What is an .amf file? AMF, old Slic3r projects, and the Mac

XML with a unit, colours and print settings

This is how a Slic3r PE project from July 2018 that I have on my disk starts:

<?xml version="1.0" encoding="UTF-8"?>
<amf unit="millimeter">
<metadata type="cad">Slic3r 1.40.1-rc2+23-feature-arrange-with-libnest2d-win64</metadata>
<metadata type="slic3rpe_amf_version">1</metadata>
<metadata type="slic3rpe_config">
; avoid_crossing_perimeters = 0
; bed_shape = 0x0,250x0,250x210,0x210
; bed_temperature = 90
…

The unit attribute gives the unit: millimetre, inch, foot, metre or micron. When it's missing, programs assume millimetres. A model is made of objects (<object>). Each has a list of vertices and one or more volumes (<volume>) of triangles. Colour can sit on materials, objects, volumes, vertices or single triangles. A constellation (<constellation>) places copies of objects with an offset and a rotation. Each vertex can carry a normal and each edge tangents, from which a program can work out a curved triangle.

Slic3r and PrusaSlicer stored their own data in AMF as well. The project above carries 171 lines of print settings in slic3rpe_config, from the bed temperature to the filament diameter. At the end of the file is a constellation with four parts laid out on the print bed.

What's in your .amf file

Three different things hide behind the .amf extension:

• plain XML, like the example above,
• XML packed in a ZIP with the same extension. Slic3r and PrusaSlicer saved projects this way, as .zip.amf,
• music: according to FileInfo, DSMI Advanced Module Format modules and the music of the Crusader games. No 3D program opens those.

Terminal tells them apart:

file model.amf

Plain XML prints XML 1.0 document text, the packed kind Zip archive data. The size difference is large: my project is 840 kB in its ZIP and 14 MB unpacked. Unpack a packed file with unzip model.zip.amf; inside is a plain .amf.

Why AMF lost to 3MF

AMF was designed by Jonathan Hiller and Hod Lipson at Cornell University. They presented it as "STL 2.0" in 2009, and in 2011 ASTM adopted it as standard F2915. Since 2013, ISO and ASTM have managed it together, and in 2026 ISO confirmed the latest edition, ISO/ASTM 52915:2020 (version 1.2), as current. The text of the standard is sold, though; only the XML schema is free.

3MF arrived in 2015 from a consortium founded by Microsoft, Autodesk, HP and others. Its specification was free, it came with a library, and slicers adopted it as their project format. PrusaSlicer stopped saving AMF in version 2.4.0, in December 2021. The stable PrusaSlicer 2.9.7 from November 2026 still opens AMF; the 3.0 development builds don't have the importer yet.

If you have old .amf projects, open them in PrusaSlicer 2.9 and save them as 3MF while you still can. The print settings, colours and bed layout stay with them.

How to open AMF on a Mac

macOS doesn't know AMF. On macOS 27.0.1 the system has no file type for it, and neither Preview, Quick Look nor the Finder shows it.

• PrusaSlicer 2.9 opens AMF, packed .zip.amf projects included.
• OrcaSlicer, Bambu Studio and UltiMaker Cura have AMF readers too.
• Preview3D, my app, opens .amf files on a Mac in its window, in Quick Look and as Finder thumbnails. On iPad, iPhone and Apple Vision Pro too.

Preview3D reads AMF through the Assimp library. I tried it in the same build the app uses, on my Slic3r project and on files I wrote from the schema. It opens plain XML; Assimp loaded the unpacked 14 MB project with 70,986 triangles in just under 4 seconds. But I ran into four limits:

• It doesn't open packed AMF and reports an XML error. Unpack it first.
• Constellation rotations aren't applied. Assimp mistakenly stores the rx, ry and rz values in the offset instead of the rotation. So a part Slic3r turned 180° on the bed shows up unrotated and 3.14 mm lower, and in a file where a part has both an offset and a rotation, the rotation numbers are used in place of the offset.
• It doesn't read the file's unit. The app always takes AMF as millimetres, and shows it at that real size in AR too. A file in inches would come out 25.4× too small.
• Curved triangles stay flat.

The app shows the geometry in one plain colour and doesn't use the file's materials or colours. Model info has the dimensions and the triangle count, plus a mesh check.

Converting AMF to STL or 3MF

Convert to 3MF in a slicer: colours, materials, units and settings stay. To STL, Preview3D converts from File → Export model as… (⌥⌘E), without uploading anything. Colours, materials and print settings are dropped, and all the parts end up in one mesh. Because of the rotation bug above, look over the result of a multi-part project before you send it to print.

Common questions

What is an .amf file?

An XML format for 3D printing under the ISO/ASTM 52915 standard: a triangle mesh with a unit, colours, materials and, optionally, curved triangles.

Is AMF or 3MF better?

3MF. It does what AMF does, its specification is free, and today's slicers save their projects in it. AMF is slowly losing support.

How do I open an old .zip.amf project from Slic3r?

In PrusaSlicer 2.9, OrcaSlicer or Bambu Studio. To view it in Preview3D, unpack it first with unzip.

Is my .amf file a 3D model?

file model.amf prints XML 1.0 document text or Zip archive data for a 3D model. Anything else is more likely a music module.

Does a Mac open .amf without another app?

No. macOS 27 has no file type for AMF, and neither Preview nor Quick Look shows it.

How do I convert AMF to STL?

On a Mac, in Preview3D with File → Export model as…, or in a slicer. Save a project with colours and settings as 3MF instead.

Related pages

• The .3mf file – the format that replaced AMF
• The .stl file – the format AMF set out to follow
• The .gcode file – what leaves the slicer for the printer

Sources

• ISO/ASTM 52915:2020 – the AMF 1.2 standard, confirmed in 2026
• ISO: the AMF schema – units, elements, curved triangles
• Cornell Chronicle: a new file format for 3D printing (2011)
• Jonathan Hiller: AMF – the 2009 STL 2.0 proposal
• PRONOM: AMF
• PrusaSlicer: AMF export removed and import temporarily removed
• OrcaSlicer: the AMF reader and Cura: AMFReader
• Assimp: reading AMF – constellation rotations
• FileInfo: the .amf extension – other formats with the same extension

Have old .amf projects from Slic3r you'd like to go through before converting them? Write to me at apps@xfanta.com and tell me what would help.