF, OBJ, GLB and the rest from the same model.
The model stays in the viewer after the download, so you can still use Section or Measure on it.
Example: a ten-body assembly
AS1 holds 10 bodies. After tessellation the mesh check runs per body and reports 10 of 10 closed, so each piece is watertight in the STL. The colors of the assembly appear in the viewer while you check it; the STL saves geometry only. The STEP is read by JavaScript in the tab and nothing is uploaded: the engine itself comes from our own domain.
Native part files from your CAD program do not open here. Export STEP from it first, then drop that file.
When the STEP does not convert cleanly
A .step or .stp file is the clear-text form of ISO 10303-21, so you can open it in a text editor and see the header. If the viewer shows Could not open, check that the file starts with that header and is not a renamed native part.
If a cylinder prints with flat bands, re-convert at Fine. If one body in an assembly reads as not watertight, the problem sits in the source model, and a fresh export from CAD usually beats a repair of the mesh. A part 25.4 times too large points to a wrong unit label. Pick Numbers are millimetres in the File units menu in Info.
After the STL: repair, cost, and the way back
If the mesh check finds holes, STL repair fixes the simple cases. To price a print, the 3D print cost calculator takes the volume straight from the model. Need the opposite direction? STL to STEP writes a faceted solid. To inspect a STEP without converting it, use the STEP viewer; for IGES parts there is IGS to STL.
Questions and answers
Is STP the same as STEP?
Yes. Both extensions name the same CAD exchange format, and the file picker on this page accepts .step and .stp alike.
Why did my STL come out 25 times too big?
The exporter labelled millimetre numbers as inches, so every length was multiplied by 25.4. Turn on Numbers are millimetres before downloading and the size returns to what the designer drew.
Can I convert STEP to STL without uploading the file?
Yes. OpenCascade runs as WebAssembly in your tab. In our test run every request went to the site's own origin, all of them GET, and no file data was posted anywhere.
How big a STEP file can this handle?
There is no fixed size cap; memory is the limit. Coarse precision keeps the triangle count down if a large assembly makes the tab slow.
Which browser should I use?
We test in Chromium browsers, currently Chrome 155. Firefox and Safari should run it but have not been tested yet.
Which precision should I choose for a print?
Fine for round parts such as bushings, knobs and threads, where flat bands would show in the plastic. Medium is enough for brackets and plates, and Coarse keeps a big assembly light when you only need to measure it.