What is an STL file?

An STL file is a 3D model saved as a list of triangles that describe its surface. It stores shape only: no colour, no texture, no materials and no unit. It is the standard file for 3D printing, because every slicer reads it, and it was created by 3D Systems in 1987 for the first stereolithography printers.

Drop your STL file here

Extension	.stl
Name	Stereolithography (also read as Standard Triangle Language)
Created by	3D Systems, 1987
Stores	Triangles only
Colour and texture	No
Units	None stored. Millimetres by convention
Variants	Binary and ASCII
Opens in	Every slicer, Blender, FreeCAD, Fusion, this page

What is inside an STL file

Each triangle is written as a normal, the direction its front faces, followed by its three corners as x, y and z numbers. That is the whole format. There are no shared vertices, so two triangles that meet at an edge each store that edge on their own.

Binary STL starts with an 80-byte header and a triangle count, then packs each triangle into 50 bytes. ASCII STL writes the same data as text, with words like facet, vertex and endloop, and is about four to five times larger. Both hold exactly the same shape.

Why 3D printing still uses STL

A slicer needs one thing: the closed surface of the part, so it can cut it into layers. STL describes exactly that and nothing more, which made it easy for every printer, slicer and CAD tool to support. Forty years on, that simplicity is why it still works everywhere.

The cost shows when you want more than shape. An STL has nowhere to store colour for a multi-material print, which part is which, or what size the numbers mean. That is why 3MF was created, and why modern slicers save their projects as 3MF.

How to open an STL file

You do not need to install anything to look at one. Use the viewer at the top of this page, or one of these:

A slicer, if you plan to print it: Bambu Studio, PrusaSlicer, OrcaSlicer or UltiMaker Cura, all free.
Blender, free, if you want to edit the mesh.
FreeCAD or Fusion, if you need to measure it or use it in a CAD model.

Common problems with STL files

Because STL stores triangles one by one, nothing in the format keeps the surface closed. Holes, triangles that face the wrong way and duplicated triangles are common, especially in scans and in models merged from several parts. A slicer may show missing walls or refuse to slice. The STL repair tool fixes the usual cases in your browser.

The other common surprise is size. STL stores numbers without a unit, so a model drawn in inches or metres can arrive in the slicer 25 times too small or 1,000 times too small. Check the dimensions before you print.

Free tools for STL files

STL viewer Open a .stl file and check the mesh before you print it. STL repair Fill holes and fix flipped triangles in an STL before you print it. 3MF to STL Turn a .3mf from your slicer into a plain .stl any printer accepts. STEP to STL Turn a STEP file from CAD into an STL for your slicer. GLB to STL Convert a GLB to a binary STL for 3D printing. OBJ to STL Convert an OBJ to a binary STL for 3D printing. FBX to STL Convert an FBX to STL for 3D printing.

Frequently asked

Is STL the same as 3MF?

No. Both describe a 3D model for printing, but STL stores only triangles. 3MF also stores the unit, colours, materials, several named objects and their position on the build plate. Most slicers read both. Use 3MF when you need colour or several parts; use STL when you need a file every tool accepts.

Can an STL file have colour?

Not in the standard. A few programs write colour into the unused bytes of a binary STL, but there are two incompatible ways to do it and most software ignores both. For a colour model, use 3MF, OBJ or GLB.

What unit is an STL file in?

None. The format stores plain numbers. Slicers treat them as millimetres, so almost every STL made for printing uses millimetres. If a model arrives tiny or huge, it was probably exported in inches or metres; scale it by 25.4 or 1,000.

How do I edit an STL file?

For mesh edits, use Blender or Meshmixer, both free. For precise changes, like resizing a hole, an STL is hard to edit because it has no features, only triangles. Go back to the original CAD file if you have it, often a STEP, and export a new STL.