IGES vs STEP File Formats: Differences and a Conversion Guide

If you exchange CAD files with manufacturers, you have almost certainly hit the moment where a supplier asks for a STEP file and all you have is an IGES — or the reverse. The IGES vs STEP question comes up constantly, because both are neutral formats designed to move geometry between different CAD systems — yet they are not interchangeable, and choosing the wrong one (or converting carelessly between them) can quietly cost you topology, assembly structure, and hours of rework.

This IGES vs STEP guide explains what each format actually is, when to use which, and how to convert between them without losing the data that matters. It is written from the perspective of a drafting team that converts these files every week, including a recent job where an IGES-to-STEP conversion went wrong in an instructive way.

What IGES is

IGES (Initial Graphics Exchange Specification) is one of the oldest neutral CAD formats still in active use. The last formal revision, IGES 5.3, dates to 1996, and that age tells you most of what you need to know about its strengths and limits. IGES was built primarily to exchange surfaces and wireframe geometry. It represents a model as a collection of entities — curves, surfaces, points — rather than as a single watertight solid. That makes it extremely flexible for moving surface data between systems, but it also means an IGES file can arrive as a loose collection of surfaces that look like a solid on screen but are not stitched into one.

The .igs and .iges extensions refer to the same format; .igs is simply the older 8.3-style short extension. There is no functional difference between them.

What STEP is

STEP (Standard for the Exchange of Product model data) is the modern successor, governed by the ISO 10303 standard. The application protocols you will see most often are AP203, AP214, and the newer AP242. Where IGES thinks in surfaces, STEP is designed to carry solid models — a proper boundary representation (B-rep) where faces, edges, and vertices are topologically connected into a closed volume. AP242 goes further again, carrying product manufacturing information (PMI) such as tolerances and annotations, plus assembly structure.

In practice this is the headline difference in the IGES vs STEP comparison: a STEP file usually arrives as a clean solid you can immediately measure, modify, and put into CAM. An IGES file often needs healing first.

IGES vs STEP: when to use each

The IGES vs STEP decision usually comes down to who is on the other end and what they are doing with the model:

• Choose STEP for manufacturing and machining. If the file is going to a CNC shop, a fabricator, or into CAM software, STEP (AP214 or AP242) is almost always the right answer because it carries a true solid and, in AP242, the tolerancing.
• Choose STEP for assemblies. STEP preserves assembly hierarchy and part relationships. IGES flattens everything into geometry and loses the structure.
• IGES still has a place for pure surface data. Some legacy systems, older CAM packages, and certain industrial-design surfacing workflows still expect IGES, and for transferring complex freeform surfaces it remains perfectly serviceable.
• When a client specifies a format, give them what they asked for. Their downstream toolchain dictates the requirement; do not “upgrade” an IGES request to STEP without checking, because their importer may be tuned for one or the other.

A real conversion that went wrong — and what it taught us

On a recent mechanical assembly job for a Brisbane manufacturer, we received the parts as IGES files. The client’s own SolidWorks installation did not have a working IGES import path that produced usable solids, so they needed the parts as STEP before they could do anything with them. On the surface a trivial conversion — open the IGES, export STEP. In reality, several of the parts came across as disconnected surface bodies rather than solids, because the original IGES export had never stitched the surfaces into a closed volume.

The fix was not to re-export and hope. We ran each part through a knit/heal step: importing the IGES surfaces, identifying the gaps where adjacent surfaces failed to meet within tolerance, knitting them into a closed boundary, and only then exporting to STEP as a true solid. Two parts had genuine geometry gaps that no automatic heal could close, and those had to be rebuilt locally before the solid would form. The lesson we took from it: always verify that an IGES import produces a single solid body before converting downstream. A surface model that looks complete on screen can still be open, and the failure only surfaces when CAM or a Boolean operation rejects it.

Common conversion pitfalls

• Surfaces vs solids. The single biggest issue. An IGES import can give you surfaces that need knitting before they behave as a solid. Always check the body type after import.
• Lost assembly hierarchy. Converting an assembly to IGES collapses it into geometry. If structure matters, use STEP from the start.
• Tolerance and gap problems. Surfaces that do not meet within the stitching tolerance leave gaps; tightening or loosening the knit tolerance is often the difference between a clean solid and a failed heal.
• Units drift. Confirm the unit system on import — a model that imports at the wrong scale is easy to miss until a dimension looks wrong.
• PMI loss. Tolerances and annotations do not survive a trip through IGES. If you need them, stay in STEP AP242.

Four practical ways to convert

• SolidWorks / Inventor. Open the source file and use Save As to the target format. Both let you set import options for whether to knit surfaces into solids automatically — turn that on and verify the result.
• AutoCAD. Capable of importing and exporting both formats, best suited where you are working with 2D-derived or simpler geometry rather than complex surfacing.
• FreeCAD. A no-cost option that handles IGES and STEP via its OpenCASCADE core. Useful for one-off conversions when you do not have a seat of a commercial package free.
• Online converters. Fine for a quick, non-confidential single part. Avoid them for anything under NDA or for assemblies where structure must be preserved.

What Australian shops tend to expect

In our experience across Australian manufacturing and fabrication, STEP has become the default request — in the IGES vs STEP choice most CNC shops, laser cutters, and fabricators will ask for a STEP file first because their CAM software ingests it cleanly as a solid. AP214 is the version you will be asked for most often; AP242 is appearing more where tolerancing needs to travel with the model, particularly in defence and aerospace-adjacent work. IGES requests now tend to come from two places: older equipment and tooling suppliers still running legacy systems, and industrial-design or surfacing workflows where freeform surface data is the point. If a supplier does not specify, sending STEP and keeping the native file on hand is the safe default.

One practical habit worth forming: when you send a neutral file, send it alongside a short note of the units and the source CAD system. A surprising number of conversion headaches are not really format problems at all — they are a part that imported at the wrong scale, or a recipient who did not know whether to expect surfaces or a solid. A one-line note prevents both.

A simple decision framework

When a file lands on your desk and the IGES vs STEP choice is not obvious, ask three questions. Is it going to manufacturing or CAM? Send STEP. Does it contain an assembly whose structure matters? Send STEP. Is the recipient on a legacy surfacing system that specifically asks for IGES? Send IGES, and verify the surfaces are clean before you do. When in doubt, ask the client which their downstream tool expects — five minutes of asking saves an afternoon of re-converting.

If you would rather hand the whole problem to a team that converts these formats daily, that is exactly the kind of work we do. See our CAD conversion services, our deeper explainer on IGES and STEP files, or our guide to converting PDF drawings to DWG. Send us a sample file and we will tell you exactly what is in it and what it will take to get it into the format you need.