STL Viewer BlogAn engineer sends you a part as bracket_rev4.step. Your phone viewer says STEP isn't supported, your slicer may not take it either, and you just want to see the part and print a test copy. The confusion is understandable: STEP and STL both describe 3D shapes, but they do it in completely different ways. Understanding STEP to STL conversion, and the one setting that matters during export, saves you from blocky circles and 200 MB files.
Short answer: STEP stores exact CAD surfaces (true circles, curves and solids); STL stores flat triangles that approximate them. Printers and most phone viewers need the triangles. Export STL (or 3MF) from your CAD program with a sensible resolution, in millimeters, then check it. STL Viewer does not open STEP files.
STEP vs STL: exact geometry vs triangles
| STEP (.step, .stp) | STL (.stl) | |
|---|---|---|
| Geometry | Mathematical surfaces (B-rep): planes, cylinders, splines | Flat triangles |
| A 10 mm hole is | A perfect cylinder | A polygon of, say, 32 to 128 sides |
| Units | Stored in the file | None |
| Editable in CAD | Yes, faces and edges can be selected | Only as a mesh |
| Assemblies | Yes, with part names | One mesh, or one file per part |
| Used for | Exchanging designs between CAD systems, machining | 3D printing and slicers |
STEP is the format to keep and exchange designs; STL is the format to print. You always go from STEP to STL, never the other way, if you want to keep a clean model. For more on the triangle format, see what an STL file is.
Why a phone viewer wants STL
Displaying STEP means evaluating exact surfaces and turning them into triangles on the fly, which is what a CAD kernel does. That is heavy machinery for a phone app, and it would still give you triangles in the end. Printing tools work with meshes, so a mesh is what a printing viewer reads. When you open a STEP or STP file in STL Viewer, it says STEP files aren't supported yet and asks you to export the model as STL, OBJ or 3MF in the app it was made with. The same goes for IGES.
The setting that matters: export resolution
When CAD exports STL, it decides how many triangles to use for every curved surface. Most programs expose this as some mix of:
- Chord tolerance (deviation): the maximum distance between the true surface and a flat triangle, for example 0.01 mm.
- Angle tolerance: the maximum angle between neighboring triangles, for example 5 degrees.
- Presets like "coarse / medium / fine".
Here is what that means for one part, a 60 mm flange with eight 6 mm holes. The numbers are illustrative, but the pattern holds for any part with curves:
| Setting | Triangles | Binary STL size | Visible in print? |
|---|---|---|---|
| Coarse (0.1 mm, 15 degrees) | ~6,000 | ~0.3 MB | Facets visible on holes |
| Medium (0.02 mm, 5 degrees) | ~40,000 | ~2 MB | Smooth at 0.2 mm layers |
| Fine (0.002 mm, 1 degree) | ~900,000 | ~45 MB | No visible difference vs medium |
For typical FDM printing, a deviation around 0.01-0.05 mm is plenty, because the nozzle can't reproduce finer detail. Going finer just makes slicing slower. Resin printers show more detail, so you might go a step finer for small resin parts.
How to export STL from CAD, step by step
- Open the STEP in your CAD program (or the original design file if you have it).
- If it is an assembly, decide what to print: hide or exclude parts you don't need, or export each part separately.
- Choose "Export" or "Save as" and pick STL (binary) or 3MF.
- Set units to millimeters. Many CAD tools default to the document unit, which may be inches.
- Set the resolution: medium or a deviation around 0.02 mm is a good start.
- Export and give the file a clear name, like
bracket_rev4_mm.stl.
3MF is a good alternative output because it stores the unit; see 3MF vs STL.
Check the exported STL
Send the STL to your phone and open it in STL Viewer for Android. In the info panel:
- Size should match the drawing. If the bracket should be 120 mm long and reads 4.72, the export was in inches. Switch the file units to inches to confirm, then re-export; our inches vs millimeters guide explains why 4.72 in is 120 mm.
- Triangles should be in the tens of thousands for a mechanical part, not millions.
- Watertight should say "Yes". CAD exports of solid bodies are almost always closed; if not, the body may have been a surface model.
- Switch to wireframe and zoom into a hole: you will see exactly how many sides your "circle" got.
With Premium, you can tap two points with Measure to verify a hole spacing, say 48.0 mm between centers, against the drawing. The measure guide shows how.
Note: STL Viewer does not open, display or convert STEP or IGES files. The conversion always happens in CAD or another desktop tool.
When you only have the STEP
If someone sends only a STEP and you have no CAD at hand, ask them for an STL or 3MF export at medium resolution in millimeters. It is a 30-second job on their side. If you must convert it yourself, free CAD programs can import STEP and export STL; follow the same steps above. Then run through the pre-print checklist before you slice.
More about which formats the app reads is on the STL Viewer website.
Frequently asked questions
What is the difference between STEP and STL?
STEP stores exact mathematical surfaces and is used to exchange CAD designs. STL approximates those surfaces with flat triangles and is used for 3D printing.
How do I convert STEP to STL?
Open the STEP in a CAD program, choose Export as STL or 3MF, set the units to millimeters and pick a medium resolution, then export.
What resolution should I use when exporting STL?
For FDM printing, a chord deviation around 0.01 to 0.05 mm is usually enough. Finer settings mostly add file size and slicing time.
Can I open a STEP file on my Android phone?
Not in STL Viewer, which shows a not supported message for STEP. Export the part as STL, OBJ or 3MF first, then open that file.