Upload the STEP file
Drop a .step or .stp file up to 300 MB onto the converter, or browse for it. Assemblies and multi-body parts are fine.
First 2 downloads free.step and .stp to .stlAny size up to 300 MB
Convert STEP toSTL online
Upload a .step or .stp CAD model and download a print-ready STL mesh. Choose the mesh quality, output units and how assemblies are split, then open the file in PrusaSlicer, Bambu Studio, Cura or any other slicer. Nothing to install.
Encrypted uploads Files up to 300 MB Opens in any slicer Deleted automatically within 7 days
Upload your STEP file
Accepts .step and .stp. Output is .stl.
Drag and drop your STEP file here
Browse filesMaximum file size: 300 MB
First 2 downloads free — any size up to 300 MB. Then $2.99 or 1 credit per file. Packs from $1.80 per file. See pricing
No STEP file to hand?
Step by step
How to convert STEP to STL
To convert STEP to STL online, upload your .step or .stp file above, pick a mesh quality (Standard suits most 3D printing), keep the units in millimetres, then click Convert STEP to STL and download the .stl file. Your first 2 downloads are free. STP to STL works exactly the same way, because .stp is just the short extension for STEP.
Choose your settings
Pick Draft, Standard or Fine mesh quality, millimetres or inches, and whether to merge parts or export each body separately.
Convert
The surfaces in your STEP file are turned into triangles using the tolerances you chose. Most parts finish in seconds.
Download and check
Download the .stl and open it in your slicer. Measure one known dimension and look at a curved face before you print.
Mesh settings
Choose the right mesh quality
STEP stores exact curves. STL can only store flat triangles, so every curved face has to be approximated. Two settings decide how closely the triangles follow the original surface, and how big the file gets.
Chord tolerance
The largest gap allowed between the true CAD surface and the flat triangle that replaces it, in millimetres. Smaller values give smoother curves and more triangles.
Angular tolerance
The largest angle allowed between neighbouring triangles. It keeps small holes, bosses and fillets round even when the chord tolerance alone would leave them blocky.
Mesh quality presets
| Preset | Chord tolerance | Angular tolerance | Best for | File size |
|---|---|---|---|---|
| Draft | 0.1 mm | 20° | Quick previews, large FDM parts, sending a model for a quote | Smallest |
| StandardDefault | 0.03 mm | 10° | Most FDM printing with 0.4 mm nozzles and 0.1–0.3 mm layers | Moderate |
| Fine | 0.01 mm | 5° | Resin (SLA, MSLA) printing, SLS, visible curved surfaces, small threads | Largest |
| Custom | You choose | You choose | Matching a print service’s or simulation tool’s mesh specification | Varies |
Draft
Facets are visible on curved faces
Standard
Smoother than a typical FDM layer can show
Fine
Follows the true curve closely
The dashed line is the exact circle stored in the STEP file. The solid outline is what ends up in the STL.
The formats
What are STEP and STL files?
STEP file (.step, .stp)
A STEP file is a CAD exchange format defined by the international standard ISO 10303. It stores exact solid geometry, assembly structure, units and product data, so almost any CAD program can open and edit it.
STP is not a different format. It is the shorter file extension for STEP, and both convert the same way.
STL file (.stl)
An STL file describes the surface of a 3D object as a list of flat triangles. It was created by 3D Systems in 1987 for stereolithography and is still the file almost every slicer, print service and mesh tool accepts.
It stores no units, colours or part names, only triangle corners and the direction each triangle faces.
This converter at a glance
- Input
- .step and .stp files (STEP AP203, AP214, AP242)
- Output
- .stl, binary or ASCII, as one file or one per body (ZIP)
- Maximum upload
- 300 MB
- Price
- First 2 downloads free, any size up to 300 MB. Then $2.99 each, or from $1.80 with credit packs
- Default settings
- Standard mesh quality (0.03 mm, 10°), millimetres, binary
- Works on
- Any modern browser on Windows, macOS, Linux, iPad and Android
- File retention
- Deleted automatically within 7 days, never published
What carries over from STEP to STL
STEP holds exact solids, assembly structure and product data. STL holds triangles and nothing else. Here is what that means for your file.
Kept
- The outer shape of every solid, within the tolerance you chose
- Dimensions, written in the units you select (millimetres by default)
- Position and orientation of each part in the assembly
- Separate bodies, as shells in one STL or as separate files
- Closed, watertight volumes, when the STEP solids are valid
Lost or changed
- Exact cylinders, NURBS and fillets, which become triangles
- Feature history, sketches and constraints
- Assembly tree and part names (kept only as file names in per-body mode)
- Colours, materials and PMI or GD&T annotations
- The unit label itself, since STL has no field for it
Units and scale
Get the size right first time
An STL file stores coordinates such as 25.4 with no unit attached, and slicers read those numbers as millimetres. If a model is written out in inches, a part that should be one inch wide shows up as one millimetre wide.
This converter reads the units in your STEP file and writes the STL in millimetres unless you choose inches, so parts open in the slicer at their real size. Before printing, measure one dimension you know, such as an overall length or a hole diameter.
If a file from somewhere else arrives at the wrong size, scale it in the slicer. In PrusaSlicer, Bambu Studio and OrcaSlicer, use the scale fields in the object panel. In Cura, use the Scale tool with uniform scaling turned on.
Scale fixes for unit mistakes
| What you see | Likely cause | Scale by |
|---|---|---|
| About 25× too small | Written in inches, read as mm | 2540% |
| About 25× too large | Written in mm, read as inches | 3.937% |
| 10× too small | Written in centimetres | 1000% |
| 1000× too small | Written in metres | 100000% |
Format comparison
STEP vs STL vs 3MF
Pick the format the next program needs. STEP is for designing and manufacturing from exact geometry, STL and 3MF are for printing, and OBJ is for visualisation.
| Feature | STEP (.step, .stp) | STL (.stl) | 3MF (.3mf) | OBJ (.obj) |
|---|---|---|---|---|
| Geometry | Exact surfaces and solids (B-rep) | Triangles only | Triangle mesh | Polygon mesh |
| Units | Stored | Not stored | Stored | Not stored |
| Colours and materials | Yes | No | Yes | Through a separate .mtl file |
| Multiple parts | Full assembly tree with names | Unnamed shells | Named objects | Named groups |
| Editable in CAD | Yes | Mesh editing only | Mesh editing only | Mesh editing only |
| Main use | Design exchange, CNC, manufacturing | 3D printing, print services | Modern slicer workflows | Rendering and visualisation |
Convert to STL when
Your slicer or print service asks for STL, or you need the one mesh format every 3D program can open. Keep the STEP as your editable master.
Keep STEP when
The part will be machined, edited, toleranced or sent to a supplier who works from CAD. Converting to a mesh would throw away exact geometry they need.
Consider 3MF when
You print with PrusaSlicer, Bambu Studio, OrcaSlicer or Cura and want units, colours and multiple named parts kept in one file.
Assemblies
Converting STEP assemblies to STL
STL has no concept of an assembly. When your STEP file contains several bodies, choose how they should come out.
Parts: Single STL
Print the assembly as one piece
Every body is placed in one STL, in its assembled position. Use this for display models, or parts that are printed together on one plate.Parts: One STL per body (ZIP)
Print each part separately
You get a ZIP with one STL per solid, named after the part in the STEP file. Use this when parts need different orientations, materials or printers.Desktop alternatives
Export STL from your CAD software
If you have the original CAD model, exporting STL directly works well. Use this converter when you only have the STEP file, don’t have a CAD licence on your machine, or want the same mesh settings every time.
In every program, look for three things: binary output, millimetre units, and a deviation or resolution setting that matches the quality presets above.
Autodesk Fusion
- Right-click the body or component in the Browser and choose Save As Mesh.
- Set Format to STL (Binary) and Unit type to Millimeter.
- Set Refinement to High, or open Refinement Settings to enter a surface deviation.
SolidWorks
- Go to File → Save As and choose **STL (*.stl)**.
- Click Options. Choose Binary output and Millimeters.
- Set Resolution to Fine, or Custom to enter deviation and angle tolerances.
- For assemblies, untick the single-file option to get one STL per component.
FreeCAD
- Open the STEP file with File → Open.
- Switch to the Mesh Design workbench, select the body and choose Meshes → Create mesh from shape.
- Set Surface deviation and Angular deviation, then click OK.
- Select the new mesh and choose Meshes → Export mesh, then save as STL.
Onshape
- Right-click the part in the Parts list and choose Export.
- Set Format to STL, STL format to Binary and Units to Millimeter.
- Choose Fine resolution, or Custom to set angular and chordal tolerance.
Before you print
Check the STL before slicing
A conversion can succeed and still give you a file that prints badly. These checks take a couple of minutes in any slicer and catch most problems before they waste filament or resin.
Open the STL viewer to inspect the file in your browser.
- 01
Measure a known dimension
Compare an overall length or hole diameter with the drawing. If it is off by 25.4×, see the scale fixes above.
- 02
Zoom into curved faces
Look at holes, fillets and arcs. If you can see flat facets where the finish matters, reconvert at Fine.
- 03
Look for repair warnings
Slicers flag open edges, flipped normals and self-intersections when the file loads. A clean STEP solid should load without them.
- 04
Check thin walls and small details
On FDM printers, walls thinner than about two extrusion widths (roughly 0.8 mm on a 0.4 mm nozzle) often print poorly or are skipped by the slicer.
- 05
Count the parts
Make sure every body you expected is there, and that parts meant to print separately are not merged.
- 06
Preview the sliced layers
Scrub through the layer preview to check supports, orientation and first-layer contact before sending the job.
Under the hood
How the converter works
Every conversion runs the same five steps on our servers, using the settings you choose. Nothing about your file is guessed or hidden.
- 01
Read the STEP file
Every solid and assembly instance is loaded with its position and the units stored in the file.
- 02
Check the geometry
Open shells and invalid faces are detected before meshing, so problems show up as a clear message instead of a broken STL.
- 03
Mesh each face
Surfaces are converted to triangles using the chord and angular tolerance of your chosen quality preset.
- 04
Write the STL
The mesh is saved as binary or ASCII, in millimetres or inches, as one file or a ZIP with one file per body.
- 05
Delete the files
The uploaded STEP and the converted STL are removed automatically within 7 days.
Common problems
STEP to STL troubleshooting
Most problems come from the mesh setting, the units, or the STEP file itself. Each fix below can be applied with the converter on this page.
Curves look faceted
The mesh is too coarse for the size of the part or the finish you need.
Fix: reconvert at Fine, or Custom with a chord tolerance of 0.01 mm.
Switch to Fine qualityThe STL is enormous
The mesh is finer than the printer can use, or the file is ASCII. A binary STL uses 50 bytes per triangle; ASCII is roughly five times bigger.
Fix: use Standard or Draft quality with Binary STL.
Use Standard, binaryThe model is the wrong size
STL has no unit field, so the slicer assumed millimetres for a file written in another unit.
Fix: reconvert with Millimetres selected, or scale in the slicer.
See the scale tableSlicer reports holes or non-manifold edges
The STEP contains open surfaces instead of closed solids, often from a surface-only export or a damaged file.
Fix: re-export the STEP as solids (AP214 or AP242) from the source CAD, or use your slicer’s repair option.
Get a designer to repair itParts are merged or missing
All bodies went into one file, or some bodies were hidden or suppressed when the STEP was exported.
Fix: choose One STL per body, and check every body was visible in the source export.
Export one STL per bodySmall holes or walls disappeared
The features are below the mesh resolution or below what the printer can produce.
Fix: use Custom with a 5° angular tolerance. If walls are under the printer’s minimum, the design needs thickening.
Set custom tolerancesSecure processing
Your design files stay yours
Engineering files often contain confidential product geometry. Conversion uploads are kept separate from the public Marathon OS CAD library, and this is exactly how they are handled.
Encrypted transfer
Files are uploaded and downloaded over an encrypted connection.
Automatic deletion
Uploaded STEP files and converted STLs are deleted automatically within 7 days.
You keep ownership
Uploading or converting a file never transfers ownership of the design to Marathon OS.
Never published
Converted files are never listed in the public CAD library.
Pricing
First 2 downloads free, simple credits after
After your free downloads, each file costs one credit, any size up to 300 MB, used only when you click Download. No subscription, credits never expire, and every purchase comes with an invoice.
Single
$2.99
1 download
Pay per fileStarter
$7.99
3 credits, $2.66 each
Pro
$12.99
5 credits, $2.60 each
Team · Most popular
$19.99
10 credits, $2.00 each
Studio
$26.99
15 credits, $1.80 each
FAQ
STEP to STL questions
Answers about quality, file size, units, assemblies, pricing and privacy.
Reviewed by the Marathon OS engineering team.
Last updated 17 October 2026Is the STEP to STL converter free?
Yes, your first 2 file downloads are free, any size up to 300 MB, with every setting available. After that, each file uses one credit, charged only when you click Download. A single download costs $2.99 and credit packs bring the price down to $1.80 per file. Credits never expire.
Is there a difference between STEP and STP files?
No. .step and .stp are two extensions for the same ISO 10303 format. This converter accepts both and treats them identically.
Does converting STEP to STL lose quality?
STL can only store flat triangles, so curved STEP surfaces are approximated. How close the approximation is depends on the mesh quality you choose. For FDM printing, Standard is finer than the printer can reproduce. For resin printing or visible curved surfaces, use Fine. Keep the STEP file as your editable master.
What mesh quality should I use for 3D printing?
Standard (0.03 mm chord tolerance, 10° angular tolerance) suits most FDM printing. Use Fine (0.01 mm, 5°) for resin printers, SLS and parts with visible curved surfaces. Use Draft only for previews and quotes.
Can I convert STEP to STL without installing software?
Yes. The converter runs in your web browser on Windows, macOS, Linux, iPad and Android, so you don’t need CAD software or a plugin. Upload the file, choose your settings and download the STL.
Why is my STL file so large?
A binary STL uses 50 bytes per triangle, so one million triangles is about 50 MB. ASCII STL is roughly four to five times larger. Choose a coarser mesh quality or binary output to reduce file size.
Why is my STL the wrong size in the slicer?
STL files do not store units, and slicers assume millimetres. If the part looks 25.4 times too small, it was exported in inches; scale it by 2540%. If it looks 25.4 times too large, scale it by 3.937%. Exporting in millimetres avoids the problem.
Can I convert a STEP assembly into separate STL files?
Yes. Under Parts, choose One STL per body and you will download a ZIP with one STL for each solid, named after the part in the STEP file. Choose Single STL to merge every body into one file.
Which STEP versions are supported?
The converter reads STEP AP203, AP214 and AP242 files exported from SolidWorks, Fusion, Inventor, CATIA, Creo, NX, Onshape, FreeCAD and other CAD programs.
Can I convert STL back to STEP?
Yes, with the STL to STEP converter, but the result is a solid made of triangular faces, not the original smooth CAD geometry. Keep your original STEP file if you will need to edit the design later.
Should I use 3MF instead of STL?
3MF stores units, colours and multiple objects in one file, and PrusaSlicer, Bambu Studio, OrcaSlicer and Cura all read it. Use 3MF when your slicer supports it and you want those extras. STL is still the safest choice when a print service or older software asks for a mesh.
What is the maximum file size?
You can upload STEP files up to 300 MB. Very large assemblies converted at Fine quality can produce STL files with millions of triangles, which may slow down your slicer.
Are my files private?
Files are transferred over an encrypted connection, are never added to the public Marathon OS CAD library, and are deleted automatically within 7 days. You keep full ownership of your design.
Need a print-ready model, not just a mesh?
Conversion gives you an STL of the geometry you have. If the model needs solid repair, thicker walls, split parts or tolerances for a specific process, a CAD designer can fix it.