STL to 3MF Converter

Why convert STL to 3MF at all

STL is the lowest common denominator — just a bag of triangles, with no units, no color, and no guarantee the mesh is even closed. 3MF is the format the 3D-printing world moved on to, and Bambu Studio, PrusaSlicer, and Cura all open it natively. Converting gets you:

• Unambiguous units. STL stores no unit, which is why a model so often imports microscopic or 25.4× too large. A 3MF declares millimeter in the file itself — what you exported is what your slicer loads.
• Manifold by specification. 3MF requires a watertight, manifold mesh, so it surfaces problems up front instead of after a failed slice. This converter welds duplicate vertices and drops degenerate triangles on the way in.
• Native to modern slicers. It's the default project/exchange format in Bambu Studio and PrusaSlicer — no "import legacy STL" detour.
• One tidy package. A single self-contained file instead of juggling one STL per part.

STL vs 3MF

The two formats describe the same triangles, but 3MF wraps them in the metadata a slicer needs and STL leaves out.

• Structure. STL is a flat, unstructured triangle soup. 3MF is a zipped OPC package holding an XML mesh, so it can carry more than raw geometry — object hierarchy, build-plate placement, and metadata all have a defined home.
• Units. STL has none, which is the whole scaling headache. 3MF states its unit (millimeter here) inside the file, so a slicer never has to guess.
• Materials and color. STL carries no color at all. 3MF has a materials-and-properties extension that can assign a color per triangle group or per object — but only if the source model already had that information (see the caveat below).
• Editability. Both are meshes, so neither is a parametric model you can re-dimension. What 3MF adds is watertightness as a rule, not a hope: an open mesh is flagged, not silently sliced.
• What's gained, what's lost. You gain correct units and a self-describing package; you lose nothing, because everything an STL held is preserved and then annotated.

One honest caveat

Converting an STL to 3MF does not add color — an STL never carried any, so there's nothing to recover. The body comes through with your slicer's default color. The 3MF color story is real, but it lives upstream: if you want a multi-color, AMS-ready 3MF, you need a model that has per-body colors to begin with. You can build one in Cadre's editor, color each body, and export a 3MF that carries those colors per body — see below.

What is a 3MF file?

A 3MF (.3mf) is a 3D Manufacturing Format file — an open standard published by the 3MF Consortium (Microsoft, HP, Autodesk, and others) to replace STL for additive manufacturing. It's an OPC zip container built around an XML model that stores the mesh plus explicit units, and optionally color, materials, and build metadata. You'll most often see it as the native project and export format of Bambu Studio, PrusaSlicer, and Cura.

Your file never leaves your browser

Plenty of "free online" converters upload your STL to a server, convert it there, and keep a copy around afterward. A few run locally now too, so "no upload" alone isn't the whole story. Here's the difference worth caring about:

• A modeling kernel is doing the work, not a converter script. The Rust kernel behind Cadre's editor is compiled to WebAssembly and packages the 3MF right here in the tab. It reads your triangles, welds them, and writes the OPC container — the file is handed straight back as a download with nothing to send onward.
• Spec-conformant output. The 3MF is built byte-for-byte against the 3MF Consortium Core specification — correct OPC package, namespaces, and plain-decimal coordinates — so slicers that read 3MF (Bambu Studio, PrusaSlicer, Cura) treat it as a standard file, not a "mostly works" approximation. A watertight mesh comes through as a solid model; an already-broken mesh comes through as a surface object, not silently faked into a solid.
• Free the whole way through. No sign-in to start, no cap on how many parts you convert, and no watermark stamped on the mesh — the print-ready 3MF you download is the entire output.

How it works

Cadre is a browser-based CAD editor whose Rust geometry kernel runs client-side via WebAssembly. Drop a file and that same kernel imports your mesh, welds and cleans it, and writes a 3MF package — almost instantly for typical print-sized models. A large mesh or a dense scan just takes a beat longer, because the packaging happens on your own CPU rather than being queued on someone else's.

Want per-body color? Model it in Cadre

A converted STL is colorless, but a model built in Cadre isn't. Open the editor, add or import bodies, give each one a color, and export 3MF from the File menu — each body keeps its color in the package, ready for an AMS or any color-capable printer. For designing parts that print right the first time, the design rules that prevent failed prints are a good starting point.

FAQ

Does converting STL to 3MF add color to my model?

No. An STL stores only triangles — it never had color — so the converted 3MF comes through with your slicer's default color. To get a color 3MF you need a model that already has per-body colors; you can build one in Cadre's editor and export a 3MF that carries them.

Does my file get uploaded anywhere?

No. Packaging your mesh into a 3MF happens on your own machine through WebAssembly — the triangles are read, wrapped in the OPC container, and returned as a download, so nothing about your part reaches a server. That matters when the STL is a client's part or an unreleased design.

Is it free?

Yes. The STL-to-3MF conversion costs nothing and asks for no sign-in; you can package as many parts as you like and the downloaded 3MF is clean, with no watermark on the mesh.

Why is 3MF better than STL for 3D printing?

3MF bakes in the millimeter unit (so models import at the right size), requires a manifold mesh, can carry color and material, and is the native format of Bambu Studio and PrusaSlicer. STL is the older lowest-common-denominator format that stores triangles and nothing else.

Will the 3MF open in Bambu Studio, PrusaSlicer, or Cura?

Yes. The output is built against the 3MF Consortium Core specification — a correct OPC package with the right content types and namespaces — so slicers that read 3MF, including Bambu Studio, PrusaSlicer, and Cura, treat it as a standard file.

Can I convert OBJ to 3MF too?

Yes. Drop an OBJ and the kernel reads its mesh the same way it reads an STL, then writes the millimetre-tagged 3MF package — same printer-ready result.

Related

• STL ⇆ OBJ converter — move between the two plain mesh formats when a tool wants OBJ.
• STL to STEP converter — wrap the same mesh as a faceted solid for a CNC quote.
• Edit an STL online — clean or repair the mesh before you package it.
• Open Cadre — model a part with real per-body colors and export a true multi-color 3MF.