What is a .step file?
STEP / ISO 10303 CAD Model · 3D model · ISO · 1994
STEP is the neutral, standardised format for exchanging precise 3D CAD models between different engineering programs. It stores exact solid geometry rather than a triangle mesh, which makes it the interoperability backbone of manufacturing and product design.
What is a .step file used for?
Defined by the ISO 10303 standard, a STEP file stores true solid geometry (a precise boundary representation), so the model stays exact and editable rather than being flattened into triangles like an STL.
Its whole purpose is interchange: a part designed in SolidWorks, Fusion 360, CATIA or FreeCAD can be handed to any of the others as a .step or .stp and keep its precision.
The file is ASCII text beginning with ISO-10303-21;, so you can even open it in a text editor, though you need CAD software to work with the geometry.
For 3D printing or rendering, a STEP is usually converted to a mesh format like STL or OBJ.
What opens a .step file?
The .step format opens in the following apps, grouped by operating system. If one app does not work, try another from the same list.
Windows
- FreeCAD
- Fusion 360
- SolidWorks
- online CAD viewers
macOS
- FreeCAD
- Fusion 360
- online CAD viewers
Linux
- FreeCAD
- online CAD viewers
Web
- Online CAD / STEP viewers
Open a .step file step by step
- Confirm the file really is a .step. Drop it into the WhatFileType identifier, which reads its signature in your browser without uploading anything.
- On Windows, open it with FreeCAD or Fusion 360.
- On Mac, use FreeCAD or Fusion 360.
- On Linux, try FreeCAD or online CAD viewers.
- Still stuck? Convert the .step to STL, which opens more widely.
Why can't I open my .step file?
Almost always because no installed app understands STEP. Install one of the apps above, or convert the file to a format your existing software already reads. If double-clicking opens the wrong program instead of nothing at all, the file association is set to the wrong app rather than the file being broken.
The other possibility is that the file is not really a .step at all. Drop it into the file identifier to check it by its bytes instead of its name. Nothing is uploaded.
How to convert a .step file
You can convert a .step file to STL, OBJ, IGES using a conversion tool or the export menu of an app that opens it.
What is the .step file signature (magic bytes)?
A .step file starts with the hex bytes 49 53 4F 2D 31 30 33 30 33 2D 32 31, which read as ISO-10303-21 in ASCII. Programs identify STEP by those bytes, not by the filename, so a renamed file still gives itself away.
These are the documented signatures for STEP.
| Hex signature | Offset | ASCII | Note |
|---|---|---|---|
| 49 53 4F 2D 31 30 33 30 33 2D 32 31 | 0 | ISO-10303-21 | 'ISO-10303-21;' header (ASCII STEP) |
See the full magic-numbers reference for every format.
Frequently asked questions
What is a .step file?
STEP is the neutral, standardised format for exchanging precise 3D CAD models between different engineering programs. It stores exact solid geometry rather than a triangle mesh, which makes it the interoperability backbone of manufacturing and product design.
How do I open a STEP or STP file?
Use CAD software such as the free FreeCAD, Autodesk Fusion 360, or an online CAD viewer. .step and .stp are the same format under two names.
What is the difference between STEP and STL?
STEP stores exact, editable solid geometry for engineering, while STL stores a fixed triangle mesh for 3D printing. Convert STEP to STL when you are ready to print; go the other way is much harder, since a mesh has lost the precise geometry.
Related formats
Sources
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