Which unit should I pick?

Millimetres, unless you know otherwise — that is what CAD exports for printing. Check the reported bounding box against the real size of the object.

Will the file be smaller?

Usually much smaller. The archive is compressed and vertices are indexed instead of repeated once per triangle.

Not from an STL, which has none to carry. 3MF supports colour, so a model that already has it keeps it.

Is my model uploaded?

No. The parsing and rewriting happen in this tab. Models are frequently confidential — an unreleased product, a client’s part, a scan of something proprietary — and none of it is transmitted. Turn your Wi-Fi off and the tool keeps working.

How large a model can I convert?

There is no imposed limit; the ceiling is your device’s memory. A desktop browser handles files of roughly 2 GB and a phone about 400 MB. A 200 MB STL — around four million triangles — converts in a few seconds on a laptop.

STL to 3MF Converter — Give Your Model Units and a Name

Converts STL to 3MF, the format designed to replace it. The immediate gain is that the file finally says what unit it is in — which is the single most common source of models printed at the wrong size.

What the upgrade actually buys

Declared units. You choose them here and they are written into the file. A slicer no longer has to assume.

Smaller files. The archive is compressed, and vertices are indexed rather than repeated per triangle. A 40 MB STL commonly becomes a 4–8 MB 3MF.

Real object structure. Objects have names and stay separate.

Room to grow. Colour, materials and print settings all have a place in the format, even though an STL source has none of them to supply.

The archive that is produced

A valid OPC package: [Content_Types].xml, _rels/.rels and 3D/3dmodel.model. That is what Cura, PrusaSlicer, Bambu Studio and Windows 3D Builder look for. Vertices are merged during the conversion, since 3MF indexes them and repeating each one per triangle would waste the format's main advantage.

Choosing the unit

Almost every STL intended for printing is in millimetres — that is what CAD packages export by default for 3D printing. Choose inches if the model came from a US-centric CAD workflow and comes out twenty-five times too small. The bounding box reported after conversion is the quickest check: a phone case should be around 150 × 75 × 10.

Other names for this

Also searched as “convert stl to 3mf”, “stl to 3mf for prusaslicer”.

Questions

Which unit should I pick?: Millimetres, unless you know otherwise — that is what CAD exports for printing. Check the reported bounding box against the real size of the object.
Will the file be smaller?: Usually much smaller. The archive is compressed and vertices are indexed instead of repeated once per triangle.
Can I add colour?: Not from an STL, which has none to carry. 3MF supports colour, so a model that already has it keeps it.
Is my model uploaded?: No. The parsing and rewriting happen in this tab. Models are frequently confidential — an unreleased product, a client’s part, a scan of something proprietary — and none of it is transmitted. Turn your Wi-Fi off and the tool keeps working.
How large a model can I convert?: There is no imposed limit; the ceiling is your device’s memory. A desktop browser handles files of roughly 2 GB and a phone about 400 MB. A 200 MB STL — around four million triangles — converts in a few seconds on a laptop.