What 3MF adds to a bare STL
STL dates from 1987, when 3D Systems created it for stereolithography. A binary STL is an 80-byte header that readers skip, a triangle count, and 50 bytes for each triangle. The standard has no field for the unit or for color. A 3MF wraps the same mesh in a ZIP package and states the unit as an attribute of the model. Files from this converter are always written in millimetres, so a 20 mm cube arrives in the slicer as 20 mm, never as 20 inches.
Get the size right before converting
Because STL has no unit, the viewer shows its numbers as millimetres, and the 3MF will state mm for them. A part drawn in inches will look 25.4 times too small. Press Edit → Inches → mm (×25.4) first, or type the length you want with scale STL. The 3MF then records the corrected size.
Convert STL to 3MF in four steps
- Drop the .stl, binary or ASCII, or press Open File.
- Check X × Y × Z in Info and fix the unit if needed.
- Press Download 3MF.
- Drop it into your slicer and confirm the size there.
The STL is read and repackaged by JavaScript in this tab, and the 3MF is built on your computer.
A 20 mm cube and a 9,024-triangle sphere both went STL → 3MF and re-opened with the same size, volume and triangle count. For big meshes, writing a 3MF of 1 million triangles took about 4 to 5 seconds in our measurement on a laptop CPU.
Example: an inch part on its way to the slicer
Take a 1 inch cube exported as STL from an inch-based program. Its numbers say 1, so Info reads 1 × 1 × 1 mm, a speck on the bed. Press Inches → mm (×25.4) in Edit and Info reads 25.4 × 25.4 × 25.4 mm. Download 3MF now, and the package tells the slicer those numbers are millimetres, so the cube arrives at its real size.
If you skip the fix, the 3MF faithfully records 1 mm. The unit label only helps when the numbers in front of you are already right.
Edit first, then save as 3MF
Edits made in the viewer go into the 3MF. You can cut a part with a plane, lay it flat on the bed, add raised text or repair holes, and then press Convert → 3MF. Undo keeps the last 20 steps if a change goes wrong. The STL repair page covers the mesh fixes.
What the mesh check reports
A 3MF of a broken mesh is still a broken mesh. Before you download, look at the check in Info: watertight yes or no, holes, open edges, non-manifold edges, flipped and degenerate triangles, inside-out normals. Holes and inside-out normals are the faults that break slicing most often. Edit → Repair welds vertices, drops degenerate triangles, turns normals outward and closes simple holes, then lists what it changed.
Weight, cost and the other directions
Before printing, the 3D print cost calculator estimates grams and price from the volume. To go back, use 3MF to STL. For CAD there is STL to STEP, and for rendering tools STL to OBJ.
Questions and answers
Why convert STL to 3MF at all?
3MF states its unit, so the slicer cannot mistake millimetres for inches. The size you set here is the size the slicer shows.
Does the 3MF get colors from the STL?
No. STL has no color to pass on, so the 3MF contains the plain mesh.
Can I convert an ASCII STL to 3MF?
Yes. Both binary and ASCII STL open, and both give the same 3MF.
Is the STL uploaded to make the 3MF?
No. The STL is read and the 3MF package is written inside this browser tab. In a logged test run no request left our own origin and nothing was posted.
Can I turn the 3MF back into an STL later?
Yes, with the 3MF to STL page. Our round-trip tests kept the size, volume and triangle count in both directions.
The slicer still shows the wrong size. Why?
The STL numbers were off before the conversion, and the 3MF kept them. Check X × Y × Z in Info against the drawing, scale in Edit, then download the 3MF again.
Can I combine several STL files into one 3MF?
Yes, through Edit: add the second file and join the parts with union, then save as 3MF. Both meshes need to be watertight for that.