ICO file signature
Magic number at the start of the file
A Windows icon starts with a 6-byte ICONDIR header: two zero bytes, then the image type 01 00 (1 = icon). A cursor file uses 02 00 in the same position.
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 00 00 01 00, it is a ICO file.
Formats that use the 00 00 01 00 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.ico
- hexdump -C -n 16 example.ico
- file example.ico
Windows (PowerShell)
- Format-Hex -Path example.ico -Count 16
Python
- open("example.ico","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 ICO file signature?
ICO files start with the hex bytes 00 00 01 00. 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.ico on Linux or macOS, or Format-Hex -Path example.ico -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the ICO signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine ICO file has these exact bytes; a file with the wrong bytes is not really ICO, whatever its name says.