STEP vs IGES: which CAD file format should you actually use?
By Rajadurai R — Founder, CadNexa · 14 years plant-head experience
A supplier emails you a part. The attachment ends in .step. Last week the same supplier sent a .igs file. Your machinist asks which one to import, your quality team asks which one to archive, and somewhere a procurement engineer is forwarding the wrong one to the wrong vendor. STEP and IGES are the two most common neutral CAD formats in manufacturing — and the confusion between them costs real time.
Here is the short answer: use STEP (.step / .stp) for almost everything in 2026. Reach for IGES (.igs / .iges) only when a legacy machine or an old CAM seat refuses to read STEP. The rest of this guide explains why, with the technical detail your shop floor actually needs.
Use first
Use rarely
What is a STEP file?
STEP stands for Standard for the Exchange of Product model data, defined by ISO 10303. The two file extensions you will see are .step and .stp — they are identical. STEP carries full solid (B-rep) geometry, assembly structure, colours, layers, and increasingly product manufacturing information (PMI) such as GD&T and tolerances. The most common application protocols are AP203 (configuration-controlled 3D design), AP214 (automotive, adds colour and layers), and the newer AP242 (merges the two and adds PMI).
In plain terms: a STEP file gives you a watertight 3D solid you can measure, section, and pull a bill of materials from. That is why nearly every CAD system — SolidWorks, Creo, CATIA, NX, Fusion, Inventor, FreeCAD — exports clean STEP today.
What is an IGES file?
IGES — Initial Graphics Exchange Specification — dates back to 1980 and was the US national standard (ANSI) for CAD exchange long before STEP existed. Its extensions are .igs and .iges. IGES stores geometry as surfaces, curves, and wireframe entities rather than a single stitched solid. There is no formal assembly tree and no native PMI.
Because IGES describes surfaces individually, an imported IGES model often arrives as a collection of faces that may have tiny gaps at the edges. Your CAD or CAM software then has to "heal" or stitch those surfaces back into a usable body. On a complex part, that stitching is where errors creep in.
STEP vs IGES: the key differences
| Factor | STEP (.step / .stp) | IGES (.igs / .iges) |
|---|---|---|
| Standard | ISO 10303 (current) | ANSI, frozen at v5.3 (1996) |
| Geometry | Solid (B-rep) + surface | Surface & wireframe only |
| Assemblies | Yes — full product tree | No native assembly structure |
| Colour / layers | Yes (AP214 / AP242) | Limited, often lost |
| GD&T / PMI | Yes (AP242) | No |
| Typical file size | Compact | Often larger for same part |
| Translation reliability | High — clean solids | Lower — needs stitching |
| Best for | Modern 3D exchange, BOM, inspection | Legacy CAM, old surface models |
When should you still use IGES?
IGES is not dead — it just has a narrow remaining job:
- Legacy CAM seats. Some older CNC and wire-EDM controllers only import IGES surfaces.
- Pure surface data. Class-A automotive surfaces or moulds modelled as trimmed surfaces sometimes transfer better as IGES.
- An old archive. If a customer's drawing vault only holds .igs files from the 1990s, you work with what you have.
For everything else — quoting, inspection, assembly review, weight and BOM extraction — STEP wins on every measure that matters.
Common mistakes engineers make with STEP and IGES
- Sending IGES of an assembly. IGES has no assembly tree, so the receiver gets one merged blob with no part names. Always send assemblies as STEP AP242.
- Assuming .stp and .step differ. They do not. Same with .igs and .iges. The extension is just truncated.
- Exporting STEP AP203 when you need colour. AP203 drops colour and layers. Use AP214 or AP242 if those matter downstream.
- Healing an IGES file and trusting it blindly. After stitching surfaces, always check for a closed solid before measuring or quoting.
How to open STEP and IGES files without installing CAD
You do not need a ₹2,00,000 CAD seat just to view a customer's model. The CadNexa online 3D viewer opens both STEP and IGES files directly in your browser — no install, no upload to a server. You can rotate the model, take measurements, create an exploded view, and auto-generate a bill of materials with Indian material prices.
If you mostly deal with one format, we also keep focused guides on the free IGES (.igs) viewer and on exporting a BOM from a STEP file. More tutorials live in the CadNexa learning center.
Open any STEP or IGES file in your browser — free
Drag in a .step or .igs file, rotate it, measure it, and pull a BOM in seconds. No installation, no card.
Open the 3D Viewer — Free →Frequently asked questions
Is STEP better than IGES?
For modern manufacturing, yes. STEP carries solid geometry, assembly structure, colour, and PMI, and it translates far more reliably than IGES, which stores only surfaces and wireframe. Use IGES only when a legacy machine or CAM system cannot read STEP.
Are .stp and .step the same file?
Yes. Both are STEP files following ISO 10303. The extension is simply shortened to three letters on some systems. The same applies to .igs and .iges for IGES files.
Can I convert IGES to STEP?
Yes, most CAD packages can import an IGES file, stitch the surfaces into a solid, and re-export as STEP. Always verify the result is a closed solid before relying on it for measurement or quoting, because surface gaps from the original IGES can carry through.
Does STEP include GD&T and tolerances?
Only the AP242 application protocol carries product manufacturing information such as GD&T, datums, and tolerances. Older AP203 and AP214 STEP files contain geometry but not PMI. Confirm your CAD exports AP242 if you need tolerances embedded in the model.
How do I open a STEP or IGES file for free?
Use a browser-based viewer such as CadNexa. You open the file locally, view and measure the 3D model, and generate a bill of materials — without installing desktop CAD software and without uploading your design to any server.