How EPUB compression works and what to expect
An EPUB is a ZIP archive of HTML/XHTML chapters, CSS, fonts and images. This tool opens that archive in your browser, optimises the HTML, CSS and images it finds, and repacks it with maximum DEFLATE compression.
What actually gets smaller
After compression you see the original size, new size, percentage saved, and how many image, HTML and CSS files were optimised.
- JPEG images: usually the biggest win, because illustrated books and comics are mostly image data. Other image formats are kept as they are.
- HTML/XHTML chapters and CSS — whitespace and comments are stripped; text-only novels are already small, so savings there are modest.
- The archive itself — rebuilt at DEFLATE level 9, the highest ZIP compression level.
Measured result from a real test book
We built an EPUB with one long text chapter and three 3000×2000 photos saved as high-quality JPEGs, then ran it through this tool.
- The saving came almost entirely from the photos. A text-only novel will shrink far less.
- JPEGs are resized to at most 1600 px and re-saved at 80% quality only when that makes them smaller. PNG, GIF and WebP images keep their original bytes.
- The output opened as a valid EPUB archive with the mimetype file first.
| Before | After | |
|---|---|---|
| File size | 11.9 MB | 0.91 MB (92% smaller) |
| Each image | 3000×2000 px, about 3.9 MB | 1600×1067 px, about 0.3 MB |
Size limit
The tool accepts EPUB files up to 200 MB. Files above that are rejected before processing because repacking a book that large in a browser tab can exhaust memory, especially on phones. If your book is larger, try a desktop browser with other tabs closed after first removing unnecessary embedded fonts or media with an EPUB editor such as Calibre or Sigil.
Sending a smaller book to Kindle
Amazon's Send to Kindle accepts EPUB files, but the email route has a lower size limit than the Send to Kindle app and website. Compressing image-heavy EPUBs is the usual way to get under it. Check Amazon's current Send to Kindle documentation for the exact limits, as they change.
If the saving is small
A text-only novel that is already a few hundred kilobytes has little left to remove. Large savings come from books with many images or unoptimised illustrations. Embedded fonts are kept as-is so the book still looks the way the publisher intended.