STEP or IGES? Which CAD file format to ask for

The short answer

Ask for STEP for solid parts and assemblies. Ask for IGES only when the system receiving the file is old or works with surfaces. And if you’ll keep changing the design, ask for the native CAD file as well: neither format keeps the feature tree.

What STEP is

STEP is the informal name of ISO 10303, the “Standard for the Exchange of Product model data”. Its first parts were published in 1994, and it is still being extended: a new edition of AP242, its protocol for model-based 3D engineering, was published in 2025.

What people call a STEP file is the plain-text encoding defined in ISO 10303-21, with the extension .step or .stp. Which data it can hold depends on its application protocol, or AP:

• AP203: configuration controlled 3D design of mechanical parts and assemblies, long supported by the aerospace and defence industries.
• AP214: core data for automotive mechanical design processes, used mainly in the automotive industry.
• AP242: managed model-based 3D engineering. Published in 2014, it combines and extends AP203 and AP214, and supersedes both.

CAD tools usually let you choose the AP when you export. AP203 and AP214 are still supported by the main CAD, CAM and CAE applications, so any of the three will carry a solid part across.

What IGES is

IGES, the Initial Graphics Exchange Specification, is older. It was first published in 1980 by the US National Bureau of Standards, now NIST. After STEP arrived in 1994, interest in developing IGES further declined, and Version 5.3, from 1996, was the last.

IGES can describe wireframe geometry, freeform surfaces, and solids. Exporters can write a solid either way, and some default to surfaces: Open CASCADE, an open-source geometry kernel, writes each face as a separate trimmed surface unless its B-rep mode is chosen. When a file holds surfaces, the receiving CAD system has to stitch them back into a solid, and small gaps between them can stop that from working.

STEP and IGES side by side

How to tell them apart

Both formats are plain text, so a text editor settles it:

• A STEP file starts with ISO-10303-21;, then has a HEADER section that names its schema, which tells you the AP, and a DATA section of numbered entities. It ends with END-ISO-10303-21;.
• An IGES file is made of fixed 80-character lines, a length that dates from punched cards. Column 73 of each line holds the letter of the section it belongs to: S (start), G (global), D (directory entry), P (parameter data), or T (terminate).

ISO-10303-21;
HEADER;
FILE_DESCRIPTION(('example shaft'), '2;1');
FILE_NAME('shaft.step', '2026-10-06T09:00:00', (''), (''), '', '', '');
FILE_SCHEMA(('AUTOMOTIVE_DESIGN'));  /* AP214 */
ENDSEC;
DATA;
#10 = MANIFOLD_SOLID_BREP('shaft', #11);
#11 = CLOSED_SHELL('', (#12, #13, #14));
/* ... faces, edges, and points ... */
ENDSEC;
END-ISO-10303-21;

What neither format keeps

Both formats carry finished geometry: faces, edges, and in STEP’s case the assembly structure. Neither, in practice, carries the feature tree: the sketches, features, and named dimensions that make a model easy to change. Open a STEP file and you get a solid you can measure, machine, and edit face by face with direct-editing tools, but there is no diameter parameter to change.

So when you’ll keep working on a design, the deliverable should include the native CAD file as well. The comparison of meshes, B-rep models, and parametric models in Scan to CAD, explained goes into the difference.

What to ask for

• A solid part or an assembly: STEP, AP242 if your tools support it, otherwise AP214.
• An older system, or a surface-only workflow: IGES, then check the model after import.
• A design you’ll keep changing: the native CAD file with its feature tree, plus a STEP for everyone else.
• Every time: when the file opens, check its units and where the part sits relative to the origin, so it lands where you expect.

What OriginShape delivers

OriginShape’s copilot, Oscar, rebuilds parts from 3D scans as editable CAD. Models go out as STEP or IGES for any CAD tool, and the construction tree is native in FreeCAD today, with more CAD tools coming. Before anything ships, every uncertain dimension is flagged for review; Design intent explains why.

Need a part rebuilt? Tell us about it.

Sources and further reading

• ISO 10303. Wikipedia, on STEP’s history and application protocols.
• ISO 10303-21. Wikipedia, on the STEP file’s structure and extensions.
• STEP-file, ISO 10303-21. Library of Congress format description, on AP203, AP214, and AP242.
• IGES. Wikipedia, on IGES’s history and its 80-column file layout.
• IGES Translator. Open CASCADE Technology documentation, on writing solids as trimmed surfaces or as B-rep.

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