AutoCAD DWG file signature
Magic number at the start of the file · AC10
AutoCAD drawings begin with AC10 and two more digits that encode the version, for example AC1027 for the 2013 format. The AC stands for AutoCAD and the number maps to a release.
What this magic number means
A magic number is a short, fixed run of bytes at a known position that tells a program what a file really is, no matter what the filename says. If a file starts with the bytes 41 43 31 30 (the text AC10), it is a AutoCAD DWG file.
Formats that use the 41 43 31 30 signature
These file types in our database carry this signature:
How to check a file's signature
You can read the first bytes of any file yourself. The magic bytes are shown in hex, the same way this page lists them.
Linux & macOS
- xxd -l 16 example.dwg
- hexdump -C -n 16 example.dwg
- file example.dwg
Windows (PowerShell)
- Format-Hex -Path example.dwg -Count 16
Python
- open("example.dwg","rb").read(4).hex()
In your browser
- Drop the file into the WhatFileType identifier, which reads the signature without uploading it.
Frequently asked questions
What is the AutoCAD DWG file signature?
AutoCAD DWG files start with the hex bytes 41 43 31 30 (AC10 in ASCII). This magic number identifies the format regardless of the file's name or extension.
How do I check a file's magic number?
Open the file in a hex editor, or run a command such as xxd -l 16 example.dwg on Linux or macOS, or Format-Hex -Path example.dwg -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the AutoCAD DWG signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine AutoCAD DWG file has these exact bytes; a file with the wrong bytes is not really AutoCAD DWG, whatever its name says.