What is a .iges file?

What an IGES file looks like

IGES predates the personal computer as a mass-market object, and it shows. The file is a sequence of exactly 80-character lines, and the letter in column 73 tells you which of five sections a line belongs to: Start, Global, Directory entry, Parameter data, or Terminate.

That rigid layout is useful for one thing: it makes IGES easy to identify whatever the file is named. A file with 80-column records and an S, G, D, P or T in column 73 is IGES, even if somebody renamed it .stp.

Surfaces, not solids

The important practical difference from STEP is that IGES was designed around surfaces. Solids were bolted on later through the MSBO (manifold solid B-rep object) entity, which not every exporter writes and not every importer reads.

The result is the single most common IGES complaint: you import a part and get a collection of surfaces that *look* like a solid but are not stitched into one. Small gaps between adjacent surfaces mean the model cannot be used for machining or simulation until it is healed. Most CAD packages have a "sew" or "stitch" command for this, with a tolerance you may have to widen.

What IGES stores

• Trimmed and untrimmed NURBS surfaces, which is what it is good at
• Curves, points and construction geometry
• A unit declaration, in the Global section
• Colour, on a limited per-entity basis
• Very limited structure: no real assembly tree

What it does not

• No feature tree or parametric history, same as STEP
• No reliable assembly hierarchy
• No product manufacturing information

When to use IGES today

When the other end asks for it. IGES remains common in older aerospace and shipbuilding toolchains, in some analysis packages, and wherever a supplier's software has not been updated in twenty years. It is also still a reasonable way to move a pure surface model, such as a car body panel or a turbine blade, between packages that disagree about solids.

For sending a solid part to a machine shop in 2026, STEP is the better choice by a wide margin, and most shops will say so if you ask.

.igs or .iges?

Identical, with the same three-character-extension history as .stp. This site treats them as one format.

Reading an IGES file here

The viewer on this site evaluates the NURBS surfaces and tessellates them for display. That is why an IGES file takes a moment to open, and why the result on screen is triangles even though the file contains exact surfaces. The dimensions shown come from the file's own unit declaration.

Where an IGES file contains unstitched surfaces, converting it to a mesh format will produce a mesh with holes, because the gaps between surfaces are real. The conversion report on this site says so, so you don't have to discover it in your slicer.