What file formats do free CAD block sites offer?

DWG: the default for 2D blocks

DWG is AutoCAD's native format and the de facto standard for 2D CAD blocks. If a site offers one format, it is almost always DWG. It is compact, opens fast, and carries full layer and block fidelity, and it is read not just by AutoCAD and AutoCAD LT but by BricsCAD, DraftSight, ZWCAD, GstarCAD, NanoCAD and most free DWG viewers.

Because it is so widely supported, DWG is the safe default for anyone doing 2D drafting. The one wrinkle is versioning — a very new DWG can refuse to open in a very old release — which good libraries handle by saving to a broadly-compatible version. The blocks here target such a version so they open across current and older CAD software without a fuss.

DXF: the universal interchange format

DXF is the open, text-based companion to DWG, published by Autodesk so other programs can read and write the same geometry. Sites offer it as the compatibility option: when DWG will not open in some niche tool, DXF usually will. It is the format of choice for feeding blocks into CNC machines, laser cutters, CAM software, vector editors and GIS tools.

DXF holds the same drawing as DWG, just stored as readable text, so it is larger and a touch slower but no lower in quality. For long-term archiving it is also the more future-proof choice, because its specification is openly documented. Offering DXF alongside DWG means a fabricator on a laser cutter and a draughtsperson in AutoCAD can both use the same block.

Revit formats: RFA and RVT

Revit, Autodesk's BIM platform, uses its own formats. RFA is a Revit family file — a parametric component like a door, window or piece of furniture that carries real data and behaviour, not just line work. RVT is a full Revit project file. These are not interchangeable with DWG blocks: a Revit family is an intelligent BIM object, whereas a DWG block is drawn geometry.

Some libraries offer RFA families for BIM users, and they are valuable if you work in Revit, because a true family schedules and behaves correctly inside a BIM model. But you cannot insert an RFA into AutoCAD as a block, nor turn a DWG block into a Revit family by renaming it. Match the format to the platform: RFA for Revit, DWG for AutoCAD.

SketchUp, 3ds Max and 3D formats

When 3D models enter the picture, more formats appear. SKP is SketchUp's native format, common for 3D furniture and architectural models. MAX is 3ds Max's format, aimed at high-detail rendering models. You may also see OBJ, FBX, 3DS and STL — interchange formats for 3D geometry that move models between modelling and rendering applications, and STL specifically for 3D printing.

These formats are about 3D modelling and visualisation, not 2D drafting. If you are rendering a scene or building a 3D model you will care about them; if you are producing 2D drawings you will not. A site focused on 2D drafting blocks, like this one, sensibly concentrates on DWG and DXF rather than spreading across 3D formats it is not built for.

Which format you actually need

Pick by your software. For AutoCAD and its compatible CAD cousins, download DWG. For a tool that does not read DWG — many CNC, laser and CAM programs, some vector and GIS tools — download DXF. For Revit, you need RFA families, not DWG blocks. For SketchUp, 3ds Max or other 3D work, you need SKP, MAX or an interchange format like OBJ or FBX.

- AutoCAD / LT / BricsCAD / DraftSight: DWG - CNC, laser, CAM, vector, GIS, archiving: DXF - Revit (BIM): RFA family files - SketchUp / 3ds Max / 3D rendering: SKP / MAX / OBJ / FBX

Matching the format to the program first saves the frustration of downloading a file your software simply cannot open.

When the format on offer isn't yours

If a block is only available in a format your software does not read, conversion is often possible. DWG and DXF convert freely into each other with AutoCAD's SAVEAS, Autodesk DWG TrueView, or the free ODA File Converter. A DWG can be imported into SketchUp and many other tools even if they do not list it as a download format.

What does not convert cleanly is intelligence: turning a dumb DWG block into a parametric Revit family is a rebuild, not a conversion, because the data and behaviour have to be authored. So for 2D geometry, format is rarely a blocker — convert and carry on. For BIM families and rich 3D models, source the right native format from the start rather than expecting a converter to manufacture the missing intelligence.