File formats · Beginner

STL file

The standard 3D printing file: a model’s surface as a list of triangles

What is an STL file?

An STL file stores the outer surface of a 3D model as a list of small triangles. It is the long-standing standard file for 3D printing, and every slicer can read it. It holds shape only, with no color, textures, materials or units, so the size has to be checked on import.

Think of it like this

Think of a mirror ball described tile by tile: every flat mirror is listed, so the shape is exact, but nothing says what color it is.

Where it fits in a 3D project

  1. Create
  2. Surface
  3. Rig
  4. Animate
  5. Render
  6. Export

Export: Saving and sharing the result: file formats, engines and printing.

How an STL file works

  1. 01The model’s surface is split into flat triangles.
  2. 02Each triangle is saved as three corner points and the direction it faces.
  3. 03The file is written as readable text or, more often, as smaller binary.
  4. 04A slicer reads the triangles and cuts the shape into printable layers.

STL file vs OBJ

PropertySTL fileOBJ
StoresTriangles onlyMesh, UVs, normals, material links
Color and textureNoYes, through a .mtl file
FacesTrianglesTriangles, quads or larger polygons
Best for3D printingMoving models between 3D apps

Use STL when a model is going to a printer; use OBJ when it needs UVs or textures, or is moving between 3D apps.

Where an STL file is used

  • Sending models to FDM and resin printers
  • Sharing printable files on model download sites
  • Exporting parts from CAD software for printing

Beginner tip

STL files carry no units. If a print arrives tiny or huge, the model was likely saved in millimeters and read as inches, or the reverse. Rescale it in the slicer before printing.

Terms related to an STL file

STL file FAQ

What does STL stand for?
It is usually traced to stereolithography, one of the first 3D printing processes. It is also often expanded as Standard Triangle Language or Standard Tessellation Language.
How to open an STL file?
Open it in any slicer, in Blender or another 3D app, or in a free online STL viewer. To print it, load it straight into the slicer that came with your printer.
STL vs OBJ: which should I use?
For a single-color print, STL is the safer default because every printer tool expects it. OBJ is better for models with UVs and textures, or when moving work between 3D apps.
Does STL store color?
No. Standard STL holds only triangle shapes. For color or multi-material prints, use 3MF, or a color-capable format such as OBJ that your printing service accepts.
Why is my STL file not printing correctly?
Usually the mesh has holes, overlapping parts or flipped faces, so the slicer cannot tell inside from outside. Run a repair tool in the slicer or a 3D app to make it watertight, then slice again.

Updated October 29, 2026 · Browse all 3D terms