3MF is what STL should have been. Convert yours and stop guessing at units, scale and object grouping every time a file changes hands.
An STL file contains triangles and nothing else — no header declaring what a unit means. A model authored in inches and one authored in millimetres are byte-for-byte indistinguishable in structure, which is why a downloaded part sometimes lands on the plate at 25.4× the size it should be, and why every slicer has a Scale button people reach for on instinct.
3MF fixes this at the format level: the specification defines millimetres explicitly, so a 3MF that says 40 mm is 40 mm in every tool that reads it. That single guarantee is most of the reason the 3MF Consortium — Microsoft, HP, Autodesk, Prusa, Bambu and others — built it.
Be clear-eyed about this: converting STL to 3MF gives you the better container, not information the STL never held. Your mesh is written into a valid 3MF with millimetres assumed — which is correct for the overwhelming majority of STLs in circulation, but it is an assumption, not a recovery. Colour, per-object grouping and print settings cannot be invented from triangles.
Where it genuinely helps: uploads to MakerWorld and Printables, handing files to someone on a different slicer, and any workflow where the file will be opened by a tool you do not control. Going the other way — pulling geometry back out of a slicer project — is 3MF to STL.
Every converter runs the same geometry engine — see all 20 conversions.