AutoCAD Drawing

DWG is the native file format of AutoCAD and the most widespread format for technical drawings: floor plans and sections, building permit documentation, site plans from surveyors, machine parts, electrical and HVAC layouts. If an architect, a builder or a supplier has just sent you a .dwg and you do not own AutoCAD, you are in the most common situation. The file is binary, so a text editor or a photo viewer shows nothing useful, but several free programs can open it.

Where DWG comes from

The format goes back to Interact, a CAD program Mike Riddle wrote in the late 1970s. Autodesk, founded in 1982, rewrote it as AutoCAD and released version 1.0 in December 1982. Autodesk has never published a DWG specification. Other programs read DWG thanks to reverse engineering by independent developers, more recently also by the GNU project LibreDWG.

The format changes with AutoCAD, though not every year. The version is written in the first six bytes of the file: open the DWG in Notepad and the very first characters read, for example, AC1032.

Newer programs open older files, never the other way round. AutoCAD 2016 will refuse a drawing saved in AutoCAD 2024. That is why offices often agree to exchange drawings “saved as 2013” or “2010”.

What a DWG file contains

• Geometry in double precision, 2D and 3D. The drawing itself lives in model space, usually at 1:1 in millimetres or metres; printable sheets with a title block, viewports and plot scale are layouts (paper space).
• Layers, line types and weights, blocks with attributes, dimensions, hatches, text styles.
• 3D solids as embedded ACIS data (binary SAB since AutoCAD 2013).
• References, not content: external drawings (xrefs), raster images and PDF underlays are only linked. Fonts are not embedded either. AutoCAD uses its own SHX shape fonts plus TrueType fonts installed in the system.
• Custom objects from Civil 3D, AutoCAD Architecture or Plant 3D. Other software shows them only as a “proxy” picture, or not at all.

The data are compressed and protected by checksums. Since AutoCAD 2007 the file also carries a TrustedDWG mark, and AutoCAD shows a warning when the drawing was last saved by other software. That is a notice, not damage.

How to open DWG without AutoCAD

• Windows: Autodesk DWG TrueView is free. It views, measures, prints to PDF and its DWG Convert tool saves drawings as older versions, also in batches.
• Any system with a browser: Autodesk Viewer after a free sign-in; on phones and tablets the AutoCAD mobile app.
• Open source on Linux, macOS and Windows: LibreCAD opens 2D DWG through its own library and copes best with simpler drawings. FreeCAD imports DWG through an external converter such as LibreDWG, converting it to DXF internally. QCAD reads DWG only in the paid Professional edition; the free Community Edition works with DXF.

Common problems

“Drawing file is not valid” or DWG won’t open

Usually the drawing is newer than your program. Check the code at the start of the file. Ask the sender to save it as an older version, use DWG Convert in TrueView, or convert the drawing to DXF, which far more programs accept.

DWG opens but parts are missing or text is wrong

Empty frames and missing plans mean xrefs were not sent along. Ask for an eTransmit package (a ZIP with all referenced files). Question marks or wrong widths of text mean a missing SHX font that the viewer substituted.

DWG file corrupted

In AutoCAD run RECOVER, then AUDIT. Next to the drawing there is often a .bak file from the previous save. Copy it and rename it to .dwg. Automatic saves end in .sv$ and sit in the temporary folder.

DWG on Mac or Linux

AutoCAD for Mac exists but is paid. For viewing, Autodesk Viewer in the browser, LibreCAD or FreeCAD are enough.

Converting DWG

Most often to PDF, for the client, the building authority or printing (see DWG to PDF), and to DXF for other CAD or CAM software, laser and CNC cutting (see DWG to DXF). SVG, PNG or JPG are handy for a website or a report.

How our converter handles it: when converting DWG to DXF, the file is audited and repaired on load. The output is always an ASCII DXF in AutoCAD 2018 format (AC1032). If a machine needs R12 or 2000, open the DXF in LibreCAD or QCAD and save it as the older version. For PDF, SVG, PNG and JPG we render model space at 200 dpi on a white background. Layouts with title blocks and viewports are not drawn, and the result is not a plot at an exact scale. If you need a sheet at 1:100, plot it from a CAD program. 3D solids may come out incomplete.

DWG at a glance

More: Library of Congress format description.