A viewer first
Bodies, faces, overall size and volume come from the exact geometry, before any mesh. Click a face to measure it, or two to measure between them.
STEP to STL converter and STEP viewer, in your browser
Drop a .step or .stp file. OpenCascade — the geometry kernel FreeCAD is built on — reads it here, in your browser: you see the part in 3D with its size and volume, choose how finely to mesh it, and download an STL your slicer opens. Before you download you see the triangle count, the file size and whether every body's mesh is closed. The file never leaves your computer.
Need the other direction, from a mesh to a CAD file? STL to STEP →
How to convert a STEP file to STL
01
Open the converter
No sign-up. The first visit downloads about 8 MB of WebAssembly (the CAD kernel and its STEP reader); your browser keeps it for next time.
02
Drop the STEP file
A .step or .stp from any CAD tool — AP203, AP214 or AP242, a part or a whole assembly. Units are converted to millimetres, which is what slicers assume for an STL.
03
Look at it, then pick the accuracy
Rotate it; click faces and edges to measure them. Choose Coarse, Standard or Fine, or type the largest deviation and angle you accept; the triangle count and file size update.
04
Download the STL
One binary STL with every body, or one STL per body in a zip. Drop it into PrusaSlicer, Bambu Studio, OrcaSlicer or Cura.
Tested on 255 real STEP files
The set: the 55 CAx-IF and OpenCascade sample files, plus 200 STEP files from public CAD projects on GitHub (two per project at most, up to 23 MB) — run with the same WebAssembly your browser runs.
- opened and meshed
- 240 / 255
- The 15 that did not open each got a reason: 11 assemblies that keep their parts in other files (the converter names those files), 2 STL files renamed .step, 2 sketches with curves only.
- closed STL for every body
- 237 / 240
- At the Standard setting. The other three are reported, not hidden: a cone face OpenCascade's mesher cannot triangulate, a surface that never was a solid, one body with 29 open edges.
- median time to read a file
- 0.09 s
- 90% under 1.2 s; meshing and writing the STL took a median 0.05 s. The largest file, 23 MB with 5,654 faces, read in 6 s.
Multi-body files are common: 110 of the 240, with up to 505 bodies in one file.
How the accuracy changes the STL
Coarse is 0.2 mm / 30°, Standard 0.05 mm / 15°, Fine 0.01 mm / 5°: the largest distance between a facet and the true surface, and the largest turn between neighbouring facets along a curve. Three CAx-IF test parts:
| Part | Coarse | Standard | Fine |
|---|---|---|---|
| io1-cm-214 (CAx-IF) · 31 × 88 × 88 mm, 29 faces | 1,888 triangles · 92 KB | 5,454 triangles · 266 KB | 25,752 triangles · 1,258 KB |
| as1-oc-214 (CAx-IF, 18 bodies) · 200 × 150 × 84 mm, 160 faces | 4,736 triangles · 231 KB | 8,584 triangles · 419 KB | 24,288 triangles · 1,186 KB |
| MAINBODY_BACK (CAx-IF) · 140 × 146 × 146 mm, 31 faces | 7,232 triangles · 353 KB | 27,234 triangles · 1,330 KB | 106,952 triangles · 5,222 KB |
What the converter does with your STEP
Accuracy you can see
Switch the view to STL facets and you see the very triangles you are about to download. Change the accuracy and the count and size follow.
A closed-mesh check per body
Every body's mesh is checked edge by edge, as the STL file will hold it. Faces that meet within the file's own tolerance but mesh a hair apart are joined; whatever is still open is counted on screen.
Assemblies
All bodies in one STL in their assembled positions, or each body as its own STL in a zip — handy for printing parts separately.
Failures that say why
A STEP that only references other files, a renamed STL, a file with only curves: the converter tells you which, instead of handing you an empty STL.
The file stays with you
It is read by WebAssembly on an origin of its own in your browser; the page uploads nothing. No sign-up, nothing stored.
Is STEP better than STL?
They do different jobs. Keep the STEP as the master copy and make an STL whenever you print.
| STEP (.step, .stp) | STL | |
|---|---|---|
| What it stores | Exact surfaces — planes, cylinders, splines — with edges and solids | Triangles only: no units, no colours |
| Curves | A hole is a true cylinder at any zoom | A hole is a ring of flat facets; how many depends on the export accuracy |
| Editing | Open in any CAD tool, change features, measure exactly | Hard to edit; made to be printed |
| Who reads it | CAD (Fusion, SolidWorks, FreeCAD, Onshape), CNC / CAM | Every slicer and 3D printing workflow |
| Size | Small: our 40 × 20 × 6 mm test bracket is 66 KB | Grows with accuracy: the same bracket is 67 KB to 1.6 MB |
FAQ
How do I convert a STEP file to STL?
Open the converter, drop the .step or .stp file, check the part in the viewer, choose an accuracy (Standard suits most prints) and click Download STL. In FreeCAD the same is selecting the body and File → Export as STL; in Fusion, right-click the body and Save As Mesh. Here nothing has to be installed.
Is STEP better than STL?
For design, yes: STEP keeps the exact geometry, so a CAD tool can measure and change it. A slicer needs triangles, so you convert to STL at the end. Converting back does not bring the exact geometry back — which is why the STEP is the file to keep.
What accuracy should I use for 3D printing?
Standard (0.05 mm, 15°) is finer than a 0.4 mm nozzle at a 0.1–0.2 mm layer height can reproduce, so the facets do not show in the print. Use Fine for large, smooth curves and Coarse for quick tests. In our tests the Fine STL was 3 to 8 times the size of the Standard one.
Can I open a STEP file without CAD software?
Yes — the converter is also a STEP viewer. Drop the file to see the part in 3D, its bodies, overall size and volume, and click faces and edges to measure them. You do not have to download anything.
Is my STEP file uploaded?
No. It is read and meshed by OpenCascade compiled to WebAssembly, running in your browser on an origin of its own; the page has no upload. All that reaches us are anonymous counts (how long it took, how many triangles), never the file or its name.
Why does my STEP file open with no geometry?
Most often it is an assembly saved as several files: the top file only lists its parts and where they are. The converter shows which files it refers to — open those, or export the assembly from your CAD tool as a single STEP file.
Why is my STL not watertight?
A surface body in the STEP that was never closed, or a face the mesher cannot triangulate, leaves a hole. The converter counts open edges per body so you know before slicing; slicers close small gaps themselves, a missing face is better fixed in the CAD tool.
STP or STEP — what is the difference?
None: both are ISO 10303-21 files, and .stp is the same format with a shorter extension. AP203, AP214 and AP242 are versions of the schema; the converter reads all three.
What units is the STL in?
Millimetres. A STEP file carries its unit, and inches or metres are converted to millimetres when it is read — the unit every slicer assumes for an STL.
Related
STL to STEP →
The other direction: flat areas merged into faces, rebuilt as a solid — and an honest account of what that is.
OpenSCAD to STEP →
A real STEP from source code: cylinders and holes as true surfaces, not triangles.
3D print generators →
Gears, boxes, brackets and more: set the parameters online and export STL or STEP.