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How to open and check an STL file before 3D printing
Open STL files on Mac, Windows or in a browser, check them before printing, and convert a GLB, OBJ or FBX model into STL with free tools.
3 min read
Quick answer
How do I open an STL file?
On a Mac, double-click it to open in Preview. On any computer, drag it into a browser viewer such as Babylon.js Sandbox. If you plan to print it, open it in your printer's slicer, which shows it on the build plate at real size. STL stores only triangles, so expect no colour or texture.
What an STL file holds
STL was created for 3D Systems' stereolithography printers. The Library of Congress format description dates its first specification to 1988. An STL is a list of triangles, each with three corner points and a normal saying which way it faces. That is all.
What it does not hold matters just as much:
- No colour or texture. There is no standard way to store colour, so a textured model arrives grey.
- No units. An STL stores numbers, not millimetres or inches. Most slicers assume millimetres, which is why a model sometimes appears tiny or enormous.
- No materials, UVs, animation or scene structure.
Files come in two encodings, ASCII text and binary. Binary is more common because it is far smaller, and almost every tool reads both.
Open it on a Mac
Double-click. Apple lists STL among the formats Preview supports.
Open it on Windows or any computer
Windows no longer includes 3D Viewer (see what replaced it), so use a browser: Babylon.js Sandbox or Online 3D Viewer. Both read STL. Drag the file in.
Open it in your slicer
If the file is going to a printer, the best viewer is the slicer you will print with, such as PrusaSlicer, Cura or Bambu Studio. It shows the model at real size on your build plate, and its warnings point to the problems that actually stop a print.
Check it before printing
General 3D checks apply (see the inspection checklist), plus three that matter for printing:
- Is it watertight? A printable mesh must be closed, with no holes. Gaps confuse the slicer about what is inside.
- Are the normals consistent? Flipped triangles make the slicer think part of the model is inside out.
- Is it the right size? Check the dimensions against what you expect. A cup that is 90 mm tall in your head may come in at 9 or 900.
Many slicers repair small problems automatically. For bigger ones, the free 3D-Print Toolbox extension for Blender checks for non-manifold edges and thin walls.
Turning another model into an STL
If you have a GLB, OBJ or FBX model and want to print it:
- Blender: import the model, apply transforms (Ctrl+A), check the scale in millimetres, then File › Export › STL.
- Online 3D Viewer: open the model and export it as STL directly in the browser.
Remember the colour is gone after conversion. Detailed painted textures do not print unless your printer handles colour through another format such as 3MF.
Generated models and printing
Vi3W exports GLB, FBX and OBJ, not STL, so printing a Vi3W model means one conversion step through Blender or Online 3D Viewer. Check the result is closed and correctly scaled before you slice. Generated models are made for screens first, so they may need thickening in places before they print well.
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