Kobo EPUB Checker

This tool fits one specific 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 reads your file and reports, in plain language, whether the pieces Kobo expects are present — a cover image, a title, a language tag, a navigable table of contents, a defined reading order, and a unique identifier — so you can fix gaps before an upload rejects or a book ships looking wrong on a Kobo e-reader.

It suits independent authors preparing a single title, small presses checking a batch one file at a time, and anyone who converted a manuscript from Word, Google Docs, or another tool and wants confirmation the export held together. It is a pre-flight check for publishing, not a general EPUB editor or converter — if you only need to see metadata, validate strict spec conformance, or fix the cover, sibling tools cover those jobs more directly.

Drop or select a single .epub file and the checker parses it in your browser. It reads the EPUB container directly — the OPF package, the mimetype file, the spine, and the NCX or nav document — then runs a fixed set of Kobo-oriented checks and shows each as pass, warning, or fail with a short explanation. There is no upload step; the file is read locally and nothing leaves the tab. You can then download a plain-text readiness report summarizing the counts and every line item.

The checks cover: cover image presence and weight, title, author (dc:creator), language (dc:language), publisher, identifier (ISBN or UUID), file size against the 1 GB Kobo Writing Life ceiling, table of contents, the mimetype file, and a non-empty spine. It also stops and warns if the file appears DRM-protected rather than attempting to read it. What it does not do: it will not edit, repair, or re-package your EPUB, it will not validate against the full EPUB specification, and it produces a readiness report — not a corrected file.

The most important limit is the cover check. Kobo recommends roughly 1600 × 2560 px, but this tool measures the cover's file weight in kilobytes, not its pixel dimensions. A small low-resolution cover saved at high quality can pass the size check while still being too small for Kobo, so treat a green cover result as “a cover exists and isn’t tiny,” not “the cover meets Kobo’s resolution.” Verify dimensions yourself in an image viewer.

Other pitfalls: it checks one EPUB at a time, so there is no bulk mode. It looks for the presence of fields and structure, not their correctness — a wrong language code or a placeholder title will pass. The DRM line always reports clean once a file is readable, because protected files are rejected earlier rather than scored. And the warnings are advisory: a missing publisher or TOC will not block a Kobo upload, but they do affect how the book presents.

Start with the summary banner: any failed checks are the items most likely to cause a rejection or a broken reading experience, so clear those first — typically a missing language tag, an empty spine, a broken or absent cover reference, or a bad mimetype. Then work through the warnings (author, publisher, identifier, table of contents, large file size) and decide which matter for your title. Open the cover image separately to confirm its actual pixel dimensions, and sanity-check that the title, author, and language strings shown back to you are the real values, not leftover defaults from your export tool.

Because this is an unofficial pre-check built against Kobo’s published guidance, it is not endorsed by Kobo and will not catch everything their pipeline does. For any title you are publishing commercially, or where contractual or financial terms ride on the file, treat a clean report as a starting point: run the file through Kobo Writing Life’s own upload preview and review the rendered result on a Kobo device or app before you confirm distribution.