STL to 3MF Converter
Wrap an STL in a 3MF so it carries its units and opens cleanly in a modern slicer.
Built and maintained by Matúš Koleják, Co-Founder, TechDraw AI
Updated
Drop a file here. It never leaves your browser.
No sign-up · no upload · no watermark · no file size limit · 3MF first, OBJ one click away
How to convert an STL to 3MF
Add your STL
Drop an STL onto the tool, or click to choose one. It is read in your browser and never uploaded.
Check the preview
Drag the model to turn it. The triangle count, the number of parts and the size in millimetres are shown under it, so a unit problem is visible before you download.
Download 3MF
Click Download 3MF. No account, no watermark, no waiting. OBJ come off the same file with one more click.
What happens to your file
The STL is read in your browser, in either the binary or the ASCII form. Which one it is gets decided by arithmetic rather than by the leading word: a binary file is exactly 84 bytes plus 50 per triangle, and several CAD packages write binary files whose header happens to begin with "solid", which is the usual reason such a file opens as empty elsewhere. The triangles then go straight out as 3MF with nothing resampled, so the mesh you get back has exactly the triangles the mesh you brought had.
Starting from a photo or a drawing instead of a mesh? That is the image to STL converter, and a 2D outline becomes a solid on the DXF to STL converter.
About formats
STL (from 'stereolithography', though the letters are also read as 'Standard Tessellation Language') was created by Chuck Hull at 3D Systems in 1987 for the first commercial stereolithography machine, and it has outlived nearly everything else from that era. The format describes a shape in the simplest way a shape can be described: as a list of triangles, each one given by three corner points and the direction its outward face points. There is no notion of a curve, so a cylinder is a fan of narrow flats and the smoothness depends entirely on how many triangles were spent on it. There is no colour, no units, no layers, no history and no way to say that two triangles belong to the same feature. That poverty is exactly why it survived: any program can write it and any program can read it, so it became the neutral handoff between modelling and printing. Files come in two flavours, a human readable ASCII form and a compact binary form that stores each triangle in fifty bytes, and the binary one is what practically everything writes today because the ASCII version runs several times larger. The one thing a printer genuinely needs from an STL is that the surface be closed, meaning every edge is shared by exactly two triangles so the solid has a well defined inside; a mesh with gaps leaves the slicer guessing where the material goes. Units are the standard trap, since the file carries none and every slicer simply assumes millimetres.
- Developer:
- 3D Systems
- Initial release:
- 1987
Frequently asked questions
How do I convert an STL file to 3MF?
Drop the STL onto the tool on this page and click Download 3MF. It is read and converted by JavaScript running in your browser, so there is nothing to install, no account to make and no queue to wait in. OBJ come off the same file with one more click.
Is this STL to 3MF converter free?
Yes. Free, no watermark, no file size limit, and no account needed to convert or to download. There is no paid tier of this tool to upsell you to.
Is my file uploaded anywhere?
No. The file is opened by code running in your browser tab and never sent to a server, which also means nothing is stored, logged or scanned. You can disconnect from the internet once the page has loaded and the converter keeps working. For a client's part under NDA that is the difference between usable and not.
Does binary or ASCII STL matter?
Both are read. Which one a file is gets decided by its length rather than by whether it starts with the word "solid", because more than one CAD package writes binary files with "solid" sitting in the 80-byte header. Guessing from that word is the usual reason an STL opens as empty in other tools.
What does wrapping an STL in a 3MF actually gain me?
A declared unit, mainly. An STL is a bare list of triangles with no statement of what one unit means, so it depends on every program downstream guessing millimetres. The 3MF written here says millimetres explicitly. You also get a container modern slicers treat as a project rather than as a dumb mesh. What it does not do is add detail: the triangles are the ones your STL already had.
Is there a file size limit?
None imposed by the tool. The practical limit is your own machine's memory, since the file is held in the browser tab; a few hundred megabytes is fine on a desktop and less so on a phone. Nothing is queued or throttled because there is no server involved to queue it.