Unpack the container
3MF is a zip with an XML model inside. The reader opens it in the tab and walks the build items.
3MF to STL when the slicer only speaks STL.
3MF carries units, parts and print settings. STL carries triangles. Drop a 3MF here and get a plain STL, with a note on what was left behind.
.3mf → .stlUnits → mm
converter local
ready
Input 3MF .3mf
Output STL .stl
Coordinates are multiplied to millimetres. Leave as they are to keep the source numbers.
Drop a .3mf file here
or drag and drop · free, and nothing leaves your browser
— —
Triangles—
Vertices—
Bounds—
Source unit changed. Convert again to update the file.
Very large models are held in memory while they convert. Close other tabs first.
A plate is more than triangles.
A 3MF plate carries units, parts and print settings; an STL carries triangles and nothing else. The conversion keeps the geometry and drops the rest.
One 3MF plate in. One STL mesh out.
A 3MF plate goes in; a single STL mesh comes out. Build items are merged and the declared unit is baked to millimetres — colour, part names and print settings do not travel.
Convert a 3MF
Separate parts become one surface.
Separate coloured parts and print settings become one grey triangle mesh. It is a narrowing conversion: STL keeps the geometry and drops the container.
Compare the formats
Measured on the plate, before it prints.
The merged STL lands on a slicer build plate at the right size. Measure a known dimension once, then slice; the print settings have to be re-entered.
Read the 3MF vs STL guideHow the handoff runs
Merge the parts
Every object referenced by the build is transformed into place and merged into one mesh. Part names and colours are not carried.
Write STL
The merged triangles are written as binary STL. The 3MF unit is converted to millimetres; STL then declares nothing, so the numbers are already in mm.
What the STL contains
| Property | In the file | Why it matters |
|---|---|---|
| Units | Converted to millimetres | Micron, mm, cm, inch, foot and metre are scaled |
| Parts | Merged | Build items flattened into one mesh |
| Colour | Dropped | No colour in STL |
| Print settings | Dropped | Slicer profile is not part of STL |
| Geometry | Kept | Triangles and coordinates unchanged |
When to keep the 3MF
If the print depends on per-part settings, colour or declared units, send the 3MF to the slicer instead of converting it.
3MF vs STL
A 3MF file is a zip archive. Inside it, an XML model describes objects, their transforms, a declared unit and, optionally, colour and slicer settings. STL predates all of that: it is a flat list of triangles with no unit, no object names and no material slots. Converting between them is a narrowing operation, not a round trip.
That makes 3MF to STL the right move when a downstream tool only accepts STL: an older slicer, a printer's web UI, a mesh editor with a narrow importer. It is the wrong move when the 3MF still carries information you need, such as separate parts, per-part colour or a print profile you intend to reuse.
The 3MF vs STL comparison explains what survives the container change, and the filament cost estimator turns the result into a material budget. The format reference lists the other containers this tool reads.
At a glance
| Property | 3MF | STL |
|---|---|---|
| Unit | Declared in the file | No field; slicers assume mm |
| Separate parts | Kept as build items | Merged into one triangle list |
| Colour and materials | Can be stored | Not stored |
| Print settings | Can be stored | Not stored |
| Reader support | Modern slicers and tools | Effectively universal |
Before you convert
- Note the unit. Open the 3MF in its slicer and check the declared unit. The converter bakes that unit into millimetre coordinates, so a wrong source unit becomes a wrong print size.
- Decide about parts. If the plate needs separate objects, split the 3MF first. This converter merges every build item into one mesh.
- Check the result. After conversion, compare the bounding box with the original. A mismatch means the unit was not what you assumed.
Reference
3MF is the better file. STL is the one everything opens. 3MF knows its units, keeps parts separate and can carry print settings. STL is universal. Convert 3MF to STL in your browser and know what is left behind. What will this print weigh and cost? Enter the bounding box and get PLA weight and cost at typical shell and infill settings, before you slice.
Why did the units disappear?
STL has no unit field. The 3MF declares a unit and this converter scales the geometry to millimetres; the STL keeps those millimetre numbers but not the declaration.
Are multiple parts kept separate?
No. The build items are merged into one triangle list. If you need parts, keep the 3MF or split the STL afterwards.
What about colours and print settings?
Dropped. STL cannot store either.
Is the file uploaded?
No. The 3MF is unzipped and converted in the tab.
Does converting change the geometry?
No. Triangles are copied with their coordinates after the declared unit is applied. What changes is the container: parts merge and print settings, colour and the unit declaration do not travel.
Why is the STL larger than the 3MF?
3MF is a compressed zip of XML; binary STL stores every triangle as 50 bytes with no compression. A larger STL does not mean more geometry, so compare triangle counts instead.