3MF vs STL: Differences and When to Convert 3MF to STL

You downloaded a model that only comes as 3MF, but the place you want to upload it, or an older tool in your workflow, only accepts STL. Or the reverse: a designer asks why you keep sending STL when 3MF "has everything". The search for 3MF to STL keeps rising because both formats are now everywhere, and they are not interchangeable without losing something.

Here is a direct comparison, a clear rule for when to convert, and what you give up when you do.

3MF vs STL at a glance

For background on each, see what an STL file is and what a 3MF file is.

When you still need STL

• Uploading to a platform or print service that lists STL as its accepted format.
• Older or specialized tools: some mesh editors, CNC or analysis tools read STL only.
• Sharing one simple part with someone who uses an older slicer.
• Scripting: STL is trivial to generate and parse, so many hobby scripts expect it.

When to keep 3MF

• Multi-color or multi-material prints, where each part already has its color.
• Plates with several parts laid out the way you want to print them.
• Anything in inches or centimeters, since 3MF carries the unit and removes guesswork.
• Your own archive of projects you might reprint later with the same settings.

What you lose converting 3MF to STL

How to convert 3MF to STL (and the reverse)

Conversion happens on a computer, in a slicer or a mesh tool. The general steps are the same everywhere:

1. Open or import the 3MF in your slicer or a free mesh editor.
2. If it has several objects, decide whether you want one STL per object or one combined file. For multi-color prints, separate files are usually what the recipient needs.
3. Choose "Export" (sometimes "Export plate" or "Export as STL"). Make sure the export unit is millimeters.
4. Name the files clearly, for example lid_60x60x6mm.stl.
5. Open the new STL in a viewer and confirm the size matches the original 3MF.

Going from STL to 3MF is the same in reverse: import the STL into the slicer, set the scale and colors, and save the project as 3MF.

Check the result on your phone

After any conversion, compare the two files. It takes a minute on a phone:

1. Open the original 3MF in STL Viewer for Android and note the size in the info panel, say 76.2 x 25.4 x 12.7 mm.
2. Open the exported STL. If the info panel shows 3 x 1 x 0.5, the exporter wrote inches without saying so. Switch the file units to inches to confirm, then re-export in millimeters.
3. Check the watertight line on both. Conversion shouldn't open holes, but merging overlapping objects can create internal faces that some checks flag.
4. With Premium, the Compare tool puts both models side by side and lists dimensions, volume and triangles for each, so a mismatch is obvious. The compare guide shows how.

The triangle count is a useful tell, too. If the 3MF shows 120,000 triangles and the STL shows 120,000, the geometry came across intact. If the STL has 12,000, the exporter decimated it, and fine details like text may be gone.

Three common myths

• "3MF prints better." The printer never sees either file; it gets G-code from the slicer. The same triangles sliced with the same settings print identically.
• "STL is obsolete." It is old and limited, but it is still the format almost every tool accepts, which is exactly why it survives.
• "Converting to 3MF fixes the units." Only if the right unit is set at import. Wrap an inch STL in a 3MF labeled millimeters and it is still 25.4 times too small, now with a confident label.

A simple rule of thumb

Keep 3MF as your master file, especially if units or colors matter. Make STL only as a delivery format for whoever needs it, and check it once before sending. If you get files in inches from US designers, our inches vs millimeters guide saves a lot of reprints, and the pre-print checklist covers the rest.

Which formats the phone app opens, and which it asks you to export first, is listed on the STL Viewer website.