# EPUBCheck PKG-016: Use only lowercase characters for the…

Source: https://epublab.com/epubcheck/pkg-016/ (updated 2026-11-03)

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.

- 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.

## How to fix it

- Rename the file to end in .epub. Nothing inside the EPUB needs to change.

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