File format guide

What a STEP (.stp) file is and how to open it on Windows

Updated 26 September 2026 · measured with 3D CAD Converter 1.0.22

A STEP file is a 3D model in the neutral format that nearly every CAD program reads and writes (ISO 10303, extensions .stp and .step). It holds exact surfaces, not triangles, which is why a slicer, a game engine or a web page cannot use it as it is.

Inside a real STEP file

ISO-10303-21;
HEADER;
FILE_NAME('HOUSING_SCD41','2026-03-23T16:24:43',...,
  'CREO PARAMETRIC BY PTC INC, 2024063',...);
FILE_SCHEMA(('CONFIG_CONTROL_DESIGN'));
ENDSEC;
DATA;
#1=CARTESIAN_POINT('',(-8.5E0,1.6E1,-8.5E0));
#9=LINE('',#8,#7);
#823=CYLINDRICAL_SURFACE('',#822,5.E0);
#833=ADVANCED_FACE('',(#832),#823,.T.);
...
ENDSEC;
END-ISO-10303-21;

70 KBfile size

54exact faces

25cylinders

23planes

AP203schema, from FILE_SCHEMA

0triangles in the file

housing_grove_scd41.stp, exported from Creo Parametric. The header names the schema; the DATA section lists points, curves and faces as exact geometry. There is not one triangle in the file.

AP203, AP214, AP242: which STEP you have

Protocol	FILE_SCHEMA says	Colors	PMI (GD&T)	Typical source
AP203	`CONFIG_CONTROL_DESIGN`	No	No	Older exports, and Creo files like the one above
AP214	`AUTOMOTIVE_DESIGN`	Yes	No	Most CAD export dialogs; 3D CAD Converter writes it
AP242	`AP242_MANAGED_MODEL_BASED_3D_ENGINEERING`	Yes	Yes	Newer exports that carry tolerances

All three carry the same exact geometry. 3D CAD Converter reads AP203 and AP214 files (both tested) and writes AP214. PMI does not survive a conversion to a mesh.

Three free ways to open a STEP file on Windows

Windows has no built-in STEP viewer. These three are free; the fourth is ours, for when opening is not enough.

eDrawings Viewer

Free viewer from Dassault Systèmes, made for SolidWorks files. View, measure, cross-section. Check that your version opens STEP and IGES before you count on it.

View + measure

CAD Assistant

Free from Open Cascade, for personal and commercial use. Opens STEP AP203, AP214 and AP242, and saves a model as STL, glTF or OBJ.

View + save as

FreeCAD

Open source. Opens STEP and lets you edit the solid (the feature history is not in the file), then export STL or OBJ, one file at a time.

Edit + export

3D CAD Converter

Not free: a trial with 10 conversions, then a one-time license. Previews and measures a STEP, then converts a whole folder in one run.

Batch convert

When you need to convert it

STEP.stp · .stepexact surfaces

STL3D printing: every slicer reads it
3MF3D printing with the units kept
glTF / GLBWeb and AR viewers such as three.js
OBJBlender and rendering
IGESOlder CAD and CAM systems
STL → STEPNo way back: the exact surfaces are gone

Mesh formats are one-way: an STL, OBJ or 3MF cannot become an exact-surface STEP again. Keep the STEP as the master.

How to open a STEP file on Windows, step by step

Open it in a free viewerCAD Assistant or FreeCAD. If you only need to look or measure, you are done.
If it won't open, check the fileAn empty or cut-off download fails in every viewer. Fix guide
Convert for where it goesSTL for printing, glTF for the web, OBJ for Blender. The Fine preset keeps curves smooth.
Batch the rest of the folderOne folder, one run: 5 STEP assemblies (38 MB) became STL in 14 s (Standard preset, Ryzen 9 3900X).

The housing from the STEP file above, converted to STL at the Fine preset — The file above after conversion: STL at the Fine preset, 20,864 triangles, 1.0 MB, converted in 0.7 s on a Ryzen 9 3900X.

Preview, measure and convert your STEP filesFree trial: 10 conversions, no credit card. Windows 10/11, 64-bit.

Download Free Trial

STEP file questions

Is .stp the same as .step?

Yes. Both extensions hold the same ISO 10303-21 text format. Renaming one to the other changes nothing.

Can Blender open a STEP file?

Not out of the box. Blender imports OBJ, STL and glTF, so convert the STEP to OBJ or glTF first.

Can I edit a STEP file?

Yes, in a CAD program such as FreeCAD. You edit the solid itself; the original feature history is not in the file.

What is the difference between STEP and STL?

STEP stores exact surfaces and assemblies; STL stores triangles only. Printers want STL or 3MF, CAD programs want STEP.

Can I open a STEP file without installing anything?

Online viewers such as Autodesk Viewer open STEP in the browser, if you may upload the file. On the PC itself you need one of the free viewers above.