.MP4 file signature
A MPEG-4 media file file begins with the bytes 66 74 79 70 (ftyp in ASCII) at offset 4 (0x4). The signature is the same whatever the file is named, which is why a renamed or extensionless MP4 file is still recognisable.
Signature
| Offset | Hex | ASCII |
|---|---|---|
| 4 | 66 74 79 70Bytes 8 to 11 name the brand: isom, mp42, M4V, avc1, and for .mov, qt . | ftyp |
MIME type: video/mp4 · usual extensions: .mp4
How to check a file's signature
Without installing anything: drop the file on our detector. It reads the bytes in your browser and shows the hex dump alongside the verdict; nothing is uploaded.
macOS and Linux (Terminal):
xxd -l 32 file.mp4
file file.mp4Windows (PowerShell):
Format-Hex -Path file.mp4 -Count 32This signature sits at offset 4, so look there rather than at the first bytes: xxd -s 4 -l 16 file.mp4.
If the bytes do not match
A file named .mp4 whose first bytes are something else is not a MP4 file. The common explanations are a wrong or deliberately misleading extension, a download that saved an HTML error page instead of the file, or a container such as ZIP wrapping the real content. The detector reports what the bytes say; the MP4 guide explains what a genuine file should contain.