What is a .stl file?
Stereolithography · 3D model · 3D Systems · 1987
STL is the standard file for 3D printing. It describes an object's surface as a mesh of triangles, with no colour or texture, which is all a slicer needs to turn a model into printer instructions.
What is a .stl file used for?
An STL stores only geometry: a shell of triangles that defines the object's surface. It carries no colour, material or units, just shape.
That minimalism is exactly why it became the universal handoff format between 3D modelling software and slicers that drive 3D printers.
For prints that need colour or texture, richer formats like OBJ, 3MF or glTF are used instead.
What opens a .stl file?
The .stl format opens in the following apps, grouped by operating system. If one app does not work, try another from the same list.
Windows
- Cura
- PrusaSlicer
- Windows 3D Viewer
- Blender
macOS
- Cura
- PrusaSlicer
- Blender
Linux
- Cura
- PrusaSlicer
- Blender
- MeshLab
Web
- Online STL viewers
Open a .stl file step by step
- Confirm the file really is a .stl. Drop it into the WhatFileType identifier, which reads its signature in your browser without uploading anything.
- On Windows, open it with Cura or PrusaSlicer.
- On Mac, use Cura or PrusaSlicer.
- On Linux, try Cura or PrusaSlicer.
- Still stuck? Convert the .stl to OBJ, which opens more widely.
Why can't I open my .stl file?
Almost always because no installed app understands STL. Install one of the apps above, or convert the file to a format your existing software already reads. If double-clicking opens the wrong program instead of nothing at all, the file association is set to the wrong app rather than the file being broken.
The other possibility is that the file is not really a .stl at all. Drop it into the file identifier to check it by its bytes instead of its name. Nothing is uploaded.
How to convert a .stl file
You can convert a .stl file to OBJ, 3MF, GLTF.
Blender and MeshLab both import STL and export OBJ, 3MF or glTF. STL stores only bare triangles, so a conversion cannot add the colour, material or units it never carried.
What is the .stl file signature (magic bytes)?
A .stl file starts with the hex bytes 73 6F 6C 69 64, which read as solid in ASCII. Programs identify STL by those bytes, not by the filename, so a renamed file still gives itself away.
These are the documented signatures for STL.
| Hex signature | Offset | ASCII | Note |
|---|---|---|---|
| 73 6F 6C 69 64 | 0 | solid | 'solid' (ASCII STL) |
See the full magic-numbers reference for every format.
Frequently asked questions
What is a .stl file?
STL is the standard file for 3D printing. It describes an object's surface as a mesh of triangles, with no colour or texture, which is all a slicer needs to turn a model into printer instructions.
How do I open an STL file?
Use a 3D printing slicer like Cura or PrusaSlicer, the built-in 3D Viewer on Windows, or Blender for editing.
Can STL files have colour?
No. STL stores only shape. For colour or texture, use OBJ, 3MF or glTF.
Related formats
Sources
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