A model downloads as a GLB, your slicer wants an STL, and the print file someone shared is a 3MF. STL vs 3MF is the question most people ask first, but GLB belongs in the same answer, because it's what AI generators and most web viewers hand you. The three formats store very different things, and choosing the wrong one costs you colour, scale or your slicer settings.
What each format stores
| STL | 3MF | GLB | |
|---|---|---|---|
| Geometry | Triangles only | Triangle meshes, several objects | Meshes in a scene |
| Units | None, slicers assume millimetres | Declared in the file, millimetres by default | Metres, by the glTF standard |
| Colour and textures | No | Colours and materials | Full textures |
| Materials | No | Base materials for printing | PBR (metal, roughness and more) |
| Print settings | No | Often, when a slicer saves the project | No |
| Up axis | Z | Z | Y |
| Main use | Getting a mesh into any slicer | Saving and sharing print projects | Web, game engines, AR, AI generators |
STL: the lowest common denominator
An STL is a list of triangles, each with a direction it faces. Nothing else. There's no colour, no material, and no unit: a number like 25 could mean 25 millimetres or 25 metres, and the file can't tell you which. Slicers read it as millimetres, which is right for files that came out of CAD tools or printing sites and wrong for a lot of files that didn't.
STL comes in two flavours. ASCII STL is readable text and much larger. Binary STL uses a fixed 50 bytes per triangle plus an 84-byte header, so you can work out the size in your head: a 500,000-triangle mesh is about 25 MB, a million triangles about 50 MB. That's why a dense AI-generated model can turn into a surprisingly heavy STL.
Its strength is that every slicer, CAD program and print service opens it. When in doubt about what a tool accepts, STL is the safe bet.
STL vs 3MF: what 3MF adds
A 3MF file is a ZIP archive of XML files, built by the 3MF Consortium as a replacement for STL in printing. The core specification puts a unit on the model, millimetre by default, with micron, centimetre, inch, foot and metre also allowed. So a part drawn at 20 mm opens at 20 mm, with no guessing.
It can also carry colours and materials for each object, several objects on one plate, and a thumbnail. Slicers such as PrusaSlicer, Bambu Studio and OrcaSlicer save their whole project as 3MF: orientation, supports, painted colours for multi-material prints and the print settings. Each slicer adds its own extras, so a project saved in one may not keep every setting when opened in another, even though the geometry comes through.
Because it's compressed, a 3MF is often smaller than the same mesh as binary STL. The cost is compatibility: older tools, some print services and some CAD programs still only take STL.
GLB vs STL: why your model arrives as GLB
GLB is the binary, single-file form of glTF 2.0, a format from the Khronos Group built for showing 3D on screens. One GLB holds the meshes, the textures as images, PBR materials that describe how metallic and rough each surface is, the scene layout and even animations. That's why AI 3D generators, including every mesh generator in our studio, return GLB by default: it's the only one of the three that keeps the model looking like the preview.
For printing, GLB has two traps. It's in metres and Y-up, while slicers expect millimetres and Z-up. A model that's 0.15 units tall in a GLB lands in your slicer as 0.15 mm, and it may lie on its back. Neither is a broken file. You scale it up and rotate it.
OBJ and PLY, the two you'll meet next
OBJ opens in almost any 3D program. Its catch is that materials live in a separate .mtl file, with textures as more files beside it, so a lone .obj someone emails you arrives grey. If you send an OBJ, zip it with its .mtl and images. PLY comes from 3D scanning and photogrammetry, and stores colour per vertex instead of as a texture image, which suits scans but makes colours blurry on a low-poly mesh. Neither has a fixed unit. For printing, both end up in the same place as GLB: converted to STL or opened in the slicer, then saved as a 3MF project.
Which one to use
- Keep the GLB as your master copy of anything generated. It's the only version with the colours.
- Use STL to get a mesh into a slicer, or to send it to a print service that asks for it.
- Save your slicer work as 3MF. It keeps orientation, supports and settings together, so you can reprint without redoing them.
- Share a print-ready model as 3MF when the other person uses a modern slicer, and as STL when you don't know what they use.
- Use GLB for the web, Blender, game engines and AR.
Our recommendation for anything you print: GLB in, STL to the slicer, 3MF out. The cost is three files per model instead of one, and that's a small price for never losing your colours or your print settings.
How to convert between them
The free 3D model converter runs in your browser and never uploads your file. It reads STL, OBJ, GLB, glTF, PLY, FBX and 3MF, and writes STL, OBJ, GLB, glTF and PLY, for files up to 100 MB.
- Drop in the file.
- Pick the output format. Choosing STL warns you that colours and textures will be dropped.
- Leave Rotate Z-up to Y-up as the converter sets it. It switches on by itself for STL and 3MF input, which are usually Z-up. If a model still lands on its back in the slicer, rotating it there takes one click.
- Keep Binary on for STL and PLY. Turn it off only if you need a text file you can read.
- Download.
Two limits to know. The converter can read 3MF but can't write it, so your 3MF has to come from a slicer. And it keeps the numbers as they are, without rescaling: a GLB in metres becomes an STL your slicer reads as millimetres, so scale it up by 1,000 there or measure first.
To measure, open the file in the STL viewer. It opens the same formats as the converter, guesses the unit from the format (millimetres for STL and 3MF, metres for GLB), lets you change that guess, and shows the size in millimetres and inches plus the triangle count. Converting 3MF to STL is worth doing only when a tool refuses 3MF, because it throws away the units, colours and settings that made the 3MF useful.
The short version
STL is triangles with no units or colour, opens everywhere, and grows by 50 bytes per triangle. 3MF adds units, colours, materials and slicer settings in a compressed ZIP, and is the better file for saving print projects. GLB is for screens: textures, PBR materials, metres and Y-up. Keep the GLB, send STL to the slicer, and save the project as 3MF.
Need an STL from a GLB right now? The 3D model converter does it in your browser, free.
