.STL

Stereolithography

Introduction

STL was created by 3D Systems in 1987 for its stereolithography printers, and it is still the most common format for 3D printing. An STL file holds one thing: a surface made of triangles. It has no colours, materials or units, so a model in millimetres and one in inches look the same to the file.

There are two forms with the same extension and the same content. ASCII STL is readable text; binary STL is smaller and far more common.

Variations

FormatVariationAboutOpen
.STLASCII STLThe text form of STL: each triangle a facet of three vertex lines.Open
.STLBinary STLThe usual form of STL: a header, a count, then 50 bytes per triangle.Open

ASCII STL

The text form: each triangle is written out with the keywords facet, outer loop and vertex. It is easy to read and to write by hand, but several times larger than the binary form.

The file starts with the word solid and ends with endsolid. Because some binary files also begin their header with solid, a reader should check further: an ASCII file has facet lines after the first line, and its size does not match the binary formula below.

Binary STL

The compact form: an 80-byte header, a count of triangles, then 50 bytes per triangle. Slicers and CAD programs write this form by default.

The file size is exactly 84 + 50 × count, where the count is the 32-bit little-endian number at offset 80.

Links

Software

Developers

  • numpy-stl, Python library that reads and writes ASCII and binary STL
  • trimesh, Python library that loads and saves STL among other mesh formats
  • Assimp, C++ library that imports dozens of 3D formats, STL among them