Starter
3credits
$7.99
$2.66 each
3 downloads of any size. Good for a one-off project.
First 2 conversions freeSTL → 3MFNo software installation
Convert STL
to 3MF online
Convert an STL (.stl) mesh into 3MF online. Upload your file in the browser and download the converted result without installing desktop CAD software.
Encrypted uploads Files up to 300 MB Automatically deleted within 7 days
Upload your STL file
Choose one file to begin the conversion.
Drag and drop your file here
or choose a file from your computer
Browse filesMaximum file size: 300 MB
First 2 conversions free — any size up to 300 MB. Then $2.99 or 1 credit per file, charged at download.
No file available? Choose a sample that matches the workflow you want to test.
Pricing
Pay as you go, cheaper by the pack
Your first 2 CAD file conversions are free, any size up to 300 MB, including the download. From conversion 3 on, each file costs 1 credit or a one-time payment, charged only when you click Download. 1 credit downloads any file up to 300 MB, and credits never expire.
Save 11%
Pro
5credits
$12.99
$2.60 each
5 downloads of any size. The everyday choice.
Team
10credits
$19.99
$2.00 each
10 downloads of any size. Best balance of price and volume.
Studio
15credits
$26.99
$1.80 each
15 downloads of any size. Lowest price per file.
Browser-based conversion
Use the converter without installing desktop software to move from STL to 3MF.
Built for 3d-printing mesh
Translate triangular surface mesh into the triangle meshes with declared units used by 3MF.
Ready for the next workflow
Open the result in PrusaSlicer, Bambu Studio, OrcaSlicer and Cura for review, editing or production.
Private file handling
Converted models are not added to the public CAD library and are removed after processing.
Continue your CAD workflow
Explore Marathon
OS CAD resources
Preview a model before conversion, find an existing engineering file, open a 2D drawing, or get specialist help when a source mesh needs to be rebuilt.
Browse CAD modelsSearch ready-to-use engineering and 3D-printing files across the Marathon OS library.Browse 2D technical drawingsExplore PDF, SVG and DXF drawing resources for engineering review and documentation.Get CAD design supportWork with a vetted designer when the model needs repair, remodelling or a production-ready native file.
Simple conversion workflow
How to convert STL to 3MF online
Move from STL mesh to 3MF print file in three clear steps.
Upload your STL file
Drag and drop the .stl file into the converter or choose it from your device. Files up to 300 MB are supported.
Use 3MF as the output
This page is preconfigured for STL-to-3MF conversion, so you do not need to search through target formats.
Download and inspect the 3MF
Download the result and open it in your preferred compatible application to confirm its geometry and scale.
Format behaviour
What changes when STL is converted to 3MF?
STL and 3MF describe geometry differently. Conversion preserves compatible shape and topology while translating or omitting format-specific data.
Normally retained
- Overall model shape
- Compatible geometry
- Model orientation
- Supported topology and layers
May not be retained
- Format-specific metadata
- Unsupported materials or textures
- Application-specific features
- Data without a target-format equivalent
STL meshSTL3MF print file3MFKeep the original STL file as the source of truth and inspect the converted 3MF file before downstream production use.
Format comparison
STL versus 3MF
Choose the output according to what the next stage of your workflow needs.
| Feature | STL STL | 3MF 3MF |
|---|---|---|
| Main purpose | 3D printing, slicing and rapid prototyping | 3D printing, slicing and multi-part print jobs |
| Geometry | Triangular surface mesh | Triangle meshes with declared units |
| Textures and UV mapping | Not supported in standard STL | Colours supported; textures through extensions |
| Materials | Not supported in standard STL | Base materials and colours can be stored |
| Object groups | Does not retain named object groups | Multiple named objects in one file |
| Common workflow | Cura, PrusaSlicer and 3D printers | PrusaSlicer, Bambu Studio, OrcaSlicer and Cura |
Choose 3MF when
Your next application or supplier specifically requires a 3MF file.
3D printing, slicing and multi-part print jobs is the next stage of the workflow.
The target geometry representation suits the downstream task.
Keep STL when
Your project still depends on data unique to the source format.
You need to continue editing in Cura, PrusaSlicer and 3D printers.
The model is still being prepared rather than handed off.
Before continuing
Check the converted
3MF before the next step
Changing the file format does not automatically make a model production-ready. Review these checks in the application used for your next workflow.
Important: File formats may not include a standard unit definition. Always verify dimensions after conversion.
- 01
Verify dimensions and scale
Confirm that the converted 3MF file uses the expected size and unit interpretation.
- 02
Inspect the complete geometry
Check that all expected bodies, surfaces, curves or drawing entities are present.
- 03
Review surface orientation
Look for flipped faces, inconsistent normals or reversed boundaries after conversion.
- 04
Check topology and open edges
Inspect gaps, overlaps, invalid edges and disconnected geometry before downstream use.
- 05
Confirm layers and object structure
Verify that supported layers, groups and model organisation were translated as expected.
- 06
Open the file in its target application
Preview the 3MF result in the software used for the next workflow before production use.
Common problems
STL-to-3MF conversion troubleshooting
Most conversion issues come from differences between file formats or problems already present in the source geometry.
The 3MF file has the wrong size
The source and destination tools may interpret units differently. Open the 3MF file and select the correct unit or scale.
Fix: Verify units and dimensionsTextures or materials disappeared
This can be expected when 3MF does not support the texture, UV or material information stored by STL.
Fix: Keep the original STL visual dataThe model has missing faces
The STL source may contain open edges, invalid polygons, overlapping geometry or inverted normals that remain visible after conversion.
Fix: Inspect and repair the source geometryThe conversion fails on a large file
High polygon counts and complex geometry need more processing. Confirm the file is below 300 MB and opens correctly in a viewer.
Fix: Simplify unnecessary geometry detailSecure processing
Your 3D files remain yours
Product models can contain confidential engineering information. Privacy and file handling stay explicit beside the tool.
Encrypted upload
Files are transferred through a secure connection.
Automatic deletion
Uploaded and converted files are deleted after 7 days.
You retain ownership
Conversion does not transfer ownership of your model.
Not publicly listed
Your uploaded files are not added to the Marathon CAD library.
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Browse engineering categories and recent 3D-printing models. Preview and validate every downloaded file before production use.
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Frequently asked questions
Everything to know before converting
Quick answers about formats, pricing, file limits, conversion quality and privacy.
How do I convert STL to 3MF online?
Upload your STL file (.stl) to Marathon OS, choose 3MF as the output format, and download the converted file in seconds — no software installation required.
Why convert STL to 3MF?
Converting STL to 3MF adds a declared unit to the mesh, so modern slicers open the part at the right size.
What STL and 3MF file extensions are supported?
This converter accepts .stl input files and outputs .3mf. Upload up to 300 MB per file.
Is the STL to 3MF converter free?
Your first 2 CAD file conversions on Marathon OS are free, any size up to 300 MB. After that, each converted file needs a small fee or 1 credit, paid only when you download it.
How is my data stored and secured?
Your files are encrypted during upload, processed securely in the cloud, and automatically deleted after 7 days.
Do I need any special software or training?
No — upload your STL file and download 3MF directly in your browser. No plugins or desktop CAD tools required.
Can I convert large STL models to 3MF?
Yes — Marathon OS is optimized for large and complex CAD models. You can upload files up to 300 MB for STL to 3MF conversion.