Check an AutoCAD DWG file without AutoCAD
An architect, a contractor or a tender sent you a .dwg and nothing on your computer or phone opens it. Drop it here to see what it is before you install anything: the DWG version and which AutoCAD releases write it, the oldest AutoCAD or free DWG TrueView that opens it, the preview picture AutoCAD stored inside, and the title, author, “last saved by”, dates and fonts recorded in the file.
The file is read by this tab on your device and is not uploaded, so a client’s plans or a tender drawing go nowhere.
What it shows
- The version, and what opens it. The first six bytes of every DWG are its version string.
AC1027means the AutoCAD 2013 format, written by AutoCAD 2013 to 2017, so AutoCAD 2012 or older refuses it with “Drawing file is not valid”, and DWG TrueView 2013 or newer opens it. The table below lists every version. - The preview stored inside. AutoCAD saves a small picture of the last view with the drawing: a bitmap (BMP) up to AutoCAD 2012, a PNG from AutoCAD 2013 on. It is shown and can be saved as PNG. It is the picture AutoCAD’s Open dialog shows, not a rendering of the drawing, and it can be blank or out of date. An old Windows Metafile (WMF) preview is named and can be saved, but is not drawn.
- Drawing properties. For files in the 2004 format and later (AC1018, AC1024, AC1027, AC1032): title, subject, author, keywords, comments, “last saved by” (the login name of whoever saved it last), revision number, the created and modified dates, the total editing time and any custom properties.
- The program that saved it. The AutoCAD release and build, for example AutoCAD 2017 build N.52.0.0 in a file saved in the 2013 format. A file in a newer format than your AutoCAD is often simply from someone with a newer subscription; they can save it down with Save As.
- Fonts and files it uses. The fonts (such as
arial.ttforromans.shx), external references (other .dwg files), images and plot style tables it refers to, with the folder they were in on the computer that saved it. - Maintenance release, code page and protection. The code page says how text in older drawings is encoded (Windows 1252 for Western Europe and the US); a password-protected or digitally signed drawing is flagged.
DWG versions and AutoCAD releases
Each DWG format is written by several AutoCAD releases. Every AutoCAD opens its own format and all older ones, so the oldest program that opens a file is the first release of its format. DWG TrueView is Autodesk’s free viewer for Windows and carries the same year numbers.
| Version string | Format | Written by |
|---|---|---|
| AC1032 | AutoCAD 2018 | AutoCAD 2018 and every release since |
| AC1027 | AutoCAD 2013 | AutoCAD 2013, 2014, 2015, 2016, 2017 |
| AC1024 | AutoCAD 2010 | AutoCAD 2010, 2011, 2012 |
| AC1021 | AutoCAD 2007 | AutoCAD 2007, 2008, 2009 |
| AC1018 | AutoCAD 2004 | AutoCAD 2004, 2005, 2006 |
| AC1015 | AutoCAD 2000 | AutoCAD 2000, 2000i, 2002 |
| AC1014 | R14 | AutoCAD Release 14 |
| AC1012 | R13 | AutoCAD Release 13 |
| AC1009 | R11/R12 | AutoCAD Release 11 and 12 |
| AC1006, AC1004 | R10, R9 | AutoCAD Release 10 and 9 |
| AC1003, AC1002, AC1001 | 2.6, 2.5, 2.2 | AutoCAD 2.6, 2.5 and 2.2 (1986–87) |
How the file is read
- The fixed header
- From R13 on, byte 0x0B holds the maintenance release, the four bytes at 0x0D the address of the preview, and the two at 0x13 the code page. The preview block starts with a fixed 16-byte marker (
1F 25 6D 07 …) that is checked before anything is read, then lists each stored picture with its type: 2 for a bitmap, 3 for a WMF, 6 for a PNG. - The 2004 format and later
- From AC1018 on, the file’s directory is a 108-byte header at 0x80 scrambled with a fixed pseudo-random sequence. It is unscrambled, then the page map and section map it points to are read, and the SummaryInfo, AppInfo and FileDepList sections are unpacked (they are compressed with an LZ77 scheme). All of this follows the Open Design Alliance’s published Open Design Specification for .dwg files; no other DWG program’s code is used.
- Tested on
- The FAA’s airport construction standard detail F-163-3 (a wildlife deterrent fence skirt, a US government work): 117,765 bytes, AC1027, saved by AutoCAD 2017, with a 256 × 76 PNG preview, “last saved by” filled in, the other properties empty, and Arial as its one font. The bitmap, WMF and 2004-format paths are tested on files generated from the specification.
What this cannot do
- Draw the drawing. This page does not read the lines, blocks, layers or layouts and cannot show, measure or print the drawing. The preview is only the small picture AutoCAD stored. The open-source programs that draw DWG files (LibreDWG, and LibreCAD, which uses it) are GPL-licensed, and this site does not use GPL code.
- Read the properties of every version. Title, author and dates are read for the 2004, 2010, 2013 and 2018 formats. The 2007 format (AC1021) encodes its sections differently, and files from R13, R14 and 2000 keep the properties among the drawing’s objects; for those the page says which details are not read.
- Open a password-protected drawing. Its contents are encrypted. The version and preview are still read; the page says it is protected.
- Edit or convert. It only reads. It cannot save a DWG in another version or as DXF.
How to see the drawing
- On this site, through DXF
- The free ODA File Converter from the Open Design Alliance (Windows, macOS and Linux) converts a folder of DWG files to DXF. Set the output to DXF and a version such as 2018, convert, then open the .dxf in the DXF viewer, which draws it with its layers, measures it and saves it as SVG or PNG, on your device.
- On Windows
- Autodesk’s free DWG TrueView opens, measures and prints DWG files. Its DWG Convert tool saves a copy in an older DWG version for someone whose AutoCAD is older than the file.
- Ask the sender
- A PDF plot is often what you need. The “last saved by” name and the dates here tell them exactly which file you mean.