What an STL file is

An STL file describes the outside of a 3D object as a mesh of small triangles. It has no colours, materials or units of its own - just the shape - which is why 3D printers, slicers and CAD programs all read it. Files come in two forms, text (ASCII) and binary; a viewer reads both.

Because there are no units in the file, a model drawn in millimetres and one drawn in inches look the same to a viewer. If the size looks a thousand times out, the model was drawn in a different unit from the one you expected.

Open it in your browser

  1. Open 3D Viewer in your browser.
  2. Drop the .stl file on the page, or choose Open and pick it.
  3. Drag to turn the model and scroll to zoom.
  4. Use Measure to click two points for a distance, and Section to cut through it and see inside.

The model's size, volume and surface area show alongside it - useful for checking how much filament or resin a print needs.

Make .stl files open with a double-click

  1. Install Metre2 3D Viewer free from the Microsoft Store.
  2. Right-click an .stl file and choose Open with > Choose another app.
  3. Pick Metre2 3D Viewer and select Always.

Other files it opens

The same viewer opens OBJ (with its .mtl materials), GLB and glTF, 3MF, PLY and FBX meshes, and STEP and IGES files from CAD programs. If you've been sent a model in a format your software won't open, try it there first.