Compress EPUB

Compress EPUB rebuilds an EPUB with maximum Deflate compression for its text files, drops system junk such as .DS_Store and __MACOSX, and can re-encode JPEG photos at 80% quality, then reports the sizes before and after. It suits a finished book that needs to be leaner: under Kobo Writing Life's 100 MB limit, cheaper on KDP's per-megabyte delivery cost, or easier to email.

Some authoring tools pack EPUBs with little or no compression, and a clean re-pack recovers that overhead without touching the reading experience.

Set expectations by what the book contains. Text already compresses well, so a text-heavy book that was packed properly changes by a few percent at most. The large files in most EPUBs are images, and without re-encoding they cannot get smaller, because JPEG and PNG data is already compressed. Photo-heavy books are where the JPEG option earns its keep.

Drop or select a single .epub file, decide whether to re-encode JPEG images, then click Compress EPUB. The tool reads the archive in your browser, writes a fresh one, and shows the original size, bytes saved and new size with a percentage. If the new file is smaller, you can download it with a -compressed suffix; if not, the tool says the book is already as small as it can make it and offers nothing to download. Nothing is uploaded, and nothing persists once the tab closes.

HTML, XHTML, CSS, XML, the package file, the NCX, SVG and TTF/OTF fonts are packed with maximum Deflate (level 9). Images (JPG, PNG, GIF, WebP, AVIF), audio, video and WOFF fonts are already compressed, so they are stored as they are rather than zipped again, which could only make them bigger. The mimetype file is written first and uncompressed, as the EPUB format requires, so the output stays a valid EPUB. With the JPEG option on, each JPEG is decoded and saved again at 80% quality, and the new version is kept only if it is smaller.

What it does not do: it does not resize images, change PNG, GIF or WebP images, subset or remove fonts, or rewrite your HTML and CSS.

DRM-protected EPUBs cannot be processed; the tool stops and says so rather than producing a broken file. The most common disappointment is expecting a dramatic shrink from a well-packed, text-heavy novel: there is little overhead to remove, and a result of a few percent, or none, is normal rather than a failure.

The JPEG option is a real quality trade. At 80% quality most photos look the same on an e-reader screen, but fine detail, gradients and text inside images can pick up artefacts, and the change is permanent in the compressed copy. Keep the original, and check image-heavy pages before you publish. Images saved as PNG are not re-encoded, and resizing oversized images before packaging usually saves far more than any setting here.

It also works on one EPUB at a time, takes longer on very large books, and will not repair a damaged EPUB.

Open the downloaded -compressed.epub in a reader and skim the table of contents, a few chapters, the cover, and any image-heavy or styled pages. Confirm fonts still render, links and navigation still jump correctly, and, if you used the JPEG option, that photos and images with text in them still look right. Compare the new size against the original to make sure the trade was worth it.

If the EPUB is bound for a store, run the relevant checker afterwards, and keep your original until the compressed copy reads cleanly.

For an official or commercial publication, have a second person check text, figures and metadata in the compressed copy.