STL (3D printing)
Also known as: .stl file, stereolithography, how to open STL
STL is the standard file format for 3D printing. It describes a model’s surface as a mesh of triangles, with no color, texture, or units — just the geometry a slicer needs to turn into printer instructions.
- Surface mesh of triangles; no color or units
- The default input format for 3D-printing slicers
- Comes in compact binary and larger ASCII forms
What an STL file stores
STL (stereolithography) reduces a 3D model to a single watertight shell of triangles. The more triangles, the smoother curved surfaces look — and the larger the file. It records only the surface; there is no color, material, or built-in scale unit.
Because it is so simple, STL is the common language between modeling software and printers. It comes in a compact binary form and a larger, human-readable ASCII form that stores the same triangles as text.
How STL fits 3D printing
To print an STL you load it into a slicer (such as Cura, PrusaSlicer, or Bambu Studio), which converts the mesh into layer-by-layer toolpaths the printer follows. You can preview an STL in Blender, the Windows 3D Viewer, or many free online viewers.
STL’s limits — no color and no real units — mean newer formats like 3MF and glTF are gaining ground for full-color printing, but STL remains the default almost everywhere.