What is the STP file format?
An .stp file contains a 3D object project saved in STEP (Standard for the Exchange of Product Data) format. .stp and .step are the same format - .stp is simply the older three-letter (DOS/Windows 8.3) spelling of a STEP file. Both share the MIME type model/step.
.stp is an ASCII text file that conforms to the ISO 10303-21 physical file format specification. (ISO 10303-11, sometimes referred to as EXPRESS, defines the underlying schema language - a separate part of the standard.) The format is platform-independent. It was developed as a universal, nonproprietary standard to facilitate 3D project data exchange between applications. Development of STEP started in 1984 with the aim to standardize the data exchange process for PLM (Product Lifecycle Management) systems, with the first formal ISO publication in 1994.
Every .stp file begins with the literal line ISO-10303-21;, followed by HEADER and DATA sections, and closes with END-ISO-10303-21;. This text header is the primary identification marker - there are no binary magic bytes.
.stp files are also used for sharing and archiving data. The parts of the STEP standard related to CAD applications are designated 2xx. Main protocols for 3D modeling were redefined in 2014 and specified in AP 242, which combined and replaced the following documents:
- AP 201 - simple 2D drawing geometry,
- AP 202 - 2D/3D drawing with associations, but with no assembly hierarchy,
- AP 203 - 3D designs of project parts,
- AP 204 - mechanical design along with its representation,
- AP 214 - core data for automotive mechanical design.
.stp files must follow their specification if used for sharing data between different applications or platforms. Projects stored in neutral .stp format may contain fewer details than those saved in an application's native format. The most commonly exchanged .stp data consists of CAD model parts for CAM and CAE workflows. A related neutral-exchange alternative is the older .igs (IGES) format, while .p21 is another common extension alias for the same ISO 10303-21 container.
Security & safety
RISK: LOWAn STP file is plain ASCII geometry data - it contains no executable code and cannot run anything, so opening one is safe. The only real-world concerns are non-malicious: large assemblies can be slow/heavy to load, and a STEP export may inadvertently include proprietary design detail you didn't mean to share (it's exact geometry, so a recipient can measure everything). As always, only the container is harmless - be cautious of a file pretending to be 'part.stp' that is actually an executable; check the true extension.
Format details
in a nutshellPrograms that open STP files
Windows5 apps
FreeCAD
Open-source File > Open the .stp to view, measure, and even edit the B-rep geometry. Free and powerful. 
eDrawings Viewer
Free Free Dassault viewer; open the STP to rotate, section, and measure. Familiar to SolidWorks users. 
CAD Assistant (Open Cascade)
Free Free offline viewer/converter; open the STP to view, measure, and export (e.g. to STL/glTF).
Autodesk Fusion 360
Freemium Insert > Insert Derive or Open to import the STP for full editing. Free for personal/hobby use.
AutoCAD / Creo Parametric / CATIA
Paid Professional CAD; File > Open / Import the STP as a solid body for engineering work. Heavy and expensive - only if you already own it. 
macOS3 apps
Linux2 apps
Technical details
deep spec| Physical file standard | ISO 10303-21 (STEP Part 21 physical file format) |
| Schema language | EXPRESS (ISO 10303-11) defines the data model and entity types; a separate part of the STEP standard from the physical file encoding |
| File identification | Plain text; first line is always `ISO-10303-21;` and last line is `END-ISO-10303-21;`; no binary magic bytes |
| MIME type | model/step |
| Extension alias | `.stp` is the 8.3 DOS/Windows spelling of `.step`; the files are byte-for-byte identical; `.p21` is a third common alias |
| Geometry representation | Boundary representation (B-rep) solid geometry; also supports surface, wireframe, and faceted (tessellated) models |
| Current dominant application protocol | AP 242 (Managed Model Based 3D Engineering, 2014) - supersedes AP 203 and AP 214 for most industrial use |
| Assembly support | Multi-level product assemblies via a part-instance / part-definition pattern encoded in the DATA section |
| PMI and GD&T | AP 242 embeds Product Manufacturing Information including GD&T annotations and 3D semantic notes alongside geometry |
| Character encoding | 7-bit ASCII body; non-ASCII characters represented as Unicode escape sequences per ISO 10303-21 §4.4 |
| Units declaration | Physical units declared in the HEADER section; SI units (millimetres, radians) most common; no mandatory default |
| Coordinate system | Right-handed Cartesian (X, Y, Z); origin and orientation defined per entity placement in the DATA section |
| Color and appearance | Color and transparency data embedded via styled-item and surface-style entities; not all importers render it |
| File size characteristic | Typically larger than equivalent native CAD formats owing to verbose plain-text entity-by-entity encoding |
| Industry mandate | Required neutral exchange format in aerospace (AS9100), automotive, and defense supply chains; widely mandated by OEMs |
| Released | 1994 (ISO 10303-21); widely adopted via AP203 (2003) and AP242 (2014) |
| Open standard | Yes · royalty-free |
| Specification | www.iso.org |
STP conversions
Community Q&A
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