What the standard defines
A list of triangular facets, each with three corner points and a direction, which together approximate the surface of a solid. Files come in a text form and a more compact binary form. That is the whole content: a shape, described by its skin.
What it does not promise
No units. The coordinates are bare numbers; whether they mean millimetres or inches is agreed outside the file. No colour, material or print settings. No guarantee of a closed, printable solid: a mesh can have holes, flipped or overlapping faces, and slicers repair or reject them. No design history: an STL cannot be edited like the CAD model it came from, only reshaped as a mesh.
What it works with
Slicers, which turn the mesh into machine instructions. 3D scanners, which commonly export it as one of their output formats. 3MF, which carries the same geometry with units, colours and settings when more than a shape has to travel with the file.
Questions makers ask before buying
Why does an STL sometimes print a hundred times too small?
Because the file carries numbers but no unit. A model drawn in one unit and read in another scales wrongly, and the slicer has to be told the intended size.
Can an STL hold colours?
Not in its common form. Multicolour prints use formats that carry colour or several objects, such as 3MF.
Last reviewed 15 September 2026