BMP file signature
Magic number at the start of the file · BM
A Windows bitmap starts with the two ASCII bytes BM (for bitmap), followed by the 32-bit file size. Only two bytes long, this is one of the shortest common signatures.
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 42 4D (the text BM), it is a BMP file.
Formats that use the 42 4D 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.bmp
- hexdump -C -n 16 example.bmp
- file example.bmp
Windows (PowerShell)
- Format-Hex -Path example.bmp -Count 16
Python
- open("example.bmp","rb").read(2).hex()
In your browser
- Drop the file into the WhatFileType identifier, which reads the signature without uploading it.
Frequently asked questions
What is the BMP file signature?
BMP files start with the hex bytes 42 4D (BM 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.bmp on Linux or macOS, or Format-Hex -Path example.bmp -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the BMP signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine BMP file has these exact bytes; a file with the wrong bytes is not really BMP, whatever its name says.