How to convert an STL to 3MF
- Drop the STL on the box above, or click Open an STL.
- Check Units in the STL. The 3MF is written in millimetres, so a part drawn in inches is multiplied by 25.4.
- Read the mesh check. If the check finds faults, tick Fix common mesh faults before converting.
- Click Download 3MF.
The sample is a flanged bushing with a 30 mm flange and a 10 mm bore, made of 1,728 triangles. Its 3MF holds the same triangles on shared vertices, in millimetres.
What the converter writes
A 3MF is a ZIP package. The converter writes the three parts that the 3MF core specification needs for one object:
[Content_Types].xml, which names the type of each file in the package._rels/.rels, which points to the 3D model.3D/3dmodel.model, the model itself: one object, its vertices and triangles, and one build item, with the unit set tomillimeter.
An STL stores the three corners of every triangle separately, so the same corner appears once for each triangle that meets there. The specification asks a producer not to repeat vertices, so the converter joins identical corners into one vertex and refers to each by number. With Millimetres chosen, the coordinates are copied as the STL holds them, to the last digit. In ThriveAI's tests, files converted to 3MF and back to STL kept every triangle and the same volume.
The output passes lib3mf, the 3MF Consortium's open source library, in strict mode, on the sample and on every other file ThriveAI tested.
Why a 3MF needs a closed mesh
The 3MF specification sets rules for the mesh of a printable object. Every triangle edge meets exactly one other triangle edge, neighbouring triangles agree on which side is out, and every face points away from the inside. The three corners of a triangle must also be three different vertices. A mesh that breaks those rules gives a file that a strict 3MF reader can refuse.
The converter always leaves out degenerate faces and exact repeats of another face, and the download message counts them. Holes and flipped faces are fixed only when you ask. Tick the fix box, and the converter runs the same pass as the STL repair tool before it writes the 3MF. The download message then lists what was fixed.
What the converter cannot do
- Add colour or material. The 3MF holds the shape, one object, and one build item.
- Know the unit. The STL does not say what unit it is in, so the converter takes the one you choose.
- Combine STL files. One STL makes one object. To combine several parts, open them together in your slicer and save a 3MF from there.
- Fix every fault. An edge shared by three or more faces cannot be fixed by the converter, and the message says so.
Your file is read and converted in this browser and never uploaded. The page counts how often the converter is used, with the number of triangles, and never records the file or its name.
Sending a part for a quote
A part sent as an STL arrives with no unit, and an estimator has to guess millimetres or inches. A part sent as a 3MF says which unit it is in. For a printed part, the quote starts from the volume and the bounding box. The 3D printing cost calculator turns those two figures into a cost from your own prices.
ThriveAI builds quoting systems that read each request for quote and its models, and price the parts from your own rates and material costs. An estimator approves each quote before it goes out.