PRT File Documentation
Summary
On this page a .prt file is a PTC Creo Parametric (formerly Pro/ENGINEER) 3D part: one component of a mechanical CAD model, stored as exact solid geometry plus its feature history. It is a proprietary PTC binary with no published signature, so full editing needs Creo, but the free Creo View or CAD Assistant will view and measure it. The extension is shared: Siemens NX, Solid Edge and CADKEY also save incompatible .prt files. To move a part to other CAD, export it to STEP.
Technical details
| Feature | Value |
|---|---|
| Full name | PTC Creo / Pro-ENGINEER Part File |
| File extension | .prt (often numbered, e.g. .prt.1, .prt.2) |
| MIME type | application/octet-stream |
| Format type | Proprietary binary 3D CAD part (parametric solid model) |
| Geometry model | Boundary representation (B-rep) exact solid |
| Developer | PTC (Parametric Technology Corporation) |
| Introduced | Pro/ENGINEER 1987; PTC Creo Parametric since 2011 |
| Open standard | No — undocumented proprietary format |
| Magic number | None published (PTC binary header) |
| Assembly counterpart | .asm (an assembly of parts) |
| Revision scheme | Numbered saves; highest number is the latest |
| Neutral exchange | .step / .stp, .iges / .igs |
| Mesh export | .stl, .obj (loses feature history) |
| Also uses .prt | Siemens NX, Solid Edge, CADKEY (incompatible formats) |
| Related extensions | .asm, .step, .iges, .neu, .xpr |
| Native editor | PTC Creo Parametric (Windows) |
| Specification | ptc.com/en/products/creo |
What is a PRT file?
On this page, a .prt file is a PTC Creo Parametric part — one component of a mechanical CAD model. PTC's Pro/ENGINEER, released in 1987, pioneered parametric, feature-based, history-driven solid modelling; the product is now PTC Creo Parametric, and both write parts with the .prt extension. A part is a single physical component. Parts are combined into an assembly, saved with the .asm extension, to build a complete product. Open the .asm to see the whole product; open a .prt for one component.
Internally a Creo .prt is exact boundary-representation solid geometry plus the parametric feature history that built it, wrapped in a proprietary PTC binary container. It is engineering data, not an image, though it usually embeds a small tessellated preview so browsers and viewers can show a thumbnail without a CAD kernel. The format is undocumented and has no published magic number, which shapes almost everything below: you identify and handle these files by their PTC origin, not by parsing a signature.
The shared .prt extension: four incompatible formats
The single most important fact about .prt is that the extension is not owned by PTC. Several major CAD systems independently chose .prt for their part files, and the binaries are mutually unreadable:
| System | What its .prt is |
|---|---|
| PTC Creo / Pro-ENGINEER | The subject of this page — PTC's parametric part |
| Siemens NX (formerly Unigraphics) | A part built on the Parasolid kernel; a different binary |
| Solid Edge | Historically .prt for parts (its current part extension is .par) |
| CADKEY / Kubotek KeyCreator | A legacy part format of its own |
A viewer or CAD package must therefore know which CAD produced the file, because opening a Siemens NX .prt in Creo will not work despite the identical extension. When a .prt refuses to open, the cause is often exactly this: the file is a different vendor's part that merely shares the suffix. The reliable escape is a neutral exchange file (STEP or IGES), which any of these systems can read regardless of who created the geometry.
B-rep solid geometry: how the part is defined
A Creo part stores its shape as a boundary representation, or B-rep. Rather than a mesh of triangles that only approximates a surface, B-rep records the exact mathematical surfaces (planes, cylinders, splines) and the topology that stitches them together: which faces bound which solid, which edges join which faces, and where vertices lie. This is watertight, precision geometry — a hole is an exact cylinder of a stated diameter, not a faceted approximation — which is why a B-rep part can be measured, sectioned and re-machined without accumulating the rounding errors a mesh would introduce.
That precision is also why exporting a part to a mesh format is a one-way loss. When you write STL or OBJ, the exact surfaces are tessellated into triangles at a chosen resolution, and the topology and dimensions are discarded. The mesh is fine for 3D printing or rendering but is no longer editable CAD.
The feature tree: parametric, history-based modelling
What separates a native Creo .prt from a plain solid is the feature history. Creo is a history-based modeller: the part is not stored merely as its final shape but as the ordered recipe of operations that produced it. A typical tree starts with a sketch on a datum plane, extrudes it into a base solid, then adds features — another extrude, a revolve, a hole, a set of fillets, a shell — each referencing geometry from earlier features.
Because features are parametric, their dimensions are editable variables. Change the base plate's thickness and every feature that depends on it regenerates: holes stay on the face they were placed on, fillets keep their edges, mating references update. The .prt stores this tree, its sketches and constraints, and the model parameters (material, part number, units, author) alongside the resulting B-rep. Neutral exports like STEP carry the final solid but not this tree, so a part re-imported from STEP is “dumb” geometry: correct in shape, but without the editable history that built it.
Numbered revisions: why a file is named part.prt.3
Creo and Pro/ENGINEER keep numbered revisions on disk. Each save can be written as part.prt.1, part.prt.2, part.prt.3 and so on, appending an incrementing integer rather than overwriting the previous file. This regularly confuses people who receive such a file and think the extension is broken or the file corrupt. It is neither. The trailing number is a version, and the highest number is the most recent revision. Open that one. Older numbers are prior saves you can usually delete once you are sure you have the latest, though in a managed workspace they may be pruned automatically.
Viewing a part and moving it to other CAD
Because Creo is expensive professional software, most people who receive a .prt want to view or convert it rather than edit it. To view, rotate and measure without a Creo licence, PTC's free Creo View opens native .prt and .asm files; the cross-platform CAD Assistant from Open Cascade and Autodesk's online viewer do the same, and Dassault's eDrawings imports many part formats. To edit, Autodesk Fusion 360 can import a Creo part.
To open the part in a different CAD package, the neutral path is STEP (.step/.stp), which virtually any CAD reads — SolidWorks, Fusion 360, FreeCAD, Onshape. Export it from Creo, or open the .prt in CAD Assistant or Fusion and re-export. STEP preserves the exact B-rep solid (though not the feature tree). IGES is the older neutral option, worth using only when a downstream tool specifically requires it. There is no direct converter from a Creo .prt to a native SolidWorks .prt-style part: SolidWorks parts are .sldprt, a different proprietary format, so you route through STEP. For 3D printing, export to STL, which tessellates the solid into the triangle mesh a slicer expects.
Frequently asked questions
What does the number after .prt mean, like part.prt.3?
It is Creo/Pro-ENGINEER's revision numbering. Each save is written as .prt.1, .prt.2, .prt.3 and so on, keeping earlier versions rather than overwriting them. The highest number is the latest revision, so open that one.
Why won't my PRT file open in my CAD program?
Either it was saved by a newer Creo version than yours, or it is actually a different CAD system's .prt (Siemens NX, Solid Edge or CADKEY) that only shares the extension. Ask the sender for a neutral STEP or IGES export, which any CAD can read.
Is a PRT the same as an assembly?
No. A .prt is a single part, one component. Multiple parts are combined into an assembly saved as .asm. Open the .asm to see the whole product and each .prt for an individual component.
References
- PTC — Creo (native Pro-E / Creo part software)
- PTC — Creo View (free viewer)
- Open Cascade — CAD Assistant (free cross-platform viewer/converter)
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