STL File Format (.stl)

An STL file is a triangle mesh created by 3D Systems in 1987 for its stereolithography 3D printers, storing nothing but a flat list of triangle facets and their normals, with no colour, material or unit of measurement attached.

.stl3D Models

STL at a glance#

Full name	Stereolithography
File extension	.stl
Category	3D Models
Developer	3D Systems
First released	1987
MIME types	model/stlmodel/x.stl-asciimodel/x.stl-binary
Compression	Uncompressed
FileFlip support	FileFlip reads and writes this format

Strengths and limitations of STL#

What STL does well

So simple that essentially every 3D printer slicer accepts it without configuration
ASCII and binary variants both exist, so a file can be human-readable or compact as needed
No practical limit on triangle count, so dense scans still export fine

Where STL falls short

Stores no colour, material, texture or unit of measurement, so scale has to be agreed on separately
No way to encode multiple parts as one object short of merging their geometry
Nothing in the format itself validates that a mesh is manifold or free of self-intersections

Use STL when the only destination is a 3D printer's slicer, which is all the format was designed to feed.

Convert STL to 3MF when colour, material or unit information needs to travel with the geometry, since STL has nowhere to put any of it.

How to open a .stl file#

A .stl file opens in the programs below, grouped by the platform you are on. If you would rather not install anything, FileFlip converts STL into a format your machine already opens, and it does it in the browser tab rather than on a server.

Platform	Programs that open it
Windows	PrusaSlicer, Cura, Autodesk Meshmixer, Blender
macOS	Blender, Autodesk Meshmixer
Linux	Blender, PrusaSlicer, Cura
Web	FileFlip

STL questions people ask#

What is an STL file used for?

An STL file is the standard input for 3D printer slicing software, storing a model's surface as a list of triangles with nothing else attached. Slicers use it to work out where to lay down material layer by layer.

Why doesn't my STL file have color?

STL was designed in 1987 purely to describe a shape for stereolithography printing, so it never included fields for colour, material or texture, only triangle geometry. 3MF was created later specifically to add colour and material data that STL can't hold.

What's the difference between ASCII and binary STL?

ASCII STL spells each triangle out as readable text starting with 'solid' and ending with 'endsolid', while binary STL packs the same triangles into a compact format with an 80-byte header followed by a triangle count and raw floating-point data. Binary STL is far smaller and is what most software writes by default.

What units does an STL file use?

STL has no built-in unit of measurement; every coordinate is just a number, and the software producing or consuming the file has to agree separately on whether it means millimetres, inches or something else. Importing an STL into the wrong unit setting is a common cause of a model coming out the wrong size.

Converting STL files#

Convert STL to another format

Every conversion runs inside your browser. The file is read from your disk, the work happens on your own machine, and nothing is uploaded. How FileFlip works names the engine behind each family and how to verify that for yourself.