What is a CAD file?

A CAD file is a drawing or 3D model saved by computer-aided design software, stored as precise geometry (lines, arcs, surfaces, solids) with units, layers and metadata rather than as pixels. The extension tells you which program made it and how it is structured. The most common ones, DWG and DXF, open directly in the viewer on this page.

CAD file meaning: what makes it different from an image

CAD stands for computer-aided design. A CAD file format stores a design mathematically: a wall is a line from one coordinate to another with a length in real units, a hole is a circle with a radius, a bracket is a solid with faces and edges. Because of that, a CAD drawing can be measured, scaled, re-dimensioned and machined, which a JPG or a PDF of the same drawing cannot. Along with geometry the file carries layers, line weights, text and dimension styles, block or component definitions, and often the design history.

CAD files split into two families. 2D drawings (floor plans, schematics, cut profiles) live in formats like DWG, DXF and DGN. 3D models (parts, assemblies, buildings) live in native formats like SLDPRT, IPT, RVT and in neutral ones like STEP and IGES. A published, non-editable copy for review is a third kind: DWF, or simply PDF.

CAD file extensions and formats compared

Rule of thumb: native formats (DWG, RVT, SLDPRT) keep everything but need the maker's software; neutral formats (DXF, STEP, IFC) travel between programs and lose some program-specific features on the way.

What are CAD programs, and which is the most used?

CAD software falls into a few types. 2D drafting: AutoCAD and AutoCAD LT (the most widely used CAD software for drawings, and the reason DWG is the default exchange format), BricsCAD, DraftSight, and the free LibreCAD and QCAD. 3D parametric modeling for mechanical parts: SolidWorks, Autodesk Inventor, Fusion, Onshape, Creo, and the open-source FreeCAD. BIM for buildings: Revit, ArchiCAD, Vectorworks. Civil CAD for roads, grading and utilities: Civil 3D and Bentley OpenRoads. Modeling and visualization: SketchUp, Rhino.

"Basic CAD" usually means 2D drafting: drawing lines and arcs to scale, organizing them in layers, and dimensioning. For someone starting out, the easiest 2D CAD to learn is one of the free tools (LibreCAD or QCAD) because they expose only the drafting core; AutoCAD skills transfer directly later, since the commands and the DWG format are the same. CAD skills on a resume typically mean producing and reading drawings in one of these programs, plus knowing drafting standards (layers, line weights, dimensioning).

How to open a CAD file without the software that made it

For DWG and DXF, drop the file into the viewer above: it renders in the browser with layers, measuring, printing and PNG/SVG/PDF export, and nothing leaves your computer. More on the CAD viewer page and in how to open a DWG file without AutoCAD.

For other formats, use the vendor's free reader: eDrawings Viewer for SolidWorks files, Autodesk Viewer online for Revit, Inventor, STEP and many others, Bentley View for DGN, and FreeCAD for STEP, IGES and STL. The free Autodesk DWG TrueView handles DWG only. For DWF use Autodesk Design Review or the Autodesk Viewer (see how to open a DWF file).

Converting between CAD formats

Conversions between 2D formats are safe: DWG to DXF and back keep the geometry. Conversions between 3D and 2D are lossy by nature: STEP to DWG gives you solids that AutoCAD can show but not parametrically edit, and DWG to STL turns solids into triangles for printing. Cross-vendor 2D conversions such as DGN to DWG or Revit to DWG are done by the originating program's export command. Every pair we cover is listed in the convert hub.

Frequently asked questions