Convert STEP to 3MF
If your slicer takes 3MF, this is a better conversion than STEP to STL, and the reason is one sentence: both formats record what their numbers mean. The size cannot be guessed wrong at either end, the parts stay separate on the build plate, and the colours come with them.
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This page is set up for STEP files. Other formats we can read work here too.
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Two formats that both state their units
Almost every size problem in 3D printing comes from a file that does not say what its numbers mean. An STL is bare numbers; a slicer assumes millimetres; a part exported in inches arrives twenty-five times too small and nobody finds out until it is on the plate.
STEP states its unit in the header. 3MF states its unit in the model. So this conversion is the rare one where the information is never lost: the unit is read out of the STEP, the geometry is converted, and the unit is written into the 3MF. Bambu Studio, OrcaSlicer, PrusaSlicer and Cura all read that declaration and place the part at its real size.
This is the whole argument for using 3MF instead of STL when you can, and it is why the 3MF Consortium created the format in the first place.
An assembly stays an assembly
Convert a five-part assembly to STL and you get one lump. Every part is welded to every other part, and if you wanted to print them lying flat and separated, you have to cut them apart again in the slicer.
A 3MF holds several named objects, each with its own place. So the assembly arrives on the plate as five things you can select, move, rotate and arrange independently — which is exactly what you want, because parts that were designed touching each other are almost never printed touching each other.
Use the object list above if you want only some of them. Everything left switched on becomes its own object in the 3MF.
Colours come across too
STEP carries a colour for each part, and 3MF has somewhere to put one. So a housing that was grey and a cover that was blue in your CAD program stay grey and blue on the plate, which makes a multi-part 3MF much easier to work with than a monochrome STL.
This is a colour per part, not a painted surface. STEP has no way to store the kind of per-triangle painting a slicer can do for multi-material printing, so anything of that sort is still something you do in the slicer afterwards.
The detail setting still applies
A 3MF holds triangles, exactly as an STL does, so the STEP file’s exact surfaces still have to be chopped up and you still choose how finely. Normal is right for almost all printing; the triangle count next to the control tells you what each choice costs.
A 3MF is a zip inside, so the same model is usually a good deal smaller as a 3MF than as an ASCII STL, and comparable to a binary one.
What survives, and what does not
Kept in the 3MF file
- The unit, written into the 3MF, so the part cannot arrive the wrong size
- Every part as its own named object, separately placeable on the build plate
- The colour of each part
Lost on the way
- The exact curved surfaces — they become flat triangles and cannot be turned back
- The nesting of the assembly tree, which becomes a flat list of named objects
- Threads, tolerances and manufacturing information
How to convert STEP to 3MF
Drop the .step or .stp file in
The CAD engine is fetched on the first CAD file you open — about 3 MB, once.
Check the unit the file declared
The input unit box shows what the STEP file itself states. This is the value that will be carried through into the 3MF, so it is worth a glance.
Choose which parts to include
Each part stays a separate object in the 3MF, so you can arrange them individually in the slicer. Switch off anything you are not printing.
Set the detail
Normal for almost everything. Fine if a curved surface will be visible on a resin print. The triangle count updates as you change it.
Convert and open in your slicer
Leave the output unit on millimetres. The 3MF will state that unit, so the part appears at its real size without you setting a scale anywhere.
STEP and 3MF compared
| Property | STEP.step .stp | 3MF.3mf |
|---|---|---|
| File contents | Plain text in the ISO 10303-21 exchange form. Readable, and enormous. | A ZIP archive holding XML. The model itself is one XML part inside it. |
| Geometry | Exact surfaces: planes, cylinders, cones, spheres, NURBS. Not triangles. | Triangles, with a build platform and placement for each object. |
| Units | Recorded in the file, as a full unit and tolerance context. | Recorded in the file. Millimetre, centimetre, inch, foot, metre or micron. |
| Colour and texture | Colour per face or per solid, through the AP214 and AP242 styling entities. | Colour groups and materials per triangle, and textures in later extensions. |
| Separate objects | An assembly tree of named parts, each with its own placement. | Several named objects, each placed on the build plate with its own transform. |
| Published by | ISO 10303, from 1994. AP242 is the current application protocol. | The 3MF Consortium, 2015, explicitly to replace STL. |
| Written by | SolidWorks, Fusion 360, Inventor, CATIA, FreeCAD, Onshape — every serious CAD program. | PrusaSlicer, OrcaSlicer, Bambu Studio, Cura, Fusion 360 and Windows itself. |
Questions
- Should I use 3MF or STL for printing?
- 3MF, if your slicer reads it — and Bambu Studio, OrcaSlicer, PrusaSlicer and Cura all do. It records the unit so the size cannot be misread, and it keeps the parts of an assembly separate. Use STL only when something older insists on it.
- Will the parts be separate on the build plate?
- Yes. Each part in the STEP becomes its own named object in the 3MF, and every slicer that reads 3MF will let you select and move them independently.
- Is this a slicer project file?
- No. It is a plain 3MF model: geometry, names, colours and a unit. Supports, print settings and plate arrangement are your slicer’s job, and this file gives it a clean starting point.
- Do the colours mean anything for printing?
- Only as labels, unless you have a multi-material printer and assign each object a filament. The colours come from the CAD file and make a multi-part plate much easier to read.
- Why is my part still the wrong size?
- Check the input unit box against what you know the part to be. If the STEP file declared the wrong unit — old files sometimes do — correct it there and the 3MF will be written correctly.
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 STEP file
Other ways to get a 3MF file
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