# What is an STL file?

> An STL file (.stl) is a 3D file that describes an object's surface as a list of triangles, each with three corner points and a normal. 3D Systems created the format in 1987 for stereolithography printers, and every 3D printing slicer reads it today. It stores only shape, with no colour, textures, units or separate parts.

URL: https://kenerateai.com/what-is-an-stl-file
Publisher: Kenerate AI (https://kenerateai.com)
Last updated: 2026-10-02

## Key facts
- Created: 3D Systems, 1987
- Stores: Triangles + facet normals
- Doesn't store: Colour, textures, units, parts
- Binary size: 50 bytes per triangle (+84-byte header)
- Opens in: Every slicer, Blender, most CAD

## In depth

### What's inside an STL
An STL is a flat list of facets. Each facet is a triangle defined by three vertices and a unit normal pointing outward [1]. There is no shared vertex list, no material and no hierarchy — which makes the format trivial to write and read, and universally supported.

### Binary vs ASCII STL
Encoding — 80-byte header, triangle count, 50 bytes per triangle [1] — Plain text: facet normal … vertex …
Size — Compact — Several times larger
Use — Almost all files today — Debugging, very old tools

### What STL can't do
- No units — a 50 can mean 50 mm or 50 inches; slicers assume mm [1].
- No colour or textures — prints come out in the filament's colour.
- No separate objects or settings — use 3MF for multi-part, multi-colour projects [2].
- No editable features — it's triangles, not CAD.

### How big will an STL be?
At 50 bytes per triangle [1], a 100,000-triangle model is about 5 MB and a million-triangle model about 50 MB. AI-generated and scanned models can be very dense; decimating them rarely changes how they print.

### How to open, edit or make an STL
- Open: the free STL viewer, any slicer (PrusaSlicer, Cura, Bambu Studio, OrcaSlicer) or Blender.
- Edit: resize, rotate, mirror and fix units in the free STL editor; remodel in Blender or CAD.
- Make: export from CAD or Blender, convert a GLB/OBJ with the free converters, or generate one with AI (Rodin outputs STL directly).
- Upgrade: save as 3MF to store units in millimetres.

References:
[1] Wikipedia — STL (file format): https://en.wikipedia.org/wiki/STL_(file_format)
[2] 3MF Consortium — 3MF specification: https://3mf.io/specification/

## FAQ
Q: What does STL stand for?
A: Usually “stereolithography”; it's also expanded as Standard Triangle Language or Standard Tessellation Language.

Q: Does an STL file have units?
A: No. Slicers assume millimetres, which is why inch-based files import 25.4 times too small.

Q: Can an STL have colour?
A: Not in the standard format. Use 3MF for colours and multiple materials.

Q: How do I open an STL file?
A: Drop it into an online STL viewer, a slicer or Blender.

Q: How do I make my own STL?
A: Model it in CAD or Blender, convert another 3D file, or generate one with AI and convert to STL.

Q: Are STL files already sliced?
A: No. An STL only describes the shape. Your slicer turns it into printer instructions — G-code or a printer-specific file — with layers, speeds and supports. Sliced files have other extensions, such as .gcode, or are saved inside a project 3MF.

Q: Are STL files safe to download?
A: The format itself holds only triangle data, not programs, so an STL can't run anything on its own. Still download from sites you trust, check the model's licence before selling prints, and preview it in an STL viewer before you print.

Q: Is STL or 3MF better?
A: 3MF stores units, colours and multiple objects; STL is simpler and opens everywhere.

## Related pages
- STL viewer: https://kenerateai.com/stl-viewer
- STL editor: https://kenerateai.com/stl-editor
- 3MF vs STL: https://kenerateai.com/3mf-vs-stl
- What is a 3MF file?: https://kenerateai.com/what-is-a-3mf-file

## Sources
- Wikipedia — STL (file format): https://en.wikipedia.org/wiki/STL_(file_format)
- 3MF Consortium — 3MF specification: https://3mf.io/specification/