What is the STEP file format?
A .step file contains 3D CAD object data saved in plain text format that conforms to the ISO 10303-21 standard, also known as the Standard for the Exchange of Product Data - STEP. The format was developed as a standardized data-sharing format for CAD and CAM applications and platforms, enabling geometry exchange between software from different vendors without loss of precision.
Every .step file begins with the literal line ISO-10303-21; and ends with END-ISO-10303-21;. The file is divided into sections:
- HEADER - file metadata organized into three required groups (
FILE_DESCRIPTION,FILE_NAME,FILE_SCHEMA) and up to three optional groups (FILE_POPULATION,SECTION_LANGUAGE,SECTION_CONTEXT). - DATA - application data stored in a format that follows the EXPRESS data-modeling language; data must follow rules for describing object instances and data mapping.
The STEP standard was published in 1994 by ISO. The most widely used application protocols are AP203 (geometry and topology of mechanical parts, standardized 2003) and AP242 (multi-discipline CAD, introduced 2014). The latest edition, Edition 3, supports references to external resources, data sharing using compressed archives based on the ZIP algorithm, UTF-8 encoding, and use of digital signatures. Because its syntax is ASCII text and largely human-readable, .step files can be opened and inspected in any text editor. The format also uses the alternative extensions .stp and .p21.
Security & safety
RISK: LOWA STEP 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.step' that is actually an executable; check the true extension.
Format details
in a nutshellPrograms that open STEP files
Windows5 apps
FreeCAD
Open-source File > Open the .step to view, measure, and even edit the B-rep geometry. Free and powerful. 
eDrawings Viewer
Free Free Dassault viewer; open the STEP to rotate, section, and measure. Familiar to SolidWorks users. 
CAD Assistant (Open Cascade)
Free Free offline viewer/converter; open the STEP to view, measure, and export (e.g. to STL/glTF).
Autodesk Fusion 360
Freemium Insert > Insert Derive or Open to import the STEP for full editing. Free for personal/hobby use. 
macOS3 apps
Linux2 apps
Technical details
deep spec| File identification string | Begins with the literal line `ISO-10303-21;` at offset 0; ends with `END-ISO-10303-21;` - no binary magic number |
| MIME type | `model/step` |
| Text encoding | Plain ASCII in Editions 1-2; UTF-8 character set supported from Edition 3 onward |
| Application protocols | AP203 (mechanical solid geometry, Edition 2 published 2003) and AP242 (multi-discipline CAD/PDM, published 2014) are the most widely implemented |
| Data modeling language | EXPRESS (ISO 10303-11) - a formal object-oriented schema language that defines all entity types and constraints |
| Geometry representation | Boundary representation (B-rep) solid geometry; supports analytic curves, NURBS surfaces, and topological shells |
| File structure | Two mandatory sections: HEADER (file metadata) and DATA (entity instances, one per line) |
| Entity instance syntax | Each entity prefixed with a `#<integer>` ID (e.g. `#12=CARTESIAN_POINT(...);`); `$` denotes a null attribute, `*` denotes a derived/unset attribute |
| Header groups | Three required (`FILE_DESCRIPTION`, `FILE_NAME`, `FILE_SCHEMA`) and up to three optional (`FILE_POPULATION`, `SECTION_LANGUAGE`, `SECTION_CONTEXT`) |
| Compressed packaging | Edition 3 allows ZIP-based compression; files commonly use the `.stpZ` or `.stp.zip` convention to reduce file size |
| Digital signatures | Edition 3 introduced support for embedded digital signatures for authorship and data-integrity verification |
| External references | Edition 3 supports `#<id>` references to entities defined in separate external `.step` files |
| Alternative extensions | `.stp` (legacy DOS 8.3 truncation) and `.p21` (ISO Part 21 reference name) are byte-for-byte identical format |
| Coordinate system | Right-handed 3D Cartesian; length units (e.g. `MILLIMETRE`, `INCH`) declared inside the DATA section via context entities |
| Released | 1994 (ISO 10303-21); widely adopted via AP203 (2003) and AP242 (2014) |
| Open standard | Yes · royalty-free |
| Specification | www.iso.org |
STEP conversions
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