# What is EPUBCheck?

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

EPUBCheck is the official, open-source validator for EPUB files, maintained by the W3C and released under the BSD-3-Clause licence. It reports every problem as a message with a code such as RSC-005 and a severity: fatal, error, warning, info or usage.

## Why it matters when you convert a PDF to EPUB

A valid EPUB is the best guarantee that a converted book opens everywhere, including stores and Send to Kindle. EPUBLab validates the EPUBs of its public benchmark with EPUBCheck 5.4.0 and publishes the result, and our EPUBCheck reference explains every message.

EPUBCheck's exit code is 1 when it finds errors and 0 otherwise; --failonwarnings makes warnings fail too. Usage messages are off by default and appear with -u (--usage).