What is a .3mf file?

What 3MF fixes

3MF was designed by the 3MF Consortium (Microsoft, HP, Autodesk, Dassault, Ultimaker, Prusa and others) specifically to replace STL for 3D printing. It fixes STL's three failings directly:

• It stores a unit. <model unit="millimeter"> is right there in the

file. A 3MF exported in inches opens at the correct physical size everywhere.

• It stores colour and materials, per object and per triangle.
• It stores multiple parts with their positions on the build plate, and it

can reference one object several times instead of duplicating its geometry.

It also stores print settings, supports and slicing metadata through extensions, which is why Bambu Studio, PrusaSlicer, Cura and Orca all use 3MF as their native project format.

What is inside the file

A .3mf is a zip archive, an OPC package like the ones Microsoft Office uses. Rename one to .zip and open it and you will find:

• [Content_Types].xml, declaring what the parts are
• _rels/.rels, pointing at the model
• 3D/3dmodel.model, which is the geometry as XML
• optionally thumbnails, textures and slicer-specific extension parts

The geometry itself is a vertex list and a triangle list, conceptually the same as STL but shared instead of repeated. A 3MF saved by a slicer is a compressed package that shares vertices, so it can be much smaller than the same model as STL. The 3MF this site writes is not compressed, and it comes out larger than a binary STL of the same model.

The build plate and the components tree

3MF handles multi-part prints with two structures.

Objects hold geometry. Components let one object include another with a transform. So a tray of twelve identical clips is stored as one clip plus twelve placements, not twelve copies.

Build items say what goes on the plate. A 3MF can contain objects that are not printed (a reference part, an alternative version) and the build section decides.

This site preserves that structure when converting to a format that can hold it, and tells you when converting to one that cannot.

Colour

3MF stores colour as sRGB, either as a colour group indexed per object or per triangle, or as base materials with a display colour. Multi-material printers use it directly.

Converting a coloured 3MF to STL discards all of it, because STL has nowhere to put it. The conversion report on this site says so.

Compatibility

Every current slicer reads 3MF. Support in CAD is good and improving: Fusion, SolidWorks, Rhino, Blender and FreeCAD all import and export it.

Where you still need STL is with older firmware-driven workflows, some online printing services, and any tool that has not been updated since about 2018.

3MF vs STL for 3D printing

Use the format your slicer or print service accepts. When both work, 3MF can keep information that STL leaves out, including a unit declaration and separate components. STL is useful when the receiving workflow only needs a triangle surface.

Changing an STL to 3MF does not add surface detail or repair its mesh. It packages the geometry with the information the converter can provide. This site's STL to 3MF converter assumes millimetres, so confirm the source dimensions first.

A slicer project saved as 3MF may also include settings specific to that slicer. Do not assume another application, or a general format converter, will preserve every project setting. Keep that original project if you need to reproduce the print setup.

How to open a .3mf file

Your slicer opens it: 3MF is the native project format of PrusaSlicer, OrcaSlicer and Bambu Studio, and Cura reads it too. On Windows, Microsoft 3D Builder opens it, since Microsoft co-wrote the specification. Blender and FreeCAD import and export it. The viewer on this page reads it in your browser and applies the unit the file declares.

A 3MF saved from one slicer opens in another, but only the geometry crosses over. Printer profiles, supports and colour assignments are stored in slicer-specific extensions, so expect to set those again.

If a 3MF refuses to open, it may be an ordinary zip that was renamed, or a zip with the model buried in a folder. A real 3MF is a zip too, but with a fixed internal layout that the reader here checks before it trusts the file.