.EPUB file signature
A EPUB document file begins with the bytes 50 4B 03 04 (PK.. in ASCII) at the start of the file. The signature is the same whatever the file is named, which is why a renamed or extensionless EPUB file is still recognisable.
Signature
| Offset | Hex | ASCII |
|---|---|---|
| 0 | 50 4B 03 04A ZIP whose first member is mimetype: application/epub+zip, stored uncompressed at offset 30. | PK.. |
MIME type: application/epub+zip · usual extensions: .epub
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.epub
file file.epubWindows (PowerShell):
Format-Hex -Path file.epub -Count 32If the bytes do not match
A file named .epub whose first bytes are something else is not a EPUB 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 EPUB guide explains what a genuine file should contain.