What is inside a 3MF file
Copy a .3mf, rename the copy to .zip and open it. You will find a few small parts:
3D/3dmodel.model, the XML model with vertices, triangles and objects[Content_Types].xmland_rels/.rels, which tell a reader where everything is- often
Metadata/thumbnail.pngand slicer settings files
The mesh is indexed. Vertices are listed once and each triangle points to three of them, so shared corners are not repeated as they are in STL. Add ZIP compression and a 3MF is often smaller than the same model as binary STL, which repeats every shared corner at 50 bytes per facet.
Units, color and several parts in one file
The model element has a unit attribute: micron, millimeter, centimeter, inch, foot or meter, with millimeter as the default. A part drawn in inches therefore arrives at its true size. That alone removes the most common STL complaint, a bracket that lands on the bed 25.4 times too small.
| Feature | 3MF | STL |
|---|---|---|
| Unit | declared in the model | none |
| Colors, materials | base materials and color groups | none in the standard |
| Several objects | yes, with names | one triangle list |
| Position on the plate | build items with transforms | raw coordinates |
| Thumbnail | optional PNG | none |
The 3MF Consortium, founded in 2015, publishes the specification openly. The core part defines the ZIP package, the XML model at 3D/3dmodel.model and the unit attribute described above.
Slicer projects and .gcode.3mf
Many slicers save a whole project as 3MF: plates, print settings, painted supports, modifiers. Those extras sit in extra files inside the ZIP and follow each slicer's own conventions. Another program opens the meshes and may ignore the settings. If you only want to share the shape, export a plain model 3MF from your slicer instead of the project.
A .gcode.3mf is a different thing. It holds sliced G-code packed for one printer family, and our viewer does not open Bambu .gcode.3mf project files. Open the plain .3mf model, or a plain .gcode in the G-code viewer.
Convert 3MF to STL and back
Going to STL keeps the triangles and drops what STL cannot hold: colors, object names, the plate layout. The numbers are written in millimeters. 3MF to STL was tested on a 9,024-triangle sphere and re-opened with the same size, volume and triangle count.
- Drop the .3mf on the viewer or press Open File.
- Check Info: size in mm, triangle count and the number of parts the package holds.
- Press Convert, choose STL binary and download the file.
The other way, STL to 3MF writes a 3MF in millimeters. Fix inch parts with Inches → mm before you save, because the unit is fixed at export. Exporting 1M triangles to 3MF took about 4–5 s in our measurement.
Open a 3MF without a slicer
Drop the file on the 3MF viewer. JavaScript in the tab unpacks the ZIP on your device, nothing is sent out, and the model shows with orbit, section plane and measure. Info gives size in mm or inches, volume, triangle count, number of parts and a weight for 17 materials. From there, the 3D print cost calculator turns that weight into a price.
Common 3MF snags and what to do
- Settings vanish in another slicer. Project data follows one program's own layout. The meshes come through; set layer height, infill and supports again.
- One file, several parts. A plate with four objects opens as four parts. Info counts them, so you know what the customer actually sent.
- Heavy scans. ZIP hides the true size. A small 3MF can unpack to more than a million triangles, which pushes the browser tab toward its memory limit.
- A print service asks for STL. Convert, then confirm the dimensions, because STL will carry bare numbers from then on.
Questions and answers
What is a 3MF file used for?
Sending models to a slicer or a print service with units, colors and part layout intact. Slicers also use it to save complete print projects.
Why is my 3MF smaller than the STL?
3MF lists each vertex once and the package is ZIP-compressed. Binary STL repeats every shared corner and stores 50 bytes per triangle.
Does 3MF to STL keep colors?
No. STL has no color field, so only the geometry survives. Keep the 3MF if the colors matter for a multi-color print.
Can I open a .gcode.3mf file here?
Not yet. Export plain G-code from your slicer and open it in the G-code viewer, or open the .3mf model itself.
Can a 3MF hold several parts?
Yes. Each object has its own mesh and name, and build items place them on the plate. Info counts the separate parts after opening.
Who maintains the 3MF format?
The 3MF Consortium, founded in 2015, publishes the specification openly at 3mf.io. The core part defines the package, the XML model and the unit attribute.
Should I send a print service 3MF or STL?
Ask which one they prefer. 3MF removes any doubt about units and keeps parts named, while STL is the format every shop accepts.