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What Is an IGES File? .IGS and .IGES Files Explained
An IGES file (.igs or .iges) is CAD geometry saved in the Initial Graphics Exchange Specification, a neutral format first published in 1980. It stores exact curves and surfaces that most CAD and CAM programs can read, but often not a closed solid, which is why STEP has largely replaced it.
What is an IGES file used for?
An IGES file moves geometry between CAD programs that cannot read each other’s native files. Today you mostly receive one from an older system: a legacy archive, a CAM package, a surface-modelling tool, or a supplier whose export settings have not changed in decades.
IGES was the first widely adopted answer to a problem every engineering company had in the late 1970s: drawings and models made on one vendor’s system were locked inside it. Aircraft and defence programmes pushed the standard hard, and for years US defence contracts asked for IGES deliveries. The last version, 5.3, was published in 1996, after which the work moved to STEP.
If someone sent you an IGES today
It is usually because their system offered it, not because it is the best choice. Ask for a STEP file if you can; it costs the sender one menu click.
What is inside an IGES file?
An IGES file is plain text in fixed 80-character lines, a layout inherited from punched cards. Each line ends with a section letter and a line number. Five sections follow each other: Start, Global, Directory Entry, Parameter Data and Terminate.
A shortened IGES file looks like this:
bracket, exported as IGES 5.3 S 1
1H,,1H;,7Hbracket,11Hbracket.igs,8HCAD 2026,3H1.0,32,38,6,308,15, G 1
7Hbracket,1.,2,2HMM,1,0.01,15H20261103.090000,0.001,500.,,,11,0; G 2
128 1 0 0 0 0 0 000000000D 1
128 0 0 12 0 D 2
128,3,3,3,3,0,0,1,0,0,0.,0.,0.,0.,1.,1.,1.,1.,0.,0.,0.,0., 1P 1
...
S 1G 2D 24P 156 T 1- Start (S). A free-text note from the program that wrote the file.
- Global (G). The file name, the sending system, the precision and the units. The
2,2HMMabove means millimetres; a1there would mean inches. - Directory Entry (D). Two lines per entity, giving its type number and where its data starts. Type 128 is a B-spline surface.
- Parameter Data (P). The numbers that define each entity: control points, knots, weights.
- Terminate (T). One line counting the lines in each section, so a reader can tell the file arrived whole.
The entities are numbered types: 110 is a line, 100 a circular arc, 126 a B-spline curve, 128 a B-spline surface and 144 a trimmed surface, which is how most 3D faces are stored.
What is the difference between .igs and .iges?
There is no difference. A .igs file and an .iges file are the same format; the three-letter version dates from systems that allowed only three characters after the dot. Any program that opens one opens the other, and renaming between them changes nothing inside.
Why is an IGES file often not a solid?
Because most IGES files store a part as a collection of separate trimmed surfaces, not as a closed volume. Nothing in the file says which edges meet, so small gaps between faces survive the translation. The part looks right on screen and still fails where a closed solid is needed.
IGES 5.x did add a true solid entity (type 186), but few programs write it, so in practice you get surfaces. That causes three common problems:
- Gaps. Faces that nearly meet leave slivers too small to see but large enough to stop CAM software computing a toolpath.
- No volume or mass. A shell of surfaces has no inside, so the receiving program cannot report weight or check interference.
- Missing faces. A surface whose trim curves the reader does not like may simply be dropped.
Most CAD programs have a sewing or knitting command that closes small gaps and turns the surfaces into a solid. It works well on clean files and poorly on old ones with large gaps.
How is an IGES file different from STEP?
Both are neutral formats that store exact geometry with declared units. The difference is that STEP records a solid with the topology saying how every face joins, while IGES usually records loose surfaces. That makes STEP the safer file for machining, quoting and editing.
| Compared on | IGES (.igs / .iges) | STEP (.step / .stp) |
|---|---|---|
| First published | 1980 | 1994 (ISO 10303) |
| Geometry | Exact curves and surfaces | Exact solids with topology |
| Closed solid | No: Often not | Yes: Yes, by design |
| Units | Yes: Declared in the file | Yes: Declared in the file |
| Still developed | No: No, last version 1996 | Yes: Yes |
| Machine shops | Accepted, reluctantly | Yes: Preferred |
The longer comparison, including STL, Parasolid and native CAD files, is in STEP vs IGES. What a STEP file holds line by line is in what is a STEP file.
Can an IGES file hold a 2D drawing?
Yes. IGES began as a drawing-exchange format, so it has entities for drawings, views, dimensions and notes. Older 2D CAD systems often exported their drawings as IGES. Dimensions and text rarely translate cleanly, though, so open the file and check it against a printed copy.
If what you have is the drawing of an old part rather than a 3D model, export or print the drawing as a PDF and upload it to Drawing to STEP from PartWork.ai. It builds the part from the drawing’s views and dimensions, checks the model against every view and returns a STEP file plus an STL in minutes, from $29.99.
How do I open an IGES file?
The quickest way is a browser viewer: drop the file into the free online IGES viewer and it renders with nothing to install. To edit or repair it, open it in a CAD program; free ones such as FreeCAD read IGES and can try to sew the surfaces into a solid.
A text editor also opens it, which is handy for checking the units in the Global section or the program that wrote the file in the Start section.
How do I convert an IGES file to STEP?
Open the IGES in a CAD program or an online converter and export STEP. The geometry carries over, but a converter can only write a closed solid if the surfaces already close. If they leave gaps, sew them first or the STEP will carry the same open surfaces.
You can do it in the browser with PartWork.ai Studio; the steps are in IGES to STEP converter. After converting, open the STEP in a viewer and check that it is one solid, not a stack of faces.
Frequently asked questions
What does IGES stand for?
Initial Graphics Exchange Specification. It was written so that CAD systems from different companies could pass drawings and 3D geometry to each other, at a time when every system had its own closed format.
Is IGES still used?
Yes, but mostly for older files. Legacy archives, older CAM systems, surface-modelling tools and long-standing suppliers still produce IGES. For new parts, STEP has replaced it, and most machine shops will ask for STEP if you can send it.
Is an IGES file 2D or 3D?
It can be either. Most IGES files you meet today hold 3D curves and surfaces, but the format can also carry a 2D drawing with views, dimensions and notes. Open it in a viewer to see which one you have.
Can I 3D print an IGES file?
Not directly. Slicers expect a mesh, so convert the IGES to STL or 3MF first. If the surfaces do not close into a solid, the mesh will have holes and the slicer may refuse it, so check that the part is watertight.
Why does my IGES file open with missing faces?
Usually because a surface failed to translate or its trim curves did not match the program reading it. The geometry is often still in the file. Try a different viewer, ask the sender to export again, or ask for a STEP file instead.
Have a drawing but no usable 3D file?
When an old IGES will not close into a solid, or there is only a drawing of the part, Drawing to STEP builds a checked 3D STEP file from the drawing. If you already have the file, look inside it in the free viewer first.