# STEP to STL converter

> Convert STEP and STP CAD files to STL (or 3MF/OBJ) in your browser with OpenCASCADE. Fine, print-ready tessellation in millimetres — free, no upload, no CAD software.

Source: https://freestlviewer.com/step-to-stl/  
Updated: October 22, 2026

Convert .step and .stp CAD files to STL without CAD software. OpenCASCADE triangulates the model on your own device — nothing is uploaded. The tool is interactive: open https://freestlviewer.com/step-to-stl/ and drop a file onto the page.

## Why convert STEP to STL?

STEP describes a part with exact mathematical surfaces. That is what CAD
programs exchange, but 3D printing works with triangles: slicers, many online
print services and most mesh tools want STL. Converting is the step that turns
the exact model into the faceted one that gets printed.

## How the conversion works

1. The first time you open a STEP file, your browser downloads the
   OpenCASCADE engine (about 7 MB) — the same kernel FreeCAD uses.
2. The B-rep surfaces are triangulated in a background worker with a linear
   deflection of **0.1 % of the bounding box** and an angular deflection of
   0.5 rad, in millimetres.
3. The converter writes the result as binary STL (or 3MF/OBJ) and your browser
   downloads it.

STEP files declare their own units, so there is nothing to choose: the STL
comes out in millimetres at the part's real size.

## Things to know

- **Assemblies become one STL.** Every body is merged into a single mesh at
  its assembled position. If you need parts separately, export them separately
  from your CAD program, or use your slicer's "split to objects".
- **Colours need 3MF.** STL has no colour; pick 3MF to keep body colours.
- **Tolerances.** The mesh is fine enough for any printer, but a bore that was
  exactly 6.000 mm in CAD is now a polygon. Measure critical fits in CAD.
- **Surfaces without volume.** Some exports contain only open surfaces. They
  convert, but the STL will not be watertight — the summary tells you before
  you download.

## STEP, STL or 3MF for printing?

If your slicer imports STEP, giving it the STEP file lets the slicer choose its
own resolution. For everything else, STL is the universal choice and 3MF the
better one when the receiver supports it: smaller, in millimetres, with
colours.

## On a phone

STEP conversion works in a mobile browser, but triangulating a large assembly
is heavy for a phone. The Mes3D app for iPhone, iPad and Mac runs OpenCASCADE
natively and can export STL, OBJ or 3MF too.

## Frequently asked questions

### How do I convert a STEP file to STL for free?

Drop the .step or .stp file onto this page, keep STL selected and click Convert and download. The file is triangulated in your browser with OpenCASCADE and saved as a binary STL in millimetres. No account or CAD software is needed.

### Will the STL be accurate enough to print?

Yes for printing. Curved faces are triangulated with a deflection of 0.1 % of the model's size, finer than an FDM printer can reproduce. The STL is a mesh, though: exact cylinders become many flat facets, which matters for CAD tolerances, not for printing.

### Can I convert STEP to STL and keep the colours?

Choose 3MF instead of STL. Body colours stored in the STEP file carry over to the 3MF; STL cannot store colour at all.

### Do I even need to convert? My slicer reads STEP.

Maybe not — recent PrusaSlicer, OrcaSlicer and Bambu Studio versions import STEP directly. Converting is still useful for older slicers, online print services that only take STL, sharing with people who have no CAD tools, and checking the exact mesh that will be printed.