IGS to STEP Converter
Upload a .igs or .iges file and get back a STEP file whose surfaces and curves are written out exactly as they came in, with no tessellation anywhere in the process. One thing to know before you start: IGES faces are not sewn together, so the STEP you download is a set of open shells rather than a closed solid body.
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Why convert IGS to STEP?
IGS is simply the three-letter spelling of IGES, the neutral CAD exchange format standardized in the 1980s. An IGES file stores a model as a collection of trimmed surface patches and curves, each written as its own entity, with no requirement that the patches form a watertight volume. That design served its purpose for decades, which is why old part libraries, supplier downloads and machine-shop archives are still full of .igs files. The problem today is downstream: many manufacturers, quoting portals and CAM departments only accept STEP, and some newer modeling tools treat IGES import as a legacy afterthought.
On this page the file is uploaded to our servers, where FreeCAD 1.1 reads it with OpenCASCADE 7.9 and writes a STEP file with the same geometry kernel. The conversion is a pure B-rep rewrite: every NURBS surface and curve is carried across with its own mathematics, so nothing is approximated by triangles and there is no chord tolerance to set. The output is AP214 (AUTOMOTIVE_DESIGN) Part 21 text with units declared as millimeters, which is the standard AP214 flavour of STEP that mainstream CAD and CAM packages import. If the IGES declares another unit on import, the geometry is scaled to millimeters before it is written.
What does not survive is worth spelling out. Because IGES faces are not stitched into a shell, the STEP file comes out with one open shell per face and zero solid bodies: a 42-face model becomes 42 open shells. Your CAD program will open it as a surface model, and if you need a solid for machining, thickness analysis or boolean work you will have to knit or sew the faces and close any gaps. Layer names, colors, entity names and any assembly grouping in the IGES are not read at all, so the STEP contains geometry and nothing else, under a single product entry written by the OpenCASCADE translator.
How to convert IGS to STEP
- Check that the IGES actually contains geometry. Some exporters write a structure-only file whose entity 416 records just point at other files; those convert to nothing, so re-export from the source CAD with the geometry included.
- Choose the .igs or .iges file. Both extensions are accepted on this page. Guests can upload up to 50 MB per file, a free account 100 MB, Plus 200 MB and Pro 1 GB, with up to 10 files in one batch (25 on Plus, 50 on Pro).
- Start the conversion. Processing happens on our servers in a FreeCAD subprocess, not in your browser; most files of a few megabytes are done in a second or two, and heavy surface models take longer.
- Download the .step file. If the tool you are feeding insists on a .stp extension, rename the download - it is the same ISO 10303 Part 21 text either way.
- Open it in your CAD program and, if you need a solid, run its sew or knit command on the imported faces and check the result for gaps before sending it to CAM or simulation.
What this converter does
- Writes AP214 STEP through the OpenCASCADE 7.9 writer in FreeCAD 1.1, with SI millimeter units declared in the file header.
- Keeps exact geometry: trimmed surfaces and curves are rewritten as B-rep, never converted to a mesh, so there is no triangle count or deviation setting to choose.
- Repairs common upload problems before reading - a gzip-compressed .igs is unpacked, UTF-16 text is re-encoded, and a leading byte-order mark is stripped.
- Screens out files that are not IGES at all. A renamed SolidWorks or Inventor part, a ZIP, PDF or image is rejected before conversion, so you get a read error instead of a broken STEP.
- The same .igs can also be sent to our IGES to STL, OBJ, PLY, 3MF and DXF pages when you need a mesh or a 2D outline instead of B-rep.
- No account or sign-up is needed to run a conversion within the guest limits.
Video Tutorial
An A-Z video guide on how to use MiConvert to change IGS format to STEP. This is the safest method to convert data without needing to download any applications to your device. 🔗 Format conversion to
IGS to STEP questions
Is .igs the same thing as .iges?
Yes. They are the same format with a long and a short extension, kept apart only because older systems were limited to three characters. This page accepts either one and treats them identically.
Why is my converted STEP a surface model instead of a solid?
IGES stores faces as independent trimmed patches and the conversion does not sew them, so you get one open shell per face and no manifold solid. Use Knit Surface in SolidWorks, Stitch in Fusion 360 or Shape Builder in FreeCAD, then close any remaining gaps. If the original CAD system is still available, exporting STEP from it directly gives you a solid.
I need a .stp file, not .step. What should I do?
Convert here and rename the downloaded file to .stp; the contents are unchanged. Use that route for now rather than our IGS to STP page, which currently rejects uploads in the browser because of a target-naming bug on our side.
The converter says no valid 3D geometry was found. Why?
It usually means the IGES is a structure-only file whose entity 416 records just reference other files, so there is no surface geometry to translate. Re-export with the geometry included; if the file really is wireframe only, send it to our IGES to DXF page, which writes curves as they are.
What does the message about not being able to read the 3D file mean?
The file was truncated before its terminate section, was corrupted in transit, or is not an IGES file despite its extension. Re-download or re-export it from the original CAD program. Our error text is deliberately generic and mentions textures even for CAD files, so ignore that wording and check the file itself.
What units and scale will the STEP have?
Millimeters. Geometry is converted to millimeters when the IGES is read, and the STEP header declares SI_UNIT millimeter. Measure one known feature after import to confirm.
Will layers, colors or part names carry over?
No. The reader loads geometry only, so colors, entity names, layer assignments and any grouping in the IGES are not written into the STEP. If you need to identify sub-parts, convert the components separately or re-apply appearances after import.
What happens to my file after the conversion?
The upload runs on our servers and is deleted as soon as the job finishes its final attempt. The converted STEP can be downloaded through its unguessable link for about 4 hours, after which it is deleted automatically. File names, sizes and outcomes are logged for operations, and signed-in users keep a conversion history row.