CAD File Formats Compared: STEP vs IGES vs STL vs DXF

Quick Reference

STEP (.step, .stp)

The modern standard for 3D CAD data exchange.

• Preserves: Solid geometry (B-rep), assembly structure, colors, product hierarchy, metadata
• Versions: AP203 (geometry only), AP214 (geometry + colors + layers), AP242 (adds PMI/GD&T)
• File size: Medium — typically 2-5x larger than native CAD files
• When to use: Sending 3D models to suppliers, cross-platform CAD exchange, archival
• Limitation: Not all STEP readers handle assemblies identically — test with your recipient's software

IGES (.igs, .iges)

The legacy standard, still widely used.

• Preserves: Surface geometry, wireframes, basic solids, annotations
• File size: Large — verbose ASCII format
• When to use: Communicating with older CAD/CAM systems that don't support STEP
• Limitation: Surface-based geometry often has gaps and stitching issues when imported. No assembly support. Effectively superseded by STEP for most use cases

STL (.stl)

Triangulated mesh format, ubiquitous in additive manufacturing.

• Preserves: Surface shape only (as triangles). No colors, no materials, no assembly structure
• File size: Can be very large for detailed parts — binary STL is ~10x smaller than ASCII STL
• When to use: 3D printing, rapid prototyping, FEA mesh input, web 3D visualization (though glTF is preferred for web)
• Limitation: No parametric data, no feature history, no dimensions. Lossy — you can't recover the original CAD geometry from an STL. Resolution (triangle count) is a tradeoff between accuracy and file size

DXF (.dxf)

The universal 2D CAD interchange format.

• Preserves: 2D entities (lines, arcs, circles, polylines, text, dimensions), layers, blocks
• File size: Small for 2D data. ASCII format is human-readable
• When to use: Laser/plasma/waterjet cutting profiles, CNC router toolpaths, 2D drawings, flat pattern export
• Limitation: Primarily 2D. 3D support exists but is poorly standardized across applications. No solid geometry

Choosing the Right Format

• Sending a 3D model to a supplier? → STEP AP214
• Sending to a 3D printer? → STL (or 3MF for multi-material)
• Sending flat patterns to a laser cutter? → DXF
• Working with legacy CAD systems? → IGES
• Displaying in a web browser? → Convert to glTF/GLB
• Archiving for long-term storage? → STEP AP242

Programmatic Handling in C#

• STEP/IGES: Use Open CASCADE (OCCT) via wrapper libraries, or SolidWorks API's SaveAs3 for export
• STL: Simple format — binary STL is just a list of triangle normals and vertices. Easy to parse manually
• DXF: Use the netDxf NuGet package for reading/writing. For simple parsing, DXF is plain-text group code/value pairs