What is the IGS file format?
An .igs file is a graphics file encoded in IGES format, used in CAD software environments to store 2D/3D vector image data. The file can contain various data for a project, such as wireframe models, surface or solid object representations, and circuit designs. .igs files are saved in plain ASCII text using a strict fixed-width, 80-column card layout - each line carries a single section-type letter (S, G, D, P, or T) and a right-justified sequence number in columns 74-80. Because the format is pure ASCII with no binary header, there is no traditional magic-byte signature; files are identified by this distinctive column structure. The extension .igs is the classic DOS three-letter spelling of .iges - both extensions refer to the identical format.
Short history of the .igs file format
The .igs format closely follows the Initial Graphics Exchange Specification (IGES), first published in 1980 by the US National Bureau of Standards (now NIST) and later standardized as ANS US PRO/IPO-100. It quickly became the standard format for storing and exchanging 3D model data across different CAD platforms, greatly facilitating collaboration among users of different software tools. In recent years it has been largely superseded by the .stp format, which offers richer solid-model support and remains widely used today. The IGES specification was last updated in 1996 (version 5.3). While newer alternatives now dominate, .igs files remain common in legacy workflows and archives, and are still broadly supported by major CAD applications. Other related neutral exchange formats include .stl for mesh geometry and .dxf for 2D/3D drawing interchange.
Security & safety
RISK: LOWAn IGS 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 to load, an IGES export may inadvertently include proprietary design detail you didn't mean to share (it's exact geometry the recipient can measure), and surface-only IGES files often arrive non-watertight and need healing before machining/printing. As always, only the container is harmless - be cautious of a file pretending to be 'part.igs' that is actually an executable; check the true extension.
Format details
in a nutshellPrograms that open IGS files
Windows5 apps
FreeCAD
Open-source File > Open the .igs to view, measure, and edit the geometry. Free; can heal surfaces and re-export. 
ABViewer
Paid Commercial CAD viewer/converter; opens IGS and exports to DXF/DWG/STL/PDF. Has a free trial. 
CAD Assistant (Open Cascade)
Free Free offline viewer/converter; open the IGS to view, measure, and export (e.g. to STEP/STL/glTF).
AutoCAD / SolidWorks / CATIA
Paid Professional CAD: File > Open / Import the IGS as surfaces/solid for full editing. Heavy and expensive - use free viewers if you only need to look. 
macOS2 apps
Linux2 apps
Technical details
deep spec| File structure | Fixed-width 80-column ASCII card layout; column 73 holds the section-type letter, columns 74-80 hold a right-justified sequence number |
| Text encoding | Plain 7-bit ASCII; no binary data, no BOM, and no binary magic-byte signature |
| Section types | Five named sections: Start (`S`), Global (`G`), Directory Entry (`D`), Parameter Data (`P`), Terminate (`T`) - each line carries exactly one section identifier |
| MIME type | `model/iges` |
| File identification | Identified by the 80-column card structure and section-letter convention, not by a binary magic number |
| Geometry types | 2D drawings, 3D wireframe, NURBS and ruled surfaces, trimmed surfaces, B-rep solids, finite-element models, and circuit schematics |
| Entity type count | Over 100 defined entity types covering curves, surfaces, annotation, and structure; type numbers range from 100 to 9999 |
| Parameter delimiters | Delimiter characters declared in the Global section; defaults are comma (`,`) for parameter separation and semicolon (`;`) for record termination |
| Measurement units | Unit system declared in the Global section; supported systems include inches, millimeters, feet, meters, and several others |
| Color support | 8 predefined color indices (1-8); custom RGB colors via Color Definition Entity (Type 314) |
| Layer support | Up to 9999 levels (layers); named layers defined via Level Function Entity (Type 406, Form 1) |
| Coordinate system | Right-handed Cartesian coordinate system; Transformation Matrix Entity (Type 124) handles placement, scaling, and rotation |
| Alternate extension | `.iges` - same format; `.igs` is the legacy three-character DOS spelling |
| Interoperability | Vendor-neutral; import/export supported natively by AutoCAD, SolidWorks, CATIA, Siemens NX, PTC Creo, and FreeCAD |
| Known limitations | No native support for assembly hierarchy or parametric feature trees; trim precision can degrade between translators |
| Typical file size | Larger than binary CAD formats due to verbose ASCII card structure; moderately complex models typically reach several megabytes |
| Released | 1980 (IGES 1.0); last version IGES 5.3, 1996 |
| Open standard | Yes · royalty-free |
| Specification | www.loc.gov |
IGS conversions
Community Q&A
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