STL (3D model file)

/ˌɛs tiː ˈɛl/ · noun · 3D file format · stereolithography

STL is the standard file format for 3D printing. It stores the surface of a model as a mesh of triangles, with no color, texture or units.

In more detail

STL was created in the late 1980s for stereolithography, one of the first 3D printing methods. Each triangle is stored as three corner points plus a facet normal that says which way the surface faces. Slicers read the triangles to work out the outline of every printed layer.

Because it is simple, nearly every CAD app, slicer and model-sharing site supports STL. The same simplicity is its weakness: there are no colors, materials, units or links between neighboring triangles.

Binary and ASCII STL

A binary STL starts with an 80-byte header and a triangle count, then stores each triangle in 50 bytes. An ASCII STL writes the same data as readable text, with lines such as facet normal, outer loop and vertex. ASCII files are often four to five times larger, so most exporters default to binary.

STL has no unit field. By convention, 3D printing models are in millimetres. If a model loads tiny in a slicer, it was probably exported in inches and needs scaling by 25.4.

STL compared with OBJ and 3MF

STL repeats every corner point for each triangle that touches it. OBJ lists each vertex once and can carry texture coordinates and materials, which makes it the usual choice for 3D editors and game engines. 3MF is a newer printing format that adds units, colors, multiple parts and print settings in one file.

Use STL when a slicer or print service asks for it. Convert to OBJ when you need the model in Blender, a game engine or a renderer.

Key points

STL describes only a model's surface, as triangles.
Binary STL is compact; ASCII STL is readable text.
STL has no units, color or texture. Most print files are in millimetres.