How to Convert a STEP File to IGES Online (Free)

Convert a STEP file to IGES for free: FreeCAD and CAD Assistant methods, the solid-to-surfaces topology trap, units, and how to check the result before sending.

To convert a STEP file to IGES for free: open the .step or .stp in FreeCAD (free), then export it as IGES (.igs / .iges) — CAD Assistant (free, Open Cascade) and some online converters do the same. Unlike a conversion to a mesh (OBJ, STL), this is a translation between two exact-geometry CAD formats: surface precision is preserved. The real thing to watch is topology: a STEP solid is often exported to IGES as a set of trimmed surfaces, which can lose the notion of a watertight volume. This guide explains how to convert cleanly and how to verify the result.

STEP vs IGES: two exact CAD formats, but not equivalent

STEP (ISO 10303, 1990s to today) and IGES (Initial Graphics Exchange Specification, frozen at version 5.3 in 1996) are both exact-geometry exchange formats: they describe NURBS surfaces and analytic curves, not triangles. The difference is what they carry around the geometry. STEP is more modern: it natively handles solids (manifold BREP), assemblies, metadata, colors, and PMI (tolerances, annotations). IGES, older, is primarily surface-oriented; it can carry solids, but in practice many translators export it as trimmed surfaces. So you convert STEP to IGES mainly for compatibility with software, a machine, or a client that only accepts IGES.

• Preserved: the exact geometry (NURBS surfaces), the units if you set them, the shape down to the micron
• Can be lost: the watertight solid (turned into a shell of surfaces), assemblies, colors, PMI, and metadata
• Reversible but imperfect: always keep the original STEP as your master file

Method 1 — FreeCAD (free, recommended)

FreeCAD is built on the OpenCASCADE kernel, known for the quality of its IGES export. It reads STEP natively and lets you set the export units and tolerances.

1. 1Download FreeCAD for free from freecad.org, then open your .step or .stp file
2. 2Check the document units (Edit → Preferences → Import-Export → IGES): IGES does not always store units reliably, so set them explicitly (mm or inches)
3. 3Select the body or bodies in the tree
4. 4File → Export, then choose the «IGES (*.igs *.iges)» format and save
5. 5Open the resulting IGES in OmniView 3D to confirm no face is missing before you send it

Method 2 — CAD Assistant (free, Open Cascade)

CAD Assistant, built by Open Cascade (the CAD kernel behind FreeCAD), opens STEP and exports to IGES. It is a lightweight alternative, available on desktop and mobile, handy for a one-off conversion.

1. 1Install CAD Assistant (free) and open your STEP file
2. 2Check units and scale in the settings before exporting
3. 3Use the «Save / Export» function and select the IGES format

Method 3 — Online STEP to IGES converter

Several services convert STEP → IGES in the browser, with no installation. Since a STEP is often a sensitive industrial deliverable (patented part, project under NDA), confidentiality comes first: prefer a tool that processes the file locally, or stay on FreeCAD / CAD Assistant locally for your proprietary models. Also check the size limit: heavy STEP assemblies are often rejected by free online converters.

The topology trap: solid vs surfaces

This is the main pitfall of a STEP → IGES conversion. A STEP often describes a closed solid (a watertight volume); on IGES export, that solid can be «exploded» into independent trimmed surfaces. The software receiving the IGES then sees a shell of surfaces, sometimes with tiny gaps at the joints, rather than a volume. For plain surface machining or a preview, that is enough; for simulation or boolean operations, you will need to stitch/heal the surfaces in the target software to recover a solid. Always check the IGES before sending it.

Do you really need to convert just to view a STEP?

No — to show, rotate, and share a part, conversion is unnecessary. OmniView 3D reads STEP, IGES, STL, OBJ, GLB, GLTF, FBX, PLY, and DAE files directly. Upload your .step as-is, get an interactive 3D viewer in 60 seconds, and share a link: your client or supplier sees the part in their browser, with no CAD software. You only convert to IGES when the destination system requires it — typically an older CAM package or a partner whose pipeline does not yet accept STEP.