What Is a DWG File? The AutoCAD Drawing Format, and Why Versions Matter

2 How Do You Find the Version of a DWG File? Read six bytes and match the code

The Library of Congress says the letters AC10 mark DWG files from AutoCAD 2.5 onward, which Autodesk released in June 1986. Older files differ. Version 1.0 starts with MC0.0, and versions 1.2 to 2.22 start with AC and then other characters. Two of our samples start with AC1.40 and AC2.10, so AC10 alone is not a full test.

FileInfo and our earlier page on this site list 41 43 31 30 as the file signature. Those bytes spell AC10, and they fit every DWG file from the 2.5 release onward. The next two bytes give the version, so six bytes give the full answer.

The file command reads those six bytes and names the AutoCAD release. It named all 141 of our DWG files, from AC1.40 to AC1032. It also named the files that we damaged, which is the weak side of a header check. The next two sections show how far a header can be trusted.

The script below prints the version of any DWG file. Save it as check_dwg_version.py and give it the name of your file.

import sys

FORMATS = {
    'AC1009': 'R11 and R12', 'AC1012': 'R13', 'AC1014': 'R14',
    'AC1015': '2000', 'AC1018': '2004', 'AC1021': '2007',
    'AC1024': '2010', 'AC1027': '2013', 'AC1032': '2018',
}

with open(sys.argv[1], 'rb') as f:
    code = f.read(6).decode('latin-1')

if code in FORMATS:
    print(code, 'is the', FORMATS[code], 'DWG format')
elif code.startswith('AC'):
    print(code, 'is an older or unlisted DWG format')
else:
    print('Not a DWG file. The first six bytes are', repr(code))

Here is what it printed for three files.

AC1032 is the 2018 DWG format

AC1021 is the 2007 DWG format

Not a DWG file. The first six bytes are '%PDF-1'

The script checks the label and nothing else. A good label does not prove that the drawing inside is complete, as the damage tests in the causes section show. A bad label does prove that something is wrong, because every genuine DWG file starts with AC.

Four more clues about your DWG file

3 What Is Inside a DWG File? A version stamp, a header and a preview picture

Autodesk lists what a drawing stores: geometry, layers, dimensions, tolerances, annotations, blocks, symbols, layouts and metadata. All of it lives in one binary file. DXF, the text twin, can describe the same content. The Library of Congress calls the two logically equivalent.

Binary files are small. For eight versions of one test drawing, our DWG files were 1.3 to 5.8 times smaller than their DXF twins. The twins came with the same sample collection, so treat the ratio as a range for our files and not as a rule.

The preview picture inside DWG files

The header of a file from the 2000 format on gives the address of a preview block. The block lists one or more pictures. We found BMP pictures in 83 files from the 2000, 2004, 2007 and 2010 formats and PNG pictures in 24 files from the 2013 and 2018 formats.

The 2018 format was the exception. Only 4 of its 21 files had a preview. So a missing preview does not mean damage. Our samples came from many sources, and programs that write DWG files can skip the picture.

This picture is the quickest way to see DWG files without CAD software. The script below saves it. The free LibreDWG tool dwgbmp does the same job in one command.

import struct
import sys

SENTINEL = bytes.fromhex('1f256d07d43628289d57ca3f9d44102b')
data = open(sys.argv[1], 'rb').read()
at = struct.unpack_from('<i', data, 13)[0]
if data[at:at + 16] != SENTINEL:
    sys.exit('No preview block found')
for i in range(data[at + 20]):
    code, start, size = struct.unpack_from('<Bii', data, at + 21 + i * 9)
    if code in (2, 6) and size:
        image = data[start:start + size]
        name = 'preview.png'
        if code == 2:
            name = 'preview.bmp'
            dib = struct.unpack_from('<IiiHHIIiiII', image)
            palette = (dib[9] or 1 << dib[4]) * 4 if dib[4] <= 8 else 0
            head = struct.pack('<2sIHHI', b'BM', 14 + size, 0, 0, 14 + dib[0] + palette)
            image = head + image
        open(name, 'wb').write(image)
        print('Saved', name, len(image), 'bytes')

Here is what it printed for two files.

Saved preview.bmp 24262 bytes
Saved preview.png 5351 bytes

The picture is small. Ours ranged from 160 to 852 pixels wide. It shows how the drawing looked when someone saved it, and it does not prove that the drawing inside opens.

4 Which AutoCAD Opens Which DWG Version? Newer programs open older files, never the reverse

Autodesk's compatibility article, dated May 2026, sets the rule. Every AutoCAD product can save, open and edit any older DWG format. It cannot open a newer one. You can change the default save format in the Open and Save tab of the Options dialog.

The article gives three examples. AutoCAD 2025 can open DWG files back to 1998, the R14 format. AutoCAD 2017 cannot open a 2018 file. AutoCAD 2018 can open DWG files from AutoCAD 2025, because AutoCAD 2025 saves in the 2018 format.

The table follows Autodesk. Wikipedia stops the 2018 row at AutoCAD 2025, and Autodesk's newer article extends it to 2027. The header codes come from Wikipedia and the Library of Congress, and our own samples confirm all of them.

Older DWG formats before R14

Autodesk lists six older version codes. They are rare today, but an archive of DWG files can contain them. We found sixteen files in our set from before the 2000 format, and the file command named all sixteen.

How to find the version your program needs

1. 1 Read the first six bytes with our header check, or use the file command.
2. 2 Match the header code to the table above and note the AutoCAD years that save that format.
3. 3 Compare those years with the program you use. If your program is older than the first year, it cannot open the file.
4. 4 Ask the sender for a copy saved to an older format, or use a viewer. The open section lists both routes.

An old file opens in a newer program, but not always unchanged. Autodesk's help on opening drawings says AutoCAD updates most objects on its own. Some need a command. Hatches and 2D polylines from before R14 use CONVERT. Lights from AutoCAD 2006 or earlier need CONVERTOLDLIGHTS. Materials from AutoCAD 2010 or earlier need CONVERTOLDMATERIALS.

12 What Else Looks Like a DWG File? .dwt, .dws, .bak, .sv$ and a lookalike

Anyone can rename files, so endings are hints and not proofs. The table lists the endings that you meet near DWG files. The FileInfo page for the DWG extension shows more programs for the main ending.

Backup and autosave files

When AutoCAD saves, it can leave a .bak copy of the last version beside the DWG file. It also writes autosave files that end in .sv$. We copied a 2018 drawing to both endings. The file command named both DWG, and the reader found the same 67 objects, so the bytes were identical.

Autodesk lists the rename as a last resort for drawings that will not open. The .sv$ default folder is %TEMP%, says its Tuesday tips article. Look there after a crash.

Files renamed to .dwg

We renamed five files to .dwg. They were DXF text, PDF, ZIP, text and empty files. The file command named all five correctly, and the reader refused all five. If a DWG file will not open and file says another type, rename it back and use the right program.

A DXL file belongs to HCL Domino and IBM Notes. If your file ends in .dxl, read the guide to Domino XML files instead.

Pictures inside DWG files

Preview pictures inside DWG files are BMP pictures or PNG pictures, as the preview notes show. You can pull the picture out with LibreDWG's dwgbmp tool when you cannot open the drawing.

13 What Is the DWG File Format Made Of? The facts we could check, and what Autodesk has not published

About the values we list

The facts about DWG files come from Autodesk's pages, Wikipedia, FileInfo, the Library of Congress and IANA. They also come from our own checks of 141 files and six databases. We read them in October 2026. Autodesk's compatibility article is dated May 2026, and its TrustedDWG blog post was last changed in December 2025.

Wikipedia lists AutoCAD 2018 to 2025 for the 2018 format, and Autodesk lists 2018 to 2027. Where two sources differ, we follow the newer Autodesk page.

What we left out

We did not test AutoCAD, AutoCAD LT, DWG TrueView, Autodesk Viewer, AutoCAD Web, any other CAD program or the SysTools and Freeviewer products. We did not read the Open Design Alliance document line by line, and we name no prices.

Our 141 sample files came from the LibreDWG test collection. We do not know who made any of them. The results show what free tools do, and not what AutoCAD does.

Sources checked

For the format and the rules, see the Autodesk file format compatibility article, the Autodesk introduction to DWG files and the Autodesk DWG viewer page. Also see the Autodesk TrustedDWG blog post and the Autodesk Security Options help.

For the extension and the records, see FileInfo for DWG, the Library of Congress format description and the IANA registration. For the unofficial specification, see the Open Design Alliance PDF. For the background, see the Wikipedia article on DWG.

Every count and label came from the files and scripts in our research folder.