What is a STEP file?
A STEP file is a 3D CAD model saved in a neutral format that almost every CAD system can read and write. Its name ends in .step or .stp. It is the usual way to pass a design between companies that use different CAD software.
What is in a STEP file
- Exact geometry. Faces, edges and solids as the CAD system defined them, not a mesh of triangles.
- The assembly. Which parts there are, how they nest and where each one sits.
- Names and colours of parts.
What is left behind is the history: the sketches, the features and the dimensions that drive them. Another CAD system can open the shape and add to it, but it cannot replay how the shape was made.
STEP is short for Standard for the Exchange of Product model data. It is the international standard ISO 10303, first published in 1994. The file itself is plain text and begins with ISO-10303-21.
.step, .stp and .p21
They are the same format. .stp is the short form of .step. A .p21 file is named after part 21 of the standard, which defines the text layout. Rename one to another and it still opens.
AP203, AP214 and AP242
Export dialogs ask which “application protocol” to use. Each defines what a STEP file may contain.
- AP203: geometry and assemblies for mechanical design. The oldest and plainest.
- AP214: written for the car industry. It adds colours and layers.
- AP242: the current one. It merges the other two and can carry dimensions and tolerances attached to the model.
If you are not sure, choose AP214 or AP242. Either keeps the colours.
STEP compared with STL, IGES and native files
| Format | What it holds | Used for |
|---|---|---|
| STEP (.step, .stp) | Exact geometry, assemblies, names, colours | Passing a design between different CAD systems |
| STL (.stl) | Triangles only. No assembly and no units | 3D printing |
| IGES (.igs, .iges) | Mostly surfaces. An older standard, from 1980 | Older systems. STEP has largely replaced it |
| Native (.sldprt, .ipt, .f3d) | Everything, with the full history | Working in the CAD system that made it |
How to open a STEP file
- With CAD software. SolidWorks, Fusion, Inventor, Creo, Onshape and the free FreeCAD all import STEP.
- Without it. Drop the file on a viewer that runs in a browser. See open a STEP file online.
How to send one
The person you send a STEP file to may have no way to open it. Sending it as a link lets them see the model in a browser first, then download the file. See how to share a STEP file.
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Open a STEP fileUpdated 6 October 2026