JPEG XL file signature
Magic number at the start of the file · ....JXL ....
JPEG XL appears in two forms: a container starting with a JXL box (....JXL ....) or a bare codestream beginning FF 0A. Both decode to the same modern image.
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 00 0C 4A 58 4C 20 0D 0A 87 0A (the text ....JXL ....), it is a JPEG XL file.
What each byte of 00 00 00 0C means
Here is the signature byte by byte, the way a hex editor shows it: the position in the file, the value in hex and in decimal, and the character that value stands for in ASCII. Bytes with no printable character show a dot.
| Byte offset | Hex | Decimal | ASCII |
|---|---|---|---|
| 0 | 00 | 0 | . |
| 1 | 00 | 0 | . |
| 2 | 00 | 0 | . |
| 3 | 0C | 12 | . |
| 4 | 4A | 74 | J |
| 5 | 58 | 88 | X |
| 6 | 4C | 76 | L |
| 7 | 20 | 32 | |
| 8 | 0D | 13 | . |
| 9 | 0A | 10 | . |
| 10 | 87 | 135 | . |
| 11 | 0A | 10 | . |
Signature variants
JPEG XL appears with more than one byte pattern. These all identify the same family:
| Hex signature | Offset | Note |
|---|---|---|
| 00 00 00 0C 4A 58 4C 20 0D 0A 87 0A | 0 | container (JXL box) |
| FF 0A | 0 | raw codestream |
Formats that use the 00 00 00 0C signature
These file types in our database carry this signature:
How to check a file's signature
You can read the bytes of any file yourself. They are shown in hex, the same way this page lists them.
Linux & macOS
- xxd -l 16 example.jxl
- hexdump -C -n 16 example.jxl
- file example.jxl
Windows (PowerShell)
- Format-Hex -Path example.jxl -Count 16
Python
- open("example.jxl","rb").read(12).hex()
In your browser
- Drop the file into the WhatFileType identifier, which reads the signature without uploading it.
Frequently asked questions
What is the JPEG XL file signature?
JPEG XL files start with the hex bytes 00 00 00 0C 4A 58 4C 20 0D 0A 87 0A (....JXL .... 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.jxl on Linux or macOS, or Format-Hex -Path example.jxl -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the JPEG XL signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine JPEG XL file has these exact bytes; a file with the wrong bytes is not really JPEG XL, whatever its name says.
Related signatures
Sources
Reuse this signature
This entry is part of the WhatFileType file signature reference, published under a CC BY 4.0 licence with the full provenance for every entry, a copy-paste embed and a JSON export. Credit WhatFileType and the data is yours to republish.