What STL Export Tolerance Should I Use?
Every CAD-to-STL export has one setting that matters more than all the others combined, and it hides under different names: tolerance, deflection, chord height, maximum deviation. It is the answer to one question — how far may a flat triangle stray from the true curved surface? Set it too coarse and your cylinders print as visible polygons. Set it too fine and a bracket becomes a 400 MB file that chokes the slicer without printing any better. This guide gives working values and explains the two failure modes.
What Deflection Actually Means
A STEP file describes a cylinder mathematically: axis, radius, exact everywhere. An STL can only approximate it with flat triangles, and deflection is the maximum gap allowed between a triangle and the true surface, in absolute units, usually millimetres. At 0.1 mm deflection, no point of the mesh is more than 0.1 mm from the real geometry. Halving the deflection roughly quadruples the triangle count on curved surfaces, which is why the setting trades accuracy against file size so steeply — and why flat faces are immune: a plane tessellates to two triangles at any tolerance.
Values That Work
| Deflection | Use case | What to expect |
|---|---|---|
| 0.01 mm | Small precise parts, threads, fine fits | Large files; only worth it when features are sub-millimetre |
| 0.05 mm | General 3D printing (finer than most printers resolve) | Good default for parts under ~100 mm |
| 0.1 mm | Default in our STEP→STL tool; fine for most FDM work | Faceting invisible after printing at typical layer heights |
| 0.25 mm | Large parts, drafts, fit checks | Slight faceting visible on small-radius curves |
| 0.5+ mm | Visualization, web viewers, renders | Clearly faceted; never for manufacturing |
The sanity check behind these numbers: an FDM printer with a 0.4 mm nozzle cannot reproduce surface detail much below 0.2 mm anyway, so exporting at 0.01 mm for FDM is paying file size for accuracy the machine will erase. Resin printers resolve finer and justify 0.02 to 0.05 mm. The rule of thumb: deflection at half your printer’s finest visible detail, and no finer.
The Two Failure Modes
Too coarse is obvious after printing: a 10 mm hole exported at 0.5 mm deflection becomes a polygon whose flats you can feel, and mating parts bind because the hole is effectively undersized. Less obviously, the error is baked in: no slicer setting recovers geometry the export threw away. Too fine fails earlier: multi-hundred-megabyte files, uploads that time out, slicers that crawl. And if a too-fine export is the file you are stuck with, decimation can rescue it — that is exactly the over-tessellated case mesh simplification handles well.
Convert STEP to STL with the deflection under your control, and see the triangle count it produces.
STEP to STL ConverterOne more parameter exists in most exporters: angular deflection, which caps the angle between adjacent triangles (useful on small-radius curves that absolute deflection alone under-tessellates). If your exporter offers it, 15 to 20 degrees pairs well with the absolute values above. When in doubt, export once at 0.05 mm, look at the file size and the slicer preview, and adjust one step in whichever direction the result argues for.
Frequently Asked Questions
What is a good STL export tolerance for 3D printing?
For typical FDM parts, 0.05 to 0.1 mm absolute deflection. Finer than 0.02 mm rarely helps any process, and coarser than 0.25 mm shows on curved surfaces. If your exporter asks for angular tolerance too, 15 to 20 degrees.
Is a higher-resolution STL always better?
No. Below the resolution your printer can physically make, extra triangles only cost file size and slicing time. The error worth eliminating is the visible faceting of a too-coarse export; past that point you are polishing numbers the nozzle will flatten.
My exported STL is huge. Do I re-export or decimate?
Re-export coarser if you still have the CAD; tessellating from exact surfaces beats approximating an approximation. Decimate when the mesh is all you have. Both paths end at the same place: enough triangles to carry the shape, no more.
Does deflection affect dimensional accuracy?
Yes, one-sided: tessellation can only cut inside a convex curve, so holes come out very slightly small and bosses very slightly slim, by up to the deflection value. At 0.05 mm this vanishes under printing tolerances. It is one reason a mesh is the wrong format for toleranced machining — covered in the STEP vs STL manufacturing guide.
This guide covers one problem. For the full set of 3D converters, checkers, and calculators, visit the hub.
Browse 3D Tools