Kobo EPUB Checker

This tool fits one moment in self-publishing: the gap between finishing an EPUB and uploading it to Kobo Writing Life or handing it to an aggregator that distributes to the Kobo store. It applies the rules Kobo publishes (a 100 MB file limit, EPUB validation on every upload, covers that are not GIFs and sit on their own page) so you can fix problems before an upload is refused or a book ships looking wrong on a Kobo e-reader.

It suits independent authors and small presses checking one title at a time, including exports from Word or Google Docs.

Kobo's rules come from the Kobo Writing Life help centre and Kobo's EPUB guidelines on GitHub; the report links both.

Drop or select a single .epub file and the check runs in your browser; nothing is uploaded, and closing the tab discards everything. DRM-protected files are refused rather than inspected.

The report covers: the file size against Kobo Writing Life's 100 MB limit; the EPUB structure, with every problem listed; whether a cover is marked, whether it is a GIF (not supported for covers), and whether a page in the book shows it; any images in GIF or unidentified formats, since Kobo lists JPG, PNG, SVG and WebP; the title, language and author; and a note on the separate store cover. Each line is a pass, warning, fail or note, and the text report lists Kobo's source pages.

Scope: it does not render the book, check fonts or CSS, or preview how Kobo's apps will display it, and it cannot see the store cover you upload separately.

A clean report is not approval: Kobo validates every upload itself, and this tool runs only the checks EPUBCheck most often fails books on.

The size limit is strict, but Kobo does not say how it counts a megabyte, so a file between 100,000,000 and 104,857,600 bytes gets a warning rather than a pass. The cover ratio is only a suggestion: Kobo's guidelines suggest 3:4 but also note that store covers average 800 × 1224 pixels, closer to 2:3, so the tool does not fail a cover on its ratio.

A cover set as a CSS background image is not found, and Kobo's own cover extraction does not support that method either.

Fix failures first: a file over 100 MB (compress the images or split the book), structure errors, a GIF cover, or a missing title or language. Then act on the warnings, such as a cover without its own page or GIF images inside the book. Re-run after each change.

Then prepare the store cover as .jpg or .png, around 300 DPI and no more than 5 MB; Kobo names its standard eReader screen, 1448 by 1072 pixels, as a good target. Open the book on a Kobo device or app if you can.

Kobo's own validation during upload is the final word, and store rules change, so check the linked pages when a number matters.