STL or 3MF: Which Format Should You Send to Your Slicer?

Download a model from Printables or MakerWorld and you will often get the choice: STL or 3MF. Bambu Studio and PrusaSlicer save their projects as 3MF, yet many model libraries still offer STL first. Both print. They do not carry the same information, and the difference shows as soon as a model has several parts, colours, or an unusual scale.

What an STL file contains

STL dates from 1987, when 3D Systems created it for the first stereolithography machines. It stores a list of triangles, each with three corners and a normal. That is all: no unit, no colour, no material, no notion of separate objects. Every slicer reads it, which is exactly why it has lasted so long.

The missing unit is the classic trap. An STL says "40", never "40 millimetres". The print world agrees to read millimetres, so a file modelled in inches or metres arrives at the wrong size without any warning.

What a 3MF file contains

3MF (3D Manufacturing Format) is published by the 3MF Consortium, founded in 2015 by Microsoft and several players of the 3D industry. A .3mf is a ZIP archive holding the model as XML. On top of the geometry, it can store:

• the unit of the coordinates, declared in the file;
• colours and materials, down to each triangle;
• several objects and their place on the build plate;
• a thumbnail and metadata;
• through extensions, print settings: Bambu Studio, OrcaSlicer and PrusaSlicer save a whole project in it, profiles and supports included.

It is also compressed, so a 3MF is usually noticeably smaller than the same model saved as STL.

STL and 3MF side by side

When STL is still the right choice

• a single-part, single-colour model;
• a printer, a print service or a tool that only accepts STL;
• a CAD workflow that does not export 3MF;
• a file that has to open everywhere, slicer or not.

When to switch to 3MF

• the model has several parts that must stay assembled;
• it has colours you want to keep for a multi-colour print;
• you want the unit settled once and for all;
• you want to keep a ready-to-print project with its settings, saved from the slicer.

Converting a model to 3MF on a Mac

Most models you want to print do not start as STL or 3MF at all: game assets in FBX, scans in PLY or OBJ, web models in GLB, AR files in USDZ. Eyemesh opens 17 formats on macOS and, since version 1.5.2, exports any of them to 3MF in one click, merged into one object, turned Z-up and with its colours kept. The full walkthrough is in exporting a 3D model to 3MF for Bambu Studio or PrusaSlicer. Going the other way, Eyemesh also exports to STL: see converting 3MF to STL.

And to look at a file before choosing anything, pressing the space bar in the Finder is enough. We explain how in previewing STL files on Mac without a slicer.

FAQ

Is 3MF better than STL?

It stores more: the unit, colours, several objects, settings. For a single-colour, single-part model, both print exactly the same.

Can every slicer open 3MF?

Bambu Studio, PrusaSlicer, OrcaSlicer and Cura all do. Some older or very specialised tools still only take STL.

Does converting an STL to 3MF improve the print?

No. The geometry is the same triangles. 3MF only adds what STL could not say, such as a colour or the unit.

Why does a downloaded 3MF open with settings I did not choose?

Because it is a slicer project: whoever saved it in Bambu Studio or PrusaSlicer stored their profile in it. A 3MF exported by Eyemesh carries no print settings, only the model.

Can I convert a 3MF back to STL?

Yes, Eyemesh exports both ways. Colours are lost on the way to STL, since the format has no place for them.