What is a .brep file?

What a BREP file is

BREP is the serialisation format of the Open CASCADE Technology geometry kernel. It is not an interchange standard like STEP or IGES. It is a kernel writing its own working data structures to disk, which is why it is fast to read and write and why almost nothing outside the Open CASCADE world opens it.

The name is the abbreviation of *boundary representation*, the same idea STEP encodes: a solid described by the surfaces that bound it.

You will meet .brep files if you use FreeCAD, which is built on Open CASCADE, or if you work with tools like KiCad's 3D export, CadQuery or Open CASCADE's own sample applications.

What it stores

• Exact geometry: planes, cylinders, cones, spheres, tori and NURBS surfaces
• The full topology: vertices, edges, wires, faces, shells and solids
• Compounds, so several solids can live in one file

What it does not store

• No unit. The file is a list of numbers. Open CASCADE and everything built

on it treat those numbers as millimetres by convention, and this site does the same, but nothing in the file says so.

• No colour or material. FreeCAD keeps appearance in its own .FCStd

project file, not in an exported BREP.

• No assembly names or structure beyond a flat compound.
• No parametric history. As with STEP, you get the shape and not the

recipe.

Text or binary?

Both exist. The text form begins with a recognisable header line, either DBRep_DrawableShape or CASCADE Topology V1, V2 or V3, and is what FreeCAD writes by default. This site identifies the format from that header, not from the file extension.

When to use it

BREP is a working format, not a delivery format. It is the right choice when:

• You are moving geometry between two Open CASCADE-based tools and want no

translation loss at all

• You are scripting FreeCAD or CadQuery and want a fast intermediate file
• You are filing a bug report against an Open CASCADE-based application and

want to hand over exactly what the kernel saw

It is the wrong choice for sending a part to anybody else, because the recipient almost certainly cannot open it. Export STEP instead.

Converting away from BREP

Because BREP carries no unit, every conversion out of it has to assume one. This site assumes millimetres, which matches Open CASCADE's own convention, and says so on every conversion report. If the part was modelled in inches, check the overall dimensions before machining: a 2-inch bracket will arrive as a 2 mm one.

Converting BREP to a mesh format tessellates the exact surfaces into triangles. Curved faces become faceted, and the model can no longer be edited as a solid. Converting BREP to STEP keeps the geometry exact and adds the unit declaration BREP lacked.