STL is the de-facto standard for 3D printing. It describes a model purely as a mesh of triangles — no color, no units, no assembly structure — which makes it simple, universally supported, and the required input for virtually every slicer, from PrusaSlicer and Cura to resin-printer tools.
Every slicer, every printable-model site — Printables, Thingiverse, MakerWorld — and every CAD "save as STL" produces STL. If someone sent you something to print, this is almost certainly what they sent.
Best for: 3D printing and rapid prototyping. There is a fuller write-up on the STL format reference page.
Every format on this site also gets the same viewport tools: orbit, pan and zoom with fit-to-view, isometric and the six standard views, a perspective/orthographic toggle, shaded, shaded-with-edges, wireframe and X-ray display, hide-selected, and a PNG screenshot of the current view. The same controls are on the general 3D and CAD viewer, which accepts every importable format at once.
Worth knowing before you rely on it. A viewer that overstates what it can read wastes more of your time than one that is specific about its limits.
| STL files | |
|---|---|
| Extensions | .stl |
| Geometry | Triangle mesh |
| Units | No — unitless numbers |
| Colors & materials | No |
| Animations | No |
| Typical use | 3D printing |
STL is unitless. Coordinates are shown exactly as stored with a millimetre label, so if the export was in inches the numbers are inches wearing the wrong suffix.
STL is both readable and writable here, so it is usually the destination rather than the source. From an STL the useful moves are re-packaging it as 3MF or lightening it. The converter runs on the same local pipeline as this viewer, so a conversion is just as private as a preview: STL to 3MF is the usual choice.
Drop it onto the viewer above. It renders in the browser with section planes, measurements, display modes and a Properties panel — no slicer, no CAD package and no account needed.
No. The mesh is parsed by a JavaScript loader in a Web Worker inside your own tab, so nothing is transmitted. A model under NDA is as safe here as it is on your desktop.
Only indirectly. You get real dimensions, triangle count, wall thickness through a section plane and a volume figure, which catches scale and hollowness problems. It does not run a manifold or watertightness check — a slicer or a repair tool does that.
Because the format carries no units, so the exporting tool’s numbers are taken at face value and labelled millimetres. A part that reads 25.4 was probably exported in inches. Scale it in your slicer, or re-export with millimetres selected.
Geometry: triangle mesh. Units: no — unitless numbers. Colors and materials: no. Animations: no. Of that, the viewer draws geometry and flat base colors; texture maps and animation are never displayed. STL is unitless. Coordinates are shown exactly as stored with a millimetre label, so if the export was in inches the numbers are inches wearing the wrong suffix.
Read the STL format reference for the format itself, open the general 3D and CAD viewer if you have several formats to look at, or start from the 3D and CAD converter.
If the model is heavier than it needs to be, the mesh simplifier reads .stl directly, decimates it in your browser and previews the result before you export. If the file is headed for a 3D printer, the G-code viewer shows the sliced toolpath layer by layer once your slicer has run.
STEP stores exact CAD geometry, STL stores triangles. Understand the difference, when each format wins, and how to convert between them for free.
3MF fixes almost everything wrong with STL — units, colors, watertightness, file size. A practical comparison, plus free tools to convert STL to 3MF.
Which file format should you actually print from? A practical ranking of STL, OBJ, 3MF and AMF for FDM and resin printing, with free conversion tools.