What is the PRT file format?
A .prt file stores a component of a 3D parametric CAD model. The file can contain geometry, surfaces, features, dimensions, and annotations. Multiple .prt component files can be assembled to recreate a complete product, then saved as an assembly file with the .asm extension. Because the .prt extension is shared across several unrelated CAD platforms, files from different systems are mutually incompatible. The .prt format is broadly divided into four families based on the originating application:
- CADKEY (later KeyCreator)
- PTC Pro/ENGINEER / Creo Parametric
- Siemens NX (formerly Unigraphics)
- SolidWorks (legacy; now uses
.sldprt)
CADKEY adopted the .prt extension around 1984 and dominated that niche until approximately 2004. After its discontinuation, KeyCreator became the primary application with full support for CADKEY-origin .prt files. PTC has used the .prt format since 1987, when Pro/ENGINEER first introduced parametric solid modeling. Despite many major releases and a rebranding to PTC Creo Parametric in 2011, the .prt extension and core format identity have remained consistent. One notable characteristic of Creo and Pro/ENGINEER .prt files is the built-in versioning system: each save appends an incrementing number to the filename (e.g., part.prt.1, part.prt.2), with the highest number representing the latest revision.
The .prt extension was also adopted by Siemens PLM for NX CAD, the successor to Unigraphics. NX .prt files use a different internal structure from PTC's format and are not interchangeable. For some time, SolidWorks saved files with a .prt suffix, but that practice led to confusion and was replaced by the dedicated .sldprt extension.
Many tools allow users to open, view, and convert .prt files. Neutral exchange formats such as STEP (.stp) and IGES (.igs) are commonly used to share .prt geometry across different CAD environments.
Security & safety
RISK: LOWA Creo .prt is passive CAD geometry/metadata - no executable code - so opening one in a viewer is safe. The practical cautions are non-malware: parts saved in a newer Creo version may not open in an older one; exporting to mesh formats (STL/OBJ) discards the editable feature history; and a .prt from a different CAD system (Siemens NX, CADKEY) won't open in Creo despite the shared extension. Treat shared parts as intellectual property and get files from sources you trust.
Format details
in a nutshell- Siemens NX / Unigraphics Part - Siemens NX (formerly Unigraphics) also saves parts as .prt; built on the Parasolid kernel and not interchangeable with a Creo .prt.
- CADKEY / KeyCreator Part - Legacy CADKEY (now Kubotek KeyCreator) used .prt for its own part files.
- Solid Edge Part - Siemens Solid Edge has historically used .prt for parts (its current part extension is .par).
Programs that open PRT files
Windows4 apps
eDrawings Viewer
Free Free Dassault viewer that imports many CAD part formats; open the .prt to view and measure. 
PTC Creo Parametric
Paid The native editor for Creo/Pro-E parts. File > Open the .prt to view and fully edit the feature tree.
Creo View (free viewer)
Free Free PTC viewer; open the Creo .prt/.asm to view, rotate, measure and mark up without a Creo license.
Autodesk Fusion 360
Freemium Insert/Upload the .prt; Fusion imports Creo parts for viewing, editing and re-export. Free for personal use. 
macOS2 apps
Autodesk Fusion 360
Freemium Import the .prt on macOS to view/edit and export to STEP/STL (Creo itself is Windows-only).
CAD Assistant (Open Cascade)
Free Free cross-platform viewer/converter; open the .prt to view, measure and export (e.g. STEP/STL/glTF). 
Linux1 app
CAD Assistant (Open Cascade)
Free Open the .prt to view and convert; one of the few free Linux options that reads native CAD parts. Technical details
deep spec| Format type | Proprietary binary parametric solid model |
| Developer | PTC (Parametric Technology Corporation) |
| File encoding | Undocumented binary; no published magic bytes or public specification |
| Versioning system | Each save creates a numbered revision file (e.g., `part.prt.1`, `part.prt.2`); the highest number is the current version |
| Geometry representation | Feature-based parametric solid and surface modeling; full design-intent history tree built on a B-rep kernel |
| Parametric constraints | Dimensions and relations stored in the feature tree drive geometry; editing a parameter propagates changes through dependent features |
| Associated assembly format | `.asm` - PTC assembly file that references `.prt` part components by file path |
| Cross-platform compatibility | `.prt` files from PTC, Siemens NX, and CADKEY share the extension but are mutually incompatible binary formats |
| MIME type | `application/octet-stream` (no dedicated IANA-registered MIME type) |
| Compression | PTC Creo `.prt` files are stored uncompressed; Siemens NX `.prt` files support optional internal compression |
| Embedded data | 3D geometry, feature history, tolerances, materials, mass properties, and user-defined parameters |
| Drawing association | Referenced by PTC drawing files (`.drw`) to generate 2D manufacturing documentation |
| Family tables | A single `.prt` file can encode a family of related part variants through an embedded family table |
| Neutral export formats | STEP (`.stp`), IGES (`.igs`), STL, and Parasolid (`.x_t`) for cross-CAD interoperability |
| Platform | Native authoring on Windows (PTC Creo Parametric); third-party viewers available on Windows, macOS, Linux, and web |
| Released | Pro/ENGINEER, 1987; continued as PTC Creo Parametric (2011) |
| Specification | www.ptc.com |
PRT conversions
Community Q&A
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