What Is an STL File? The 3D Printing Format Explained
Definition
STL is a 3D file format that describes a surface as a list of triangles, each with three corner points and a normal. It stores no units, colors or materials, which makes it simple and almost universally supported by slicers and CAM software.
Also called: stereolithography file, .stl, standard triangle language.
STL dates from the first stereolithography printers in the late 1980s and is still the lowest common denominator of 3D printing. Every slicer reads it.
Binary and ASCII
Binary STL stores each triangle in 50 bytes and is the norm. ASCII STL is human-readable text and much larger.
Watertight meshes
A slicer needs to know what is inside and outside the model. That only works if the surface is closed. Tools on Depthue, like the image to STL tool, always export watertight meshes. Check any STL with the STL viewer.
Questions
What units does an STL use?
None. STL stores numbers without units. Nearly all 3D printing software assumes millimetres.
What makes an STL printable?
A closed, watertight surface where every edge is shared by exactly two triangles, with normals pointing outward and no self-intersections.