Convert STL to 3MF
STL is the oldest 3D printing format and it stores nothing but triangles. 3MF stores the same triangles plus the units, the colour and the separate objects. This tool writes a 3MF that opens in PrusaSlicer, Bambu Studio, OrcaSlicer and Cura.
Drag a 3D file here, or.
This page is set up for STL files. Other formats we can read work here too.
The file is read on your own computer. It is never uploaded.
Your file is read by your own browser and never sent to a server. There is no upload, no account and no copy of your model anywhere but on your computer.
What a 3MF has that an STL does not
An STL is a list of triangles. There is no unit, no colour, no name, no way to say that two shapes are separate objects. Everything that is not a coordinate has to be guessed by whatever opens the file, and the guess is usually millimetres because that guess is right most of the time.
3MF writes the unit into the file. It has a place for a colour, a place for a name, and a proper list of objects. It is also a compressed package, so the same model is usually a third of the size of the STL it came from.
The mesh is repaired on the way
An STL writes every triangle on its own, with all three corners listed again from scratch. A cube arrives as thirty-six separate points sitting on top of each other, and nothing in the file says that any two triangles touch. Before the 3MF is written, points in the same place are joined, triangles that faced inwards are turned round, and triangles with no area are dropped.
That matters more for 3MF than for STL. The 3MF specification expects a proper mesh with shared points and consistent winding, and a slicer reading one is entitled to assume it. The measurements beside the 3D view show what the repair found: how many duplicate points were merged, how many faces were turned, and whether the result is closed.
Setting the unit before you write it
The second unit box decides what goes into the 3MF. Leave it on millimetres for printing. If your STL was modelled in inches, set the first box to inches and the second to millimetres, and the numbers are converted once, here, instead of being fixed by scaling in the slicer every time you open it.
From then on the file carries the answer. Anyone who opens that 3MF gets the size you intended, because the file states it rather than leaving it to be inferred.
The result is a model, not a project file
There are two things people mean by a 3MF. One is the standard model package: geometry, units, maybe colour, written to the 3MF specification and readable by any slicer. The other is what Bambu Studio or PrusaSlicer saves when you press Save Project — the same package with a great deal more inside it, including your print settings, the plate layout, the printer profile and often the sliced result.
What is written here is the first kind. It opens in every slicer as an object you place on the plate, exactly as an STL would, but carrying its unit. It will not restore anybody’s print settings, because those settings are specific to a printer and a filament and were never in your STL to begin with.
This matters if you were hoping to rebuild a project from an STL someone sent you. The geometry is all that survived; the rest was never in the file.
What survives, and what does not
Kept in the 3MF file
- The exact shape, triangle for triangle
- The size, now written into the file as a real unit
- A clean mesh with shared points and consistent winding
Lost on the way
- Nothing from the STL: it holds only triangles, and all of them survive
- No colour is added, because an STL never had one
How to convert STL to 3MF
Drop the .stl file in
Binary or ASCII. The tool checks the file rather than trusting the extension, so a mislabelled file still opens.
Read what the repair found
The panel beside the 3D view says how many duplicate points were merged and how many faces were turned round. A large number of flipped faces means the original STL was worse than it looked.
Set the input unit if the STL was not millimetres
This is your one chance to fix it permanently. Once written into the 3MF the answer travels with the file and nobody downstream has to guess again.
Check that the model is closed
The open-edge count is beside the 3D view. 3MF expects a proper mesh, and a slicer reading one is entitled to assume it is closed.
Download the .3mf
A compressed package, usually about a third the size of the STL it came from. It opens in PrusaSlicer, Bambu Studio, OrcaSlicer, Cura and Windows 3D Builder.
STL and 3MF compared
| Property | STL.stl | 3MF.3mf |
|---|---|---|
| File contents | Binary or plain text. Binary is an 80-byte header, a triangle count, then 50 bytes per triangle. | A ZIP archive holding XML. The model itself is one XML part inside it. |
| Geometry | Triangles only. Curves are approximated when the file is made, and cannot be recovered. | Triangles, with a build platform and placement for each object. |
| Units | None. The file stores bare numbers, and every reader has to guess what they mean. | Recorded in the file. Millimetre, centimetre, inch, foot, metre or micron. |
| Colour and texture | None. There is no place in the format for a colour, a material or a texture. | Colour groups and materials per triangle, and textures in later extensions. |
| Separate objects | One body. Several parts written into one STL become one undivided lump of triangles. | Several named objects, each placed on the build plate with its own transform. |
| Published by | 3D Systems, 1987, for the first stereolithography machines. | The 3MF Consortium, 2015, explicitly to replace STL. |
| Written by | Every CAD program and every slicer, as the common denominator. | PrusaSlicer, OrcaSlicer, Bambu Studio, Cura, Fusion 360 and Windows itself. |
Questions
- Will PrusaSlicer and Bambu Studio open this file?
- Yes. The package is written to the 3MF specification: a content types part, a relationship pointing at the model, and the model itself in the core namespace. It is a plain model rather than a project file, so it opens as an object you can place on the plate.
- Is the 3MF smaller than the STL?
- Usually about a third of the size. 3MF is a compressed package and STL is not, and the repair step also removes the duplicated corners that make up most of an STL.
- Can I put a colour in the 3MF?
- Only if the source has one, and an STL does not. Converting from an OBJ with a material file, or from a GLB, does carry the colour into the 3MF as a base material.
- Why would I do this at all?
- Because the size stops being a guess. A 3MF that says it is in millimetres cannot be opened at the wrong scale, and it also keeps separate objects apart, which STL cannot.
- Will this give me back a project file I can edit in my slicer?
- No. It writes a standard 3MF model, not a slicer project. Print settings, plate layout and printer profiles are not in an STL and cannot be invented from one.
Having it printed
If the part is larger than your bed, needs a material your printer cannot handle, or you do not have a printer, these will print it and post it to you. Work out what it would cost you first — the cost calculator on this site gives you a figure to compare against.
- JLC3DPCheapest for resin and nylon, slow shipping
- CraftcloudCompares dozens of printers at once, including local ones
- PCBWayMetal printing and CNC as well as plastic
These are ordinary links, not paid referrals — we earn nothing if you use them. The converter above is the same whether you do or not.
Where to go next
The other direction
Other things to do with a STL file
Other ways to get a 3MF file
Related guides and tools
- Gridfinity generatorBins and baseplates, any size, built in the browser
- Turn text into an STLType a word and print it, without drawing anything
- Turn an image into an STLA photo or a logo, turned into something you can print
- Lithophane generatorA photo printed as thickness, lit from behind
- Repair an STL that will not sliceClose the holes and separate the bad edges so a slicer accepts it
- STL to G-codeGuide
- How to convert an STL file to G-codeGuide