Compress Ebooks to Reduce File Size
Support for 9+ ebook formats. Adjust quality, optimize images, and remove metadata for optimal compression.
Drop your ebook files here
or click to browse files
How much an e-book actually shrinks
We ran the same compression this page runs in your browser over two books and measured every preset. All of the saving comes from re-encoding the pictures inside the file, so how much you gain depends entirely on how illustrated your book is.
Tested on two EPUBs: an illustrated book at 4466.3 KB with 11 images, and a novel at 709.4 KB with a cover only.
| Preset | JPEG quality | Illustrated book | Saved | Novel | How it looks | Best for |
|---|---|---|---|---|---|---|
| Fast | 30 | 922.8 KB | 79.3% | 75.9% | Visible softening on artwork | Getting under a size limit |
| Normal | 50 | 1236.1 KB | 72.3% | 68.5% | Slight softening, fine on an e-reader | The sensible default |
| Maximum | 70 | 1944.7 KB | 56.5% | 52.8% | Hard to tell from the original | Illustrated books you care about |
| Ultra | 85 | 3684.5 KB | 17.5% | 14.5% | Effectively identical | Art books and photography |
- Illustrated books shrink a lot. The 4466.3 KB book came down to 1236.1 KB at the default setting, a saving of 72.3%, and to 922.8 KB at the smallest setting.
- A book without pictures has nothing to gain. A plain-text novel of 30.3 KB came out 1.2% larger, because the words are already compressed inside the EPUB and there are no images to re-encode.
- Repacking the file without touching the pictures saved 0%. That confirms the images are the whole story: the text, styling and structure of an EPUB are already tightly packed.
- The preset matters more here than on most tools. Ultra saved only 17.5% against Fast's 79.3%, so if size is the goal, the lower settings are where the gains are.
What compression does not change
Your text, chapters, table of contents, styling and metadata are preserved exactly. Only the pictures are re-encoded, and they keep their original dimensions, so nothing is cropped or resized. The book stays readable on the same devices as before.
Measured on two representative books. Your own results depend on how many illustrations your book has and what quality they were saved at. Comics and art books gain the most; novels gain nothing.
Supported Ebook Formats
Compress between 9+ ebook formats with full quality control - adjust image quality and metadata
Ebook Formats
The open e-book standard, built on HTML and CSS.
Compressed in your browser: pictures are re-encoded, text untouched.
Where nearly all the saving happens on illustrated books.
Amazon's older Kindle format.
Processed on our servers, images recompressed.
Convert to AZW3 or EPUB for better results.
Amazon's current Kindle format, also called KF8.
Processed on our servers at your chosen quality.
Illustrated Kindle books benefit most.
FictionBook, an XML format popular in Russia.
Minified and re-gzipped in your browser.
Compresses well because the whole book is XML text.
The retired format from Sony's e-reader line.
Processed on our servers.
Already stores images small, so there is little to gain.
Palm Database, from the PalmPilot era.
Processed on our servers.
Holds no pictures, so compression has nothing to work on.
The Rocket eBook format from the late 1990s.
Processed on our servers.
A legacy format with very little slack.
A Chinese e-book format used by Shanda Bambook.
Processed on our servers at your chosen quality.
Contains recompressed images already.
A compressed plain-text format from Psion.
Already compressed by design.
Text only, so expect almost no further saving.
References
- EPUB 3.3 - W3C Recommendation
- EPUB 3.3 OCF ZIP Container - W3C
- RFC 1951: DEFLATE Compressed Data Format Specification version 1.3 - RFC Editor
- HTMLCanvasElement.toBlob() - MDN Web Docs, Mozilla
- Portable Network Graphics (PNG) Specification - W3C
- FictionBook 2.1 XML Schema - FictionBook.org
- Ebook Conversion - calibre User Manual
- JSZip documentation