What is the P21 file format?
.p21 is a neutral 3D CAD exchange file governed by ISO 10303-21, the clear-text encoding layer of STEP (Standard for the Exchange of Product model data). It is mechanically identical to .step and .stp - same ASCII structure, same parsers, same content - with .p21 designated by ISO as the preferred extension for general-purpose Part 21 exchange that is not tied to a specific application protocol.
Every .p21 file is plain ASCII text. The first line is always the literal string ISO-10303-21;, which identifies the format. A HEADER section follows, recording the file description, authoring tool, and the application protocol name in the FILE_SCHEMA entry. The bulk of the file is the DATA section: numbered entity records such as #1=CARTESIAN_POINT(…); that encode exact boundary-representation (B-rep) solid geometry - analytic surfaces, precise edges, and solids, not a triangle mesh.
Because .p21 carries true B-rep geometry, it preserves the exact dimensions and topology computed by the originating CAD system. This makes STEP the standard neutral interchange format in aerospace, automotive, and industrial engineering. The FILE_SCHEMA entry names the application protocol: AP203 for configuration-controlled design, AP214 for automotive parts, and AP242 (the current recommended protocol) for managed model-based 3D engineering with PMI annotations and kinematics.
Any STEP-capable application opens .p21 files; FreeCAD, Autodesk Fusion 360, and CATIA are common examples. For 3D printing, tessellate the geometry first to an STL or similar mesh format. Compared with IGES, the older neutral-exchange alternative, STEP is preferred for all new work.
Security & safety
RISK: LOWA .p21 STEP file is plain ASCII geometry data - no executable code, safe to open. Real-world concerns are non-malicious: large assemblies load slowly, and a STEP export reveals exact geometry, so a recipient can measure everything (watch what you share). As always, confirm a 'part.p21' isn't an executable in disguise. If the file is actually a PAR recovery volume, it's binary parity data, also harmless on its own.
Format details
in a nutshell- PAR / Parchive recovery volume - Parity-archive recovery files are numbered .p01, .p02 … so .p21 can be the 21st PAR recovery volume (QuickPar/MultiPar); these are binary parity data, unrelated to STEP.
- Numbered split-archive / volume part - Some tools name sequential split parts .pNN, so a stray .p21 may be one piece of a multi-volume archive set.
Programs that open P21 files
Windows4 apps
FreeCAD
Open-source File -> Open the .p21 (rename to .step if the dialog filters it out) to view, measure, and edit the B-rep geometry. Free and powerful. 
CAD Assistant (Open Cascade)
Free Free offline viewer/converter; open the .p21 to view, measure, and export (e.g. to STL/glTF).
Autodesk Fusion 360
Freemium Insert/Open to import the .p21 for full editing. Free for personal/hobby use. 
macOS2 apps
Linux1 app
Autodesk Viewer
Free Upload the file in a browser to view, rotate, and measure - no install (rename to .step if needed). Technical details
deep spec| Encoding | Plain ASCII text (ISO 10303-21 clear-text encoding); no binary sections |
| Format signature | First line is always the literal text `ISO-10303-21;` - no binary magic bytes |
| File terminator | Last line is always `END-ISO-10303-21;` |
| File structure | `HEADER` section (FILE_DESCRIPTION, FILE_NAME, FILE_SCHEMA) followed by one or more `DATA` sections of numbered entity records, then the end marker |
| Entity referencing | Each geometry entity carries a unique integer reference (`#1`, `#2`, …) that later entities cite by number, forming a directed graph of relationships |
| Geometry representation | Exact boundary-representation (B-rep) solid geometry - analytic surfaces, edges, and vertices; not a polygon or triangle mesh |
| Application protocol | Declared in `FILE_SCHEMA`; common values: `AP203` (design), `AP214` (automotive), `AP242` (model-based 3D engineering, current recommended) |
| MIME type | `model/step` (primary IANA type); also accepted: `application/step`, `application/p21` |
| Compression | None in a raw `.p21` file; a ZIP-compressed variant is distributed as `.stpz` |
| Typical file size | Tens of KB for simple parts; hundreds of MB for large assemblies |
| Relationship to .STEP / .STP | Mechanically identical format and parsers; ISO guidance reserves `.stp`/`.step` for AP-specific files and suggests `.p21` for general Part 21 exchange |
| Supported data scope | 3D solid geometry, assembly hierarchy, product metadata, tolerances; AP242 adds PMI annotations and kinematic joints |
| Governing technical committee | ISO/TC 184/SC 4 (Industrial data) |
| Cross-platform support | Format is OS-independent plain text; supported on Windows, macOS, Linux, and via web-based viewers |
| Released | 1994 (ISO 10303-21); .p21 designated for general Part 21 exchange files |
| Latest version | ISO 10303-21:2016 (3rd edition); application protocols up to AP242 (2014, ed.2 2020) |
| Open standard | Yes · royalty-free |
| Specification | www.iso.org |
P21 conversions
Community Q&A
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