How to convert a 3MF to STL
- Drop the 3MF on the box above, or click Open a 3MF.
- Under Objects to convert, keep All objects for one STL of the whole plate, or pick one object.
- If the 3MF is not in millimetres, choose whether the STL is written in millimetres or in the 3MF's own unit.
- Check the size and the mesh, then click Download STL.
The sample is a 3MF with four objects: a spur gear, a flanged bushing, a star knob, and a pair of hex standoffs. The pair is one standoff placed twice. Pick each object in turn to see its own size and volume.
What a 3MF file holds
A 3MF is a ZIP package of XML files. The Library of Congress format description says the format extends the packaging rules of the Open Packaging Conventions. The same description says the 3MF Consortium built the specification from work that Microsoft donated. Inside the package, a model file lists the objects. Each object is a mesh, or a set of other objects placed as components. A build list then places objects on the plate, each with a transform.
Unlike an STL, a 3MF states its unit. The 3MF core specification allows six: micron, millimeter, centimeter, inch, foot, and meter, with millimeter as the default. The 3MF Consortium's FAQ adds that STL holds no colour or material information and has no way to add any.
A 3MF can also keep each object in a separate model file inside the package, a layout from the 3MF production extension. Bambu Lab's wiki says Bambu Studio saves 3MF files this way by default, and that some other programs cannot open them for that reason. The converter follows the links between model files, so a project saved this way converts like any other 3MF.
What the STL keeps and what it loses
The STL keeps the shape of every object you chose, placed as the build list places it. Components are put together with their transforms, and copies of one object become separate pieces. The 3MF specification says a mirroring transform must not turn a shape inside out. The converter therefore reverses the corner order of each mirrored triangle, so the mirrored faces still point out.
The STL loses everything else: colours and materials, the names of the objects, the print settings, and the unit. An STL has no unit field, so a slicer that opens it reads millimetres. For that reason the converter writes millimetres unless you ask for the 3MF's own unit.
What the converter cannot do
- Convert a sliced file. A 3MF that holds only sliced print instructions, with no model part, has no shape to convert.
- Read every extension. Beam lattices and slice stacks from 3MF extensions are not read, and neither are textures.
- Fix the mesh. The STL holds the triangles as the 3MF stores them. If the mesh check finds faults, run the STL through the STL repair tool.
- Open very large files. The limit is 150 MB and 10 million triangles across the build.
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 objects and triangles, and never records the file or its name.
Pricing the parts on a plate
To price a plate of printed parts, pick each object to read its own volume and size. A material and machine estimate starts from those figures. A NIST review of additive manufacturing costs says that the size of the build envelope, and how fully that envelope is used, both affect cost. The number of parts on a plate therefore changes what each one costs. The 3D printing cost calculator works out the cost from your own material price and machine rate.
ThriveAI builds quoting systems that read each request for quote and the models attached to it. Each system prices every part from your own rates and material costs, and an estimator approves each quote before it goes out.