# EPUBCheck PKG-016: uppercase .EPUB extension, how to fix it

Source: https://epubcc.com/epubcheck/pkg-016/ (updated 2026-11-04)

EPUBCheck PKG-016 is a warning message: “Use only lowercase characters for the EPUB file extension for maximum compatibility.” The file extension is written in uppercase (.EPUB); lowercase .epub is the most compatible. The content of the book is not affected, only how some apps and stores recognise the file.

- Official message (EPUBCheck 5.4.0): Use only lowercase characters for the EPUB file extension for maximum compatibility.
- Default severity: Warning
- EPUBCheck's suggestion: For maximum compatibility, use ".epub".

## Typical causes

- A file saved or renamed on a system or tool that uppercased the extension.
- A file name copied from an old archive or a download system that uses uppercase names.

## How to fix it

1. Rename the file so it ends in .epub, in lowercase.
2. Nothing inside the EPUB needs to change; validate the renamed file if you want a clean report.
3. If a script produces the file, fix the extension in the script so the next build is right.

## When it's not this

If the extension is something other than .epub (.zip, .epub3), the message is PKG-017.

## Sources

- EPUBCheck 5.4.0 message texts (MessageBundle.properties, w3c/epubcheck on GitHub): https://github.com/w3c/epubcheck/blob/v5.4.0/src/main/resources/com/adobe/epubcheck/messages/MessageBundle.properties (read 2026-11-04)

Message text and severity: EPUBCheck (W3C, BSD-3-Clause). Explanation: EpubCC.