What Is an IGES File? A Guide to the IGES File Format

An IGES file is a neutral CAD file, saved as .igs or .iges, that moves 2D and 3D geometry between CAD, CAM and CAE programs. Nearly every CAD system can open the IGES file format, but the model often arrives as loose surfaces instead of a solid part.

Those loose surfaces usually come from the export setting, so that is where the fix starts: choose the solid (MSBO, type 186) option where both programs support it, check the units and send a 2D drawing with the file (because IGES carries no tolerances). If the part is new and machined, the STEP file format, which usually opens as a solid, gets a cleaner quote from custom manufacturing companies.

The reason this matters at the quoting stage is cost. A supplier cannot calculate a part's volume, and therefore its material cost, until an engineer has knitted the loose surfaces into a solid, so an IGES quote often takes longer than a STEP quote for the same part.

The rest of this guide covers how IGES stores geometry, why it imports as surfaces, which export setting to choose and how to open, fix, convert and send one.

What this guide covers:

• What an IGES file is and how the format is structured
• Why IGES files import as surfaces, not solids
• Which export setting to choose, and how to fix a file that will not form a solid
• How to open, convert and send an IGES file for custom parts production

 

TL;DR / Key Takeaways

• IGES stands for Initial Graphics Exchange Specification. .igs and .iges are the same format with 2 interchangeable extensions
• Check the units before sending. A unit mismatch makes a part arrive 25.4 times too large or too small
• The most common IGES export is trimmed surfaces (type 144), not solids (type 186). That is why IGES files so often import as loose surfaces that usually need knitting into a solid before quoting
• Most IGES problems start at the export setting. Choosing the solid (MSBO) option, where both the sending and receiving software support it, avoids most repair work
• For custom parts production, IGES is accepted but STEP is preferred. Either way, send a 2D drawing with the 3D file, because IGES cannot attach tolerances or GD&T to the 3D geometry

 

What Is an IGES File?

An IGES file is a neutral CAD file that moves 2D and 3D geometry between different CAD, CAM and CAE programs without needing the software that created it. To work across so many systems, the format describes points, curves, surfaces and, in later versions, solids as numbered "entities" written in plain text. That's why a model exported from CATIA as an IGES file can still open in SolidWorks, Mastercam, Rhino or FreeCAD.

That cross-software purpose is built into the name: IGES stands for Initial Graphics Exchange Specification. The format grew out of a U.S. Air Force manufacturing program, and its first version was published by the National Bureau of Standards (now NIST) in 1980. It became an American national standard in 1981, the first U.S. national standard for exchanging CAD data.

Is .IGS the Same as .IGES?

An IGS file and an IGES file are the same format with 2 different extensions. Software that opens one opens the other, and renaming .igs to .iges changes nothing inside the file. SolidWorks, for example, lists the format as IGES (*.igs, *.iges) and reads both.

The choice between IGS and IGES comes down to habit: SolidWorks saves IGES files as .igs, while some other programs use .iges.

IGES File Format Versions: From 1.0 to 5.3

Version	Year	What changed for users
1.0	1980	Wireframe geometry, basic surfaces and 2D drafting annotation
2.0-3.0	1983-1986	More curve, surface and annotation entities, including NURBS surfaces
4.0	1988	Early solid modeling entities
5.x	1990-1996	Boundary-representation (B-rep) solids, including entity type 186 (MSBO)

Version 5.3, published in 1996, was the last official release, and development then moved to STEP. Solid support arrived late, after most CAD vendors had already built their IGES exporters around surfaces, and many still default to surfaces today.

For most new 3D part handoffs, the STEP file format (ISO 10303) has replaced IGES, and STL, DXF and DWG each cover a narrower job when exporting CAD files for CNC machining.

How Is the IGES File Format Structured?

The IGES file format is plain text split into 5 sections, so you can open it in Notepad to check details such as the units, but it is not practical to edit by hand.

File Section	Letter in Column 73	What the Section Contains	What to Look For
Start	S	Free-text notes from the sender	Any notes on the part, such as a part number or revision
Global	G	File-wide settings: sending system, units, precision, author, date	Which software wrote the file, the units flag and the minimum resolution
Directory Entry	D	An index: 2 lines per entity, with type, form, color and layer	Which entity types the file actually uses
Parameter Data	P	The geometry itself: coordinates, knot vectors, control points	Rarely read directly
Terminate	T	A single line with the line count of each section	A missing T line usually means a truncated file

To check an IGES file’s units without CAD software, open it in a text editor and read the Global (G) section near the top of the file. One of its lines carries the unit string, a short code written as 2HMM (millimeters), 2HCM (centimeters) or 4HINCH (inches). If that string does not match the units the part was modeled in, a millimeter model read as inches arrives 25.4 times too large, and an inch model read as millimeters arrives 25.4 times too small.



A correct unit string can still be undone on the receiving side. When a Rhino user reported IGES files that arrived scaled down, Tim Hemmelman of McNeel (the company behind Rhino), pointed to the likely cause: importing into a template set to different units. He advised opening the file rather than importing it, and his reply, below, shows the unit string from 2 real files, one in centimeters (2HCM) and one in inches (4HINCH).

The file’s size is not a reliable quality signal either. IGES files of the same part can differ widely in size depending on how the exporter writes them: duplicate entities, extra attributes and the number of digits per coordinate all add bulk. A smaller file is not automatically worse, and a larger one is not automatically more accurate.

Why Does an IGES File Import as Surfaces Instead of a Solid?

An IGES file imports as surfaces instead of a solid because most exporters write each face as an independent trimmed surface (type 144) with no record of which edges it shares with its neighbors. The receiving software has to guess which faces belong together and stitch them within its own tolerance. If any gap exceeds that tolerance, the result remains an open set of surfaces.

Another IGES limitation makes this worse: the whole file uses a single accuracy setting, even for a large assembly that mixes very large and very small parts. Roman Lygin, founder of CAD Exchanger, a CAD conversion software company, calls that setting "essentially useless" for large multi-part assemblies.

For quoting, this is the IGES problem that matters most, because a factory has to close those gaps before it can price or machine the part, and it is the main reason STEP is preferred over IGES for solids.

Trimmed Surfaces (Type 144) vs Solids (Type 186)

The difference between a type 144 and a type 186 export is the difference between a set of panels and a sealed box:

IGES Export Type	How the Geometry Is Stored	Can Volume & Weight Be Calculated?	Best Used For	What You See When It Opens	Ready for Machining & Quoting?
Trimmed surfaces (type 144)	Independent faces, each trimmed by its own boundary curves	Not calculable until stitched	Surface modeling, mold and styling work	Surface bodies or a "quilt"	Not until knitted into a solid
MSBO solid (type 186)	Shells, faces, edges and vertices with shared topology	Calculable on import	Machining, inspection, mass properties	A single solid body	Yes, if it imports without errors

The catch with type 186: some older programs cannot read it, so senders often fall back to type 144 to be safe.

How to Import IGES into SolidWorks

To import IGES into SolidWorks, open the file through File > Open and set the import options before it loads:

1. Open the file: go to File > Open and set the file type to IGES (*.igs, *.iges).
2. Click Options before opening the file.
3. Keep "Surface/solid entities" selected so faces and solids both come in.
4. Tick "Try forming solid(s)" so SolidWorks attempts to build a solid on import.
5. Tick "Perform full entity check and repair errors" for files from unknown sources.

Note: SolidWorks's import options warn that the full check slows the import, but it catches faulty entities before they become modeling problems.

If the import still lands as surface bodies, run Import Diagnostics to repair faulty faces, then Knit Surface with "Create solid" checked. The repair table further down covers the other common failures.

How to Import IGES into AutoCAD

To import IGES into AutoCAD, use the IMPORT command in full AutoCAD for Windows:

1. Run IMPORT (or IGESIMPORT) from the command line.
2. Select the .igs or .iges file.
3. Check what came in: AutoCAD brings the geometry in as surfaces, solids or wireframe, depending on what the file contains.

Note: AutoCAD LT does not support IGES import or export, and AutoCAD for Mac does not import it either.

When the model arrives as surfaces, AutoCAD's SURFSCULPT command combines surfaces that enclose a watertight volume into a solid. If SURFSCULPT fails, there's a gap, and you'll need the repair steps covered later, or you'll need to re-export from the sender.

Which IGES Export Setting Should You Choose?

For a part going to a machine shop, choose the solid (MSBO, type 186) export setting if both the sending and receiving software support it, and fall back to trimmed surfaces (type 144) only when the receiver cannot read solids. Most IGES problems start at export, because CAD programs offer many IGES export variants and the default is often a surface export.

Export setting	What the receiver gets	Use it when
Manifold solid (MSBO, type 186)	A closed solid with shared edges	Sending a machined part to a factory or CAM system
Trimmed surface (type 144)	Independent faces needing stitching	The receiver's software cannot read type 186
Bounded surface (type 143)	Faces with more explicit edge mapping than 144	A specific receiving system asks for it
Wireframe / 3D curves only	Edges and curves, no faces	2D profile work or older CAM that machines from curves

Units and the target system matter at export too. Set the units yourself instead of relying on the template's default, so the part does not arrive at the wrong size.

Second, the target system: many CAD programs let you tailor the IGES export to the software that will open it. If you know which machining software your supplier uses and it appears in the list, choose it.

 

What Are IGES Files Used For?

IGES files are used to exchange CAD geometry between different software, mainly for legacy data, surface-heavy models and compatibility with systems that cannot read STEP.

For new solid parts, STEP has taken over. What IGES is used for today comes down to the 5 jobs below.

Where IGES Files Show Up Today

Where	Why IGES persists	What to watch
Legacy CAD archives and older supplier data	Decades of drawings and models were saved as IGES, and many suppliers still send it by habit	Re-export critical parts to STEP when they're revised
NURBS and surface-heavy models (wing skins, styling surfaces)	IGES handles freeform surfaces well, and surface workflows do not need a closed solid	Fine for surfacing; check closure before machining
Contract-required deliverables (aerospace, defense, automotive)	Older contracts and data standards named IGES; U.S. defense specification MIL-PRF-28000 sets the requirements for delivering product data in IGES	Follow the contract; add STEP AP242 where allowed
2D and wireframe exchange	Older CAM and tube-bending software reads IGES curves	Confirm the receiver's expected entity structure
Compatibility fallback	Some programs charge extra for STEP import, so IGES is the free option nearly everyone can open	Send it as a backup alongside STEP

In practice, loose surfaces and curves go as IGES, and closed solid parts go as STEP. Parasolid (.x_t) is a common alternative when both sides use software built on it, such as SolidWorks or Siemens NX.

Which file to send: IGES or STEP?

Your Situation	File Format to Send	Why This Format Works
New machined part, receiver's software unknown	STEP (AP214 or AP242)	Solid geometry that nearly every CAM system reads
Part with GD&T that should travel in the model	STEP AP242 plus a 2D drawing	AP242 can carry PMI; the drawing stays the safe reference
Freeform surfaces for a mold or styling partner	IGES (or both IGES and STEP)	Surface data without forcing a closed solid
Contract or customer specifies IGES	IGES, plus STEP if the contract allows	Meets the requirement while giving CAM a solid
Old supplier software cannot read STEP	IGES as solid (type 186), else type 144	Best compatibility the receiver can handle

 

How Do You Open an IGES File?

You can open an IGES file in 3 ways, depending on what you have installed: 3D CAD software, a free viewer or a plain text editor (which shows the units and header, not the model). Each option is covered below.

Free IGES Viewers (FreeCAD, eDrawings, Autodesk Viewer)

3 free tools cover most viewing needs:

• FreeCAD: free, open-source CAD for Windows, macOS and Linux that opens, edits and exports IGES
• eDrawings Viewer: free desktop viewer from Dassault Systèmes that opens IGES 5.1 to 5.3 files for viewing and measuring
• Autodesk Viewer: a free browser-based viewer whose supported file types include .iges and .igs, with nothing to install

Viewers are enough to check shape, measure a dimension or confirm the right revision before forwarding a file. If the file is confidential, use a desktop viewer: it keeps the file on your computer, while a browser viewer uploads it to the provider's servers.

Open an IGS File in SolidWorks, Inventor, Fusion or AutoCAD

All 4 open IGES directly, with small differences:

CAD Program	How to Open the File	What You Get After Opening
SolidWorks	File > Open, file type IGES	Solid or surface bodies, depending on import options
Autodesk Inventor	File > Open or Import, select .igs/.iges	A solid, or a set of joined surfaces if the file does not form a solid
Autodesk Fusion	Upload or Insert into a design	Solid or surface bodies in the browser tree
AutoCAD (full version, Windows)	Run IMPORT and select the .igs or .iges file	Solids, surfaces or wireframe; AutoCAD LT cannot open IGES

In all 4, an imported IGES file has no feature history. You can edit the geometry directly, but the sketches and dimensions that built the original part do not exist in the file.

 

How Do You Fix an IGES File That Does Not Form a Solid?

To fix an IGES file that does not form a solid, stitch (knit or sew) its surfaces into one closed shell within a small tolerance, patch any missing faces, then convert the shell to a solid. If that fails after a reasonable try, the faster fix is usually a new export from the sender.

What You See	What Usually Causes It	How to Fix It
Imports as many surface bodies	Exported as trimmed surfaces (type 144)	Knit/stitch surfaces into a solid
Stitch fails with open edges	Gaps wider than the stitch tolerance	Find the gap; raise tolerance slightly or patch the face
Missing faces	Bad trim curves in the source export	Rebuild the face with a fill or boundary surface
Overlapping or duplicate edges	Duplicate entities in the file	Delete duplicates, then stitch again
Part is 25.4x too big or too small	Unit mismatch	Check the G section units; re-import with correct units

When to Ask for STEP or Parasolid Instead

Ask the sender for STEP or Parasolid when a repair takes longer than a re-export would, or when the part has tight tolerances that a stitched model might no longer match. For solids, STEP beats IGES whenever there's a choice, and Parasolid does too when both sides run Parasolid-based software.

For CNC quoting, that request costs a single email. Skipping it means every factory quoting the part spends its own time repairing the file, and each may repair it a little differently, so you end up comparing quotes for slightly different parts.

When a repaired model does go out, a stitched surface can drift slightly from the original design, so the drawing becomes the reference. On Haizol, Drawing Collaboration lets a buyer and a factory pin notes to a specific feature on the drawing, so a questioned edge or radius is identified exactly instead of described in an email.

How Do You Convert IGES to STEP or STL?

To convert IGES to STEP or STL, open the IGES file in CAD software or a converter, fix it if needed, and export in the target format. Conversion does not fix bad geometry, though: an IGES file that imports as open surfaces becomes a STEP file full of open surfaces unless it's knitted into a solid first.

The same rule applies to every target format:

Convert IGES To	How to Convert It	What to Watch Out For
STEP	Import, knit into a solid, export STEP AP214 or AP242	Heal before exporting, or the STEP stays a surface model
STL	Import, close the model, export STL at a fine mesh tolerance	Open surfaces produce a leaky mesh that will not slice cleanly
DWG	Full AutoCAD: import, then save as DWG (3D); or create 2D views and export	DWG from IGES keeps geometry only, no feature history
DXF	Create 2D drawing views or a flat projection, then export DXF	DXF holds outlines only; state thickness and material separately
PDF	Export a 3D PDF from a CAD or conversion tool, or a 2D drawing PDF from the drawing views	A 3D PDF is for viewing; a 2D PDF is what carries dimensions

To convert IGES to DWG or DXF, which are used for 2D drawings and laser or waterjet cutting in sheet metal fabrication, the cleaner route is to create 2D drawing views of the model instead of converting the 3D file directly. To convert IGES to PDF, first decide whether the reader needs a rotatable 3D view or a dimensioned drawing; a factory quoting the part needs the dimensioned drawing.

To go the other way, convert from a native CAD file to IGES using Save As or Export, and choose the solid (type 186) option where available.

Can You Send an IGES File for Custom Parts Production?

You can send an IGES file for custom parts production, and most factories will quote from it, because nearly every machining (CAM) system reads IGES. A solid STEP file plus a 2D drawing still gets a cleaner first quote: STEP arrives as a solid more reliably, and the drawing carries what IGES cannot, such as tolerances, GD&T, tapped hole callouts and surface finish.

A factory can see a 10 mm hole in the model but cannot tell whether it is a clearance hole or a precision bearing fit (H7). Haizol's Quick RFQ accepts IGES alongside STEP, DWG and PDF files.

RFQ Checklist: Units, Material, Revision, Quantity

Before sending an IGES file for custom parts production, check these 4 items:

1. Units: confirm the G-section units match the intended size, and state units in the RFQ.
2. Material: give the full grade, such as 6061-T6 aluminum or 304 stainless, not just the family. Missing material and tolerance information is a common cause of inflated CNC machining quotes.
3. Revision: put the part number and revision letter in both the IGES file name and the drawing.
4. Quantity: state the prototype quantity and expected production volume.

If the geometry is confidential, set the NDA level on the RFQ as well. Haizol offers 3 options: no NDA, Haizol's standard digital NDA or your own NDA.

On Haizol, a Quick RFQ takes CAD, PDF and ZIP files up to 200 MB, and the RFQ package is checked for completeness before it is matched with verified CNC machining factories. 90% of RFQs receive quotes from 8+ verified factories within 24 hours, and buyers pay nothing to submit an RFQ or compare quotes.

Need quotes for a custom part? Upload your IGES, STEP, drawing and production requirements through Haizol's Quick RFQ.

IGES File FAQs

Is an IGES file a CAD file?

An IGES file is a CAD file, though a neutral exchange format rather than a native one. It holds geometry any CAD program can read, but not the feature history of the program that created it.

Is IGES obsolete?

IGES is no longer developed, since version 5.3 in 1996 was the last, but it is not obsolete in practice. It is still widely read and written, especially for surface models and legacy data. For new solid parts, STEP is the better default.

Can you 3D print an IGES file?

An IGES file cannot be 3D printed directly. A 3D printer's slicing software needs a mesh, so the IGES file has to be converted to STL or 3MF first, and the model must be closed for the mesh to print correctly.

Can you edit an IGES file?

An IGES file can be edited as geometry in any CAD program: faces can be moved, cut or filleted. The original sketches and parametric features cannot be recovered, because the IGES file never contained them.

Why is my IGES file so large?

IGES files are large because they are uncompressed text in fixed 80-character lines, with entity type numbers repeated on every line and padding between columns. The format has no built-in compression, so zipping the file before sending is the standard fix.

Is IGES 2D or 3D?

IGES handles both. The format covers 2D drawings and wireframes as well as 3D surfaces and solids, which is why it shows up in older drafting workflows and in 3D surface exchange.