What Is an IGS File? CAD Geometry on Its Way to Someone Else
IGS carries a model between CAD programs. Jump to your question below, or work through it in order.
IGS carries a model between CAD programs. Jump to your question below, or work through it in order.
An IGS file carries CAD geometry from one program to the next. It holds Initial Graphics Exchange Specification data, a neutral format published in 1980 so that rival CAD systems could trade models.
It is plain ASCII text, so you can open it and read the header. It carries curves, surfaces and sometimes solids. Whether what arrives is a surface or solid model decides most of what happens next.
It often sends surfaces rather than a solid, which is why imported models arrive with holes.
IGS stands for nothing on its own. It is the three-letter form of IGES, the Initial Graphics Exchange Specification. The short name survives from the days when DOS allowed three characters. The IGS file extension and the longer .iges spelling name one format, and how to open the IGES file is the same question either way.
IGES carries geometric elements in 2D, 3D or both. It exists because computer-aided design software could only read its own files. It was built for one job: transferring a part between two CAD systems. Two CAD applications could hold the same part and neither could open the other. Nothing moved between CAD programs.
In September 1979 an Air Force ICAM Industry Days meeting gave a General Electric engineer his chance. He challenged the CAD vendors to fix it. ComputerVision answered that vendors would open their database structures if Boeing and General Electric handed over their translators.
The National Bureau of Standards created the IGES Organization in spring 1980, run by Roger Nagel from NBS, Walt Braithwaite from Boeing and Philip Kennicott from General Electric. NIST's own account records what starting it cost: an Air Force, Navy and NASA backer, willing to fund $25,000 for such an effort.
Seventy-five thousand dollars and three engineers produced a format all CAD programs on earth still export, forty-six years later. Version 1.0 arrived in March 1980 and became an ANSI standard a year after that.
.igs and .iges, same formatmodel/iges, registered with IANAA solid model is faces plus a record of which face meets which. IGES exporters often send the faces and drop the record, so what arrives is a bag of separate surfaces that happens to be part-shaped.
Think about a cardboard box. Six flat panels, plus tape holding panel one to panel two. Solid modeling is based on just that idea. Remove the tape and you still own six panels. You no longer own a box, and nothing you put inside stays there.
CAD software calls that tape topology. Solids are trimmed surfaces plus the record of which edge belongs to which two faces. IGES has entities for all of it. Face is 510, Shell is 514, Edge is 504, Vertex is 502.
Almost nothing writes them. Exporters reach for entity 144, the trimmed surface, then stop. Your program gets the patches with no tape, and works out which edge meets which by measuring how close they sit. That guesswork is the connectivity, rebuilt on arrival.
| What | How it travels |
|---|---|
| Curves and wireframe | Faithfully. Lines, arcs, conics and splines all have entities |
| NURBS surfaces | Faithfully. Entity 128, and this is what IGES is good at |
| Trimmed surfaces | Faithfully. Entity 144, the workhorse of all exports |
| Solids | Rarely. Entity 186 exists and almost no one writes it |
| Which face touches which | Often lost. The cause of all gaps in an IGS import |
| Assemblies | Badly. Multi-level structure normally arrives flat |
| Colors and layers | Sometimes, and rarely mapped the way you expect |
| Tolerances and GD&T | Never. Only flat annotations and leader lines |
| Feature history | Never. No format in this class carries it |
Shapes. Handing a supplier the exact curved surface of a part when their software cannot read yours.
Everything else. Assemblies, tolerances, materials, history and all you plan to keep for decades.
This is why the same file behaves differently in two programs. Rebuilding a solid from loose surfaces is guesswork with a tolerance attached, and no two CAD kernels land on the same answer. The STP file format avoids all of this by sending the topology along with the faces.
Nothing on a normal computer opens CAD files. Windows and macOS have never shipped an IGES viewer. Install FreeCAD or drop the file into a browser viewer. Damage is rare.
Someone emails you an IGS file, you double-click it and nothing happens. The instinct is that it arrived broken. It rarely did. CAD files need CAD software, and no operating system ships any.
The older version of this page listed four common errors instead. They are worth walking through, because those same four sentences sit on file-help pages all over the web.
| What you see | What it often means | Start with |
|---|---|---|
| Double-click does nothing | No CAD software on the machine | FreeCAD, which is free |
| It opens as walls of numbers | Working just as intended | Nothing. IGS files are text. See section 7 |
| Model appears with holes | Surfaces arrived without their topology | Heal it. See section 6 |
| Model is a shell, not a part | Same cause. It never closed into a solid | Knit the surfaces, then check |
| Assembly arrives as loose parts | Subfigure entities read differently here | Ask for STEP instead |
| Fillets look faceted | Fixed precision written into the file | Ask for a re-export at finer tolerance |
| Nothing visible after import | Units mismatch, so the IGS model lands tiny or huge | Zoom to fit, then check the Global section |
| Viewer opens then stalls | An ASCII IGS assembly can pass 500 MB | A viewer built for large files |
Check the IGS file before you blame it. Open it in Notepad. Its first line should end with S followed by a number, on all IGS files ever written. A few lines down, Global names which program wrote it, what units it uses and what tolerance it claims. Thirty seconds there saves an afternoon of guessing.
FreeCAD, free on Windows, Mac and Linux, is the answer for most people. To look without installing a thing, drop the file into a browser viewer. To check what the file claims, open it in Notepad.
Programs that open IGS files split by what you need. Big CAD systems read them. Autodesk Inventor and SolidWorks both open one, and so do small free viewers. CADSoftTools and a handful of others build nothing but viewers for IGS files.
Looking at the shape inside an IGS file is one job. Measuring it, healing it and building on it is a second job. Most of the big directories mix the two into one long list of paid trials.
| Software | Runs on | Why choose it |
|---|---|---|
| FreeCAD | Windows, Mac, Linux | The answer. Real CAD software that imports IGS, heals surfaces and exports STEP |
| ShareCAD.org | Any browser | For one quick look. Upload and view, nothing installed |
| CAD Exchanger Cloud | Any browser | Viewing plus conversion, with a free tier |
| Any text editor | Everywhere | Reads the Global section, which names the exporter, the units and the tolerance |
| eDrawings Viewer | Windows, Mac | From the SolidWorks family, and built for reviewing rather than editing |
| ideaMK IGS Viewer | Windows | Small, quick, view and print only |
You may need to be careful here. Do not upload private files to a browser viewer. For the sample part that hardly matters. For your customer's tooling or an unreleased product it matters a great deal, and FreeCAD installs in two minutes.
| Software | On import |
|---|---|
| SolidWorks | Reads trimmed surfaces and B-splines, with healing you can configure |
| Siemens NX | Strong IGES 5.3 support, with automatic healing offered on import |
| Autodesk Inventor | Attempts to rebuild a solid on its own |
| PTC Creo | Four assembly export modes: flat, one level, all levels, all parts |
| Solid Edge | Limited to bounded surfaces. Check the stitching yourself |
| CATIA V5 | Reads it, and loses solid topology often enough to expect it |
| Rhino | The best NURBS importer here. High-curvature surfaces survive |
| AutoCAD | Mostly 2D and wireframe. Weak on surfaces |
| ABViewer, SpinFire, 3DViewStation | Viewers with measurement and conversion, built for large assemblies |
One file, once? ShareCAD.org shows you the shape in a browser. If you need to work on it, install FreeCAD and skip the trials.
The big lists have rotted. One of them still leads with Autodesk AutoCAD with IGES Import plug-in, and AutoCAD is the weakest tool on the page for this job. Another leads its full answer with Ignition Spreadsheet, then admits it has not analyzed what those files contain.
The older version of this page named Canvas X16, TurboCAD Deluxe V9 and SpinFire Professional. It also misspelled Nemetschek Vectorworks. Its steps were select the file, double-click, choose Open with. No program was named in the steps at all.
Because the exporter sent faces without recording which face meets which. Your CAD program measures the gaps between loose edges and joins the ones close enough. Anything wider than the tolerance stays open.
Your imported IGS model looks right until you try to use it. Shelling fails. Adding a fillet fails. The mass properties report no volume. Somewhere in that shape sits an edge belonging to one surface and not to the one beside it.
The sending program held a watertight solid. Exporting to IGS broke it into trimmed surfaces, entity 144, one per face. Topology entities that would have said face A meets face B along this edge were available and were not written.
Your program now has to rebuild that. It compares the edges, and joins two surfaces whose edges sit closer together than the stitching tolerance. Most do. A few do not, and those are the free edges that leave your model open. A shape with free edges is not watertight, so it is not a solid and it has no volume.
| Cause | What you see |
|---|---|
| Missing topology | Fragmented surfaces that will not knit into a solid at all |
| Tolerance mismatch | Most faces join, a handful leave naked edges |
| Sloppy source model | Disjoint faces that were already apart before the export |
| Micro-edges | Slivers shorter than tolerance, leaving micro-gaps that block the knit |
| Duplicate surfaces | Two faces in the same place, one of them untrimmed |
| Flipped normals | Model closes, then machining or simulation behaves oddly |
| Fixed precision | Fillets and blends look faceted or drift off their neighbors |
| Unit mismatch | Everything imports at the wrong scale, so nothing lines up |
McNeel puts this plainly in its own notes for Rhino users: most applications that produce IGES files do not match the unit and tolerance settings without user intervention. The number written into your file and the number your kernel works to are two different numbers, and nothing reconciles them for you.
An open solid is not a model you should machine from. An unclosed shape has no volume, so mass, center of gravity and material cost are all wrong. Toolpath software will refuse it or produce something dangerous.
Fix the geometry before the next step touches it. Section 6 is the procedure.
Stitch, check, then close. Run your program's heal or knit command, look for naked edges, widen the tolerance a little if a few remain. If the file is badly broken, ask the sender for STEP.
Every serious CAD package ships IGS healing and calls it something different. SolidWorks knits. Rhino joins and reports naked edges. Inventor rebuilds solids on its own. NX offers healing on import. FreeCAD has a shape-fixing workbench. Underneath, the sequence is identical.
If naked edges survive all that, stop working the IGS. Patch the holes with new surfaces only when you must, because a hand-patched face is your geometry, not the designer's. Asking for the model again costs one email.
STEP AP242. It carries the topology, so nothing has to be rebuilt. Almost all CAD programs can write it.
IGS 5.3, solids on. If IGS is the only option, ask for the newest flavour with solid entities enabled.
Only need the shape, not the engineering? An STL mesh file turns all of it into triangles and never has this problem, because it was never a solid to begin with.
Aerospace and automotive suppliers do not leave this to chance. The SASIG criteria in Europe and JAMA in Japan set quality rules for just this: smooth joins between surfaces, controlled stitching tolerance and no degenerate faces. If you exchange IGS files regularly, writing the tolerance into the purchase order is cheaper than healing all files that arrive.
Five sections. Every line runs 80 characters, with the section letter sitting in column 73. Open the IGS file in Notepad and the layout is obvious within seconds.
The layout of an IGS file gives away when the format was designed. Lines run 80 characters because that is how many fit on a punched card. The section letter sits in column 73 because columns 73 to 80 were the part of the card reserved for sequence numbers.
| Letter | Section | What it holds |
|---|---|---|
S | Start | A human-readable note. Often empty, sometimes the only clue about the part |
G | Global | Units, precision, scale, author, and which program wrote the file |
D | Directory Entry | Two lines per entity. Its entries give the type number, where the data sits, color, level and form |
P | Parameter Data | The actual numbers. Almost the full file |
T | Terminate | One line counting how many lines the other sections used |
A sixth, optional Flag section can sit before Start in binary and compressed variants, which almost nothing writes. Everything in the file after that follows the same five-letter order. Two of the four biggest guides get the structure wrong. One says the file has three parts. Another calls the D section Data Entry rather than Directory Entry.
Global is the useful one, because it carries the information you need before opening the IGS anywhere else. Its contents list the delimiters, file name, sending program, units, precision and IGES version. The first field is the comma itself, and every field after it runs in order, closed by a semicolon. Your CAD program reads this section before anything else, because an import cannot start without that information.
That single line includes all you need before you import. Reading it takes a few seconds and settles most of the guesswork. Text inside uses Hollerith notation, which means a number, then H, then that many characters. 4HSLOT spells SLOT.
Read that as a comma delimiter, a semicolon terminator, a file named SLOT, its full path, then the software that produced it. That is the full Start and Global section. Further along come the units, the model space scale and the smallest gap it can tell apart, which is the tolerance that decides whether your import knits.
| Type | Entity | Why it matters |
|---|---|---|
| 100 | Circular arc | The oldest geometry in the format |
| 110 | Line | Wireframe and drawing work |
| 116 | Point | The simplest entity in the format |
| 126 | Rational B-spline curve | Free-form edges, and it can represent a circuit trace as easily as a wing |
| 128 | Rational B-spline surface | NURBS. Where IGES is still strong |
| 144 | Trimmed surface | The one all exporters write |
| 186 | Manifold solid B-Rep object | A real solid. Almost nothing writes it |
| 502, 504, 510, 514 | Vertex, edge, face, shell | The missing topology. See section 2 |
| 212, 214 | General note, leader | Flat annotation. No real tolerancing |
| 308, 408 | Subfigure definition and instance | Assemblies, read differently everywhere |
| 314 | Color definition | Travels sometimes, maps rarely |
Entity 128 arrived in IGES 3.0, in 1986. An IGES file written before that carries no NURBS at all, so an old archive file may be wireframe only. The version is in the Global section, which is one more reason to open the file and look before you decide it is broken.
STEP, in almost every case. It carries real solids, assembly structure and tolerancing. It is also still being developed. Ask for STEP first and accept IGES when the sender cannot produce it.
STEP is ISO 10303, and it did to IGES what IGES did to nothing at all. Its current application protocol, AP242, carries all IGES carries plus the parts IGES drops. Both are neutral formats. Only one is still maintained. There is no category where IGS wins on merit.
| IGS | STEP | |
|---|---|---|
| Arrived | 1980 | 1994 |
| Last updated | 1996 | Still being developed |
| Solids | Rarely | Yes, with topology |
| Surfaces | Yes, true to shape | Yes |
| Assemblies | Flat and hit or miss | Proper hierarchy |
| Tolerances and GD&T | No | Yes. AP242 carries semantic PMI |
| Healing needed on import | Often | Rarely |
| File size | Large, fixed 80-column ASCII | Smaller |
| Standards body | Withdrawn | Active ISO standard |
Three cases, and defense and aerospace archives account for most of them.
Suppliers on software old enough that STEP export is missing or broken. The pure surface handover, where NURBS geometry is all anyone needs and IGES carries it cleanly. And the archive full of files written before STEP existed, which cannot be re-exported because nothing still opens the native model.
Outside those, asking for STEP costs one email and saves all the healing. Our STP file format guide covers what arrives when you do.
Anything with solids. Parts going to manufacturing, assemblies, work with tolerances on it, files being archived.
Only when asked. Legacy software at the other end, or pure surface work where nothing else survives anyway.
Working across CAD formats? the STEP format carries solids, STL meshes carry triangles for printing and plotter PLT files carry drawings on their way to paper.
IGES 5.3 shipped in 1996 and nothing has been added since. The standard itself was formally withdrawn on 23 July 2013. Every major CAD program still writes it.
Formats often die when their software dies. IGS did the reverse. Its committee wound down, its standard stopped and its standards body struck it off. No one using it noticed, because all CAD vendors kept that exporter in the menu.
| Year | Version | What arrived |
|---|---|---|
| 1980 | 1.0 | Points, lines, arcs and basic surfaces. ANSI Y14.26M the following year |
| 1983 | 2.0 | Parametric splines and surfaces of revolution |
| 1986 | 3.0 | NURBS surfaces, entity 128 |
| 1988 | 4.0 | Solid entities: 186, 514, 510, 504, 502. Optional binary form |
| 1991 | 5.1 | Solid support firmed up, colors and levels improved |
| 1993 | 5.2 | Entities settled and fixed |
| 1996 | 5.3 | The last version. Nothing has been published since |
| 1998 | 6.0 draft | Baselined in January and never published |
| 2013 | None | The standard is withdrawn on 23 July |
Look at what that means for your IGS file. IGES gained the entities for describing a real solid in 1988, and thirty-eight years later most exporters still do not write them. The feature arrived and the tools never followed. The committee that would have pushed them closed.
Because removing IGS would break someone. A vendor who drops IGS export gets support calls from customers whose supplier runs 1998 software. Keeping it costs a menu entry and some maintenance. One technical guide sums the situation up: in 2026, IGES persists in digital workflows through historical inertia.
Two directories on the first page of results get the history wrong in opposite directions. One credits the Air Force ICAM programme as the developer, when ICAM convened the meeting and helped pay for the work while the National Bureau of Standards published the standard. Another calls IGES simply obsolete, on a page listing eleven current programs that read it.
Send STEP unless someone asked for IGS. Keep the IGS files you were given, because they may be the only surviving copy of geometry whose native model nothing can still open.
Sending and keeping pull two ways. No new IGS file should leave the building, and almost no old IGS file should be deleted.
An old IGS file is often the last readable version of a part, and IGS outlives the software that made it. The native model needs software that no longer runs. The drawings sit on paper in a drawer and the plot file is a picture. The IGS still opens in FreeCAD today.
If those files matter, convert them now and keep both. Storage costs nothing next to remeasuring a part no one has the drawings for.
Import the IGS, heal the surfaces into a closed solid, then export as STEP. The order matters. Converting a broken IGS file produces a broken STEP file, and now two formats carry the same holes.
Only need to look at the part? Export a PDF file or an STL 3D file instead. Neither can be edited back into engineering, which is sometimes the point.
Published, registered and finished. IGES has an official media type at IANA and a published standard, and that standard has not moved since 1996.
Nothing here helps you open an IGS model. It is on record because IGS is one of the best documented formats covered on this site. The paperwork also tells the story better than any description of the format does.
| Item | Value |
|---|---|
| Media type | model/iges, registered with IANA |
| Registered | 10 January 1997, by Curtis Parks of NIST |
| Superseded | application/iges, registered 14 April 1995 |
| Encoding note, verbatim | IGES is a clear text ASCII encoded format. Compression is recommended |
| Published standard | ANS/US PRO/IPO-100, U.S. Product Data Association |
| Standard designation | ANSI/US PRO/IPO 100:1996, Initial Graphics Exchange Specification IGES 5.3 |
| Earlier designation | ANSI/ASME Y14.26M, from 1981 |
| Status | Withdrawn, 23 July 2013 |
| Successor | ISO 10303, known as STEP |
That media type is worth a moment. Most formats on this site have no registered media type at all and their software invents one. IGS has two in the record, and the current one sits in a model branch RFC 2077 created specifically for 3D data.
| Columns | What sits there |
|---|---|
| 1 to 72 | The data, whatever the section is |
| 73 | The section letter: S, G, D, P or T |
| 74 to 80 | Columns 74 to 80 hold a sequence number, right-justified inside the section |
Eighty columns, with the last eight reserved for numbering, is the layout of a punched card. IGS has carried it since 1980 and carries it still. That is why an IGS file of a complex assembly can run past 500 MB while the same model as STEP is a fraction of that.
Two directories on the first page of results describe the structure incorrectly. One calls Directory Entry the Data Entry section throughout, including in a short form it defines.
The older version of this page said IGES divides into three sub-parts and named one of them Entities. Its explanation was that entities are supported for import into ICE, a sentence lifted from another program's help file.
Two moves settle most of what is left. Try them in order.
Install FreeCAD. Free on all systems, and it reads IGS properly. See section 4.
Heal, then check. Knit the surfaces and hunt naked edges. Section 6 is the sequence.
Comparing CAD formats? STP files carry solids and should be the first thing you ask for, while STL files turn all of it into triangles for 3D printing.
Holding a drawing rather than a model? the PLT plotter format and HPGL files are plotter output, and PDF files are where most of it ends up.
Working in a drawing program instead? CDR files come from CorelDRAW and AI files come from Adobe Illustrator, and both handle curves rather than parts.
Need a picture of the part? TGA files and PCX files store pixels, which is the reverse of what an IGS file does.
Stuck on your computer rather than your file? Our Windows fixes and macOS guides cover the system side.
New to all of this? What a file format is covers the ground under it, and our file format guides list the rest.
Found something here that does not match your own file? Send us the Global section from it and we will check it against the standard.