STL's two weaknesses, fixed
No units. STL stores numbers without saying what they mean. Slicers assume millimetres, so a part exported in inches opens 25.4 times too small. 3MF writes the unit into the file.
Size. STL repeats every corner for every triangle and is uncompressed. 3MF stores each vertex once and packs the whole model in a ZIP container.
How to convert
- Drop the
.stlon the box above. - Look at the size in the summary. If a part that should be 60 mm long reads 2.36 mm, it was exported in inches — set The file is in to inches. The converter suggests this when the model looks suspiciously small.
- Keep 3MF selected and click Convert and download.
The 3MF contains one object, in millimetres, with the same triangles as the STL.
Is the conversion lossless?
For geometry, yes: every triangle is kept at the same position (after the unit scaling you choose). Identical corners are merged into shared vertices, which is how 3MF stores meshes; that changes the file's structure, not its shape.
When to keep STL
Send STL as well when the receiver uses old software, a CNC or CAM tool, or an online service that asks for it. For your own printing and for sharing with people on current slicers, 3MF is the better default.
On a phone
STL to 3MF works in a phone browser. The Mes3D app for iPhone, iPad, Mac and Android also sets a file's unit with one tap — including an inch-to-millimetre fix — and exports 3MF, STL or OBJ.