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Max 20 files • Max file size: 200MB
Accepted formats: .epub
Image optimization + ZIP repacking for smaller ebooks
EPUB file compression reduces the size of ebook publication files (.epub) by optimizing embedded images (cover, illustrations, photos) and repacking the ZIP container with maximum compression. Unlike PDF compression which handles complex page layouts, EPUB compression is straightforward: EPUB is a ZIP archive containing HTML chapters, CSS stylesheets, and images—the images are 40-90% of file size in most ebooks. Compression targets three areas: image dimensions (resize 2000x3000px covers to 1200px), image quality (JPEG 60-90%), and ZIP repacking (DEFLATE level 9 instead of default level 6).
The EPUB format specification (developed by International Digital Publishing Forum) defines EPUB as a ZIP-based container with specific file structure requirements, making it highly compressible. Most EPUB file size comes from: 1) High-resolution cover images (often 2000x3000px, 5MB+), 2) Uncompressed illustrations/photos throughout chapters, 3) Suboptimal ZIP compression used by ebook creation tools. E-reader screens (Kindle Paperwhite: 1072x1448px, Kobo Clara: 1072x1448px) don't need 2000px+ images—resizing to 1200px provides identical visual quality on devices while reducing file size by 50-70%.
This compressor works with both EPUB 2 (original standard with NCX navigation) and EPUB 3 (modern standard with nav.xhtml). Both versions fully supported, compression preserves version-specific features, chapter structure, table of contents, and all formatting intact.
Browser-based convenience with better compression than Calibre
Calibre ebook management software has a built-in compression feature ("Polish books" → "Compress images"), but Reddit discussions (#2 rank, 79 traffic: "How to compress an epub : r/Calibre") show users seeking better compression or simpler methods.
This web tool provides one-click compression optimized for e-readers, with presets tailored to Kindle, Kobo, Nook screen resolutions—no Calibre installation, no library management, just upload and download. All compression happens in your browser—EPUB files never leave your device.
Drag and drop EPUB files onto the page, or click "Choose EPUB Files" to browse. Up to 20 files at once (200MB maximum per file). File info automatically displayed: EPUB version (EPUB 2 or EPUB 3), image count, estimated chapter count, current file size. Batch processing available. Files processed locally using browser-image-compression v2.0.2 + JSZip—files never leave your browser, ensuring privacy for unpublished manuscripts or DRM-free purchased ebooks.
Five presets: Lossless (no image compression, only repack ZIP with DEFLATE level 9, 5-15% reduction—for archival quality). Balanced (80% quality, 1920px, 40-60% reduction—recommended default, high-quality images preserved). E-Reader Optimized (75% quality, 1200px, 50-70% reduction—recommended for Kindle, Kobo, Nook—images resized to match e-reader screen resolutions 1072-1264px typical, e-ink displays hide compression artifacts). Maximum (60% quality, 800px, 60-80% reduction—removes metadata author/publisher/dates, for email sharing). Target Size: Auto-adjust quality to hit specific size limit (e.g., compress 100MB → 25MB for email, uses binary search algorithm 6 attempts max).
Processing time: 5-15 seconds for text-heavy books (few images), 20-60 seconds for illustrated books, 60-120 seconds for image-heavy books (photography, comics, textbooks with 100+ images). Files get "_compressed" suffix (e.g., my-novel_compressed.epub). Compatible with all e-readers (Kindle via Send to Kindle, Kobo, Nook, Apple Books). Files never returned larger (if compression doesn't help, warning shown). EPUB structure preserved: chapter order, table of contents, navigation, formatting—only images optimized. EPUB extracted as ZIP (JSZip), images compressed in parallel via Web Workers, then repacked as new EPUB with DEFLATE level 9 and mimetype file correctly placed first (uncompressed, as required by EPUB specification).
Image resolution is the key to EPUB compression
In most EPUB ebooks, images are 40-90% of total file size: cover image (often 2000x3000px, 5MB+), chapter illustrations, author photos, charts, decorative graphics. Example breakdown for typical 80MB illustrated novel: cover image 6MB (7.5%), 50 chapter illustrations at 1.5MB each (93.75%), HTML/CSS text 2MB (2.5%). Compressing images = compressing the EPUB. ebooks.stackexchange.com discussions (rank #7, 28 traffic) focus on "How can I reduce the resolution of images in an epub or mobi file?"—proving technical publishers recognize image optimization as the key compression method.
E-reader screen resolutions: Kindle Paperwhite (1072x1448px, 300 PPI), Kobo Clara HD (1072x1448px, 300 PPI), Nook GlowLight (768x1024px, 212 PPI). A 1200px image is MORE than sufficient for any e-reader. Images in EPUB are displayed in the flow of text (not fullscreen like comic books). A cover image at 1200x1800px (1.5MB) looks identical to 2000x3000px (6MB) on Kindle—but saves 4.5MB (75% reduction). This tool's E-Reader Optimized preset (1200px max, 75% JPEG quality) is calibrated for e-ink displays, which have lower contrast than LCD screens—compression artifacts that are visible on phones/tablets are imperceptible on Kindle/Kobo.
15MB EPUB (13MB cover at 2000x3000px, 2MB text). E-Reader preset compresses cover to 1.5MB (1200x1800px, 75% quality) → Total: 3.5MB (77% reduction).
80MB EPUB (6MB cover, 70MB illustrations at 1800x1200px each). E-Reader preset resizes to 1200x800px, 75% quality → Total: 25MB (69% reduction).
200MB EPUB (150 high-res photos at 2400x1600px, 1-2MB each). Balanced preset (1920px, 80% quality) → Total: 80MB (60% reduction). Maximum preset (800px, 60%) → 40MB (80% reduction) but visible quality loss.
50-70% reduction vs Calibre's 20-40%
Calibre is THE ebook management software (Reddit discussions at #2 rank 79 traffic), with a built-in compression feature called "Polish books" that reduces image quality and cleans metadata. However, Reddit users at #2 ask "How to compress an epub?" and "Is there a simple way to reduce the file size of an EPUB?"—proving Calibre users want MORE compression or SIMPLER methods than Calibre's built-in feature. Calibre's "Polish books" typically achieves 20-40% reduction (JPEG quality adjustment only). This tool achieves 50-70% reduction (E-Reader preset: quality + resize) by addressing image dimensions, which Calibre doesn't optimize.
File size limits and delivery fees for self-published authors
For authors uploading to Amazon KDP: Large EPUBs (50-200MB with photos/illustrations) incur delivery fees ($0.15/MB over 10MB). A 100MB EPUB costs $13.50 in delivery fees per sale at 70% royalty—compressing to 25MB with E-Reader preset reduces fees to $2.25, saving $11.25 per sale.
Compress before uploading to KDP, not after—KDP converts EPUB to Kindle format automatically, and if your source file is large, the converted Kindle file is also large (triggering delivery fees).
Apple Books, Google Play, and Kobo don't charge delivery fees, but large files = slow downloads for readers on cellular connections, abandoned downloads, storage complaints. Compress to <50mb best experience.< for p reader> Everything about compressing EPUB files Yes, EPUB files compress very well—typically 30-70% reduction depending on content. EPUB is a ZIP archive containing HTML chapters and images. Most file size comes from high-resolution images (cover, illustrations). Compressing images to e-reader-appropriate resolutions (1200px) reduces file size 50-70% with no visible quality loss on Kindle, Kobo, Nook. Image-heavy books (photography, comics, illustrated textbooks) achieve 60-80% compression. Text-heavy novels with cover only: 10-30% reduction. Upload EPUB to this web tool, select E-Reader Optimized preset (50-70% reduction, perfect for Kindle/Kobo), download compressed file. Alternative: Use Calibre's "Polish books" feature (right-click book → Polish books → Compress images), but Calibre only adjusts JPEG quality—doesn't resize images, so achieves less compression (20-40% vs 50-70%). This tool combines quality reduction + image resizing for maximum compression without quality loss on e-readers. Use EPUB compression tool with dimension settings: E-Reader preset resizes images to 1200px max (perfect for Kindle's 1072x1448px screen), Maximum preset to 800px (for basic Kindles). Manual method: 1) Extract EPUB as ZIP, 2) Compress images with Photoshop/GIMP, 3) Repackage as EPUB (complex). Image resolution is #1 factor in EPUB file size—reducing 2000px cover to 1200px saves 75% on that image alone (ebooks.stackexchange.com #7, 28 traffic, 7 answers). Slight quality reduction for images, text and formatting completely unaffected. E-Reader preset (75% JPEG quality, 1200px) is imperceptible on e-ink displays (Kindle, Kobo)—e-readers have lower contrast than LCD screens, hiding compression artifacts. Balanced preset (80% quality, 1920px) preserves high quality for tablets/phones. You control quality level via presets. Lossless mode (no image compression) available if you need zero quality loss (only repack ZIP, 5-15% reduction). In Calibre: 1) Right-click book in library, 2) Select "Polish books", 3) Check "Compress images" option, 4) Set JPEG quality (80-90 recommended), 5) Click OK. Calibre compresses images but doesn't resize dimensions, limiting compression to 20-40%. For maximum compression (50-70%), use this web tool which resizes images to e-reader resolutions (Calibre lacks image resizing feature). Reddit #2 "How to compress an epub : r/Calibre" (79 traffic) shows users seeking more compression than Calibre offers. Typical: 30-70% reduction depending on content. Image-heavy books (photography, comics, illustrated textbooks): 60-80% reduction with Maximum preset. Text-heavy novels with cover only: 10-30% reduction. Example: 150MB comic book (100 full-page images) → 45MB with E-Reader preset (70% reduction), → 30MB with Maximum preset (80% reduction). Lossless mode (ZIP repack only): 5-15% reduction. Balanced preset (most use cases): 40-60% reduction. Main causes: 1) High-resolution cover image (often 2000x3000px, 5MB+), 2) Uncompressed illustrations/photos throughout book (1.5-3MB each if not optimized), 3) Embedded fonts (1-5MB), 4) Suboptimal ZIP compression by ebook creation tools. Images are 40-90% of EPUB size—compress images = compress EPUB. Check cover image resolution first (should be 1200x1800px max for e-readers). Solution: Use E-Reader preset to resize cover to 1200px (75% reduction on cover alone) + compress all illustrations. Yes, compress EPUB first (E-Reader preset), then: 1) Send to Kindle via email (Kindle converts automatically), or 2) Use Kindle app (iOS/Android/desktop) which supports EPUB directly, or 3) Convert to MOBI/AZW3 with Calibre. Most Kindle e-readers don't support EPUB natively, but Kindle app does. Compressing first ensures fast email delivery (<25mb "compress (40 amazon and attachment authors delivery epub fees for global kdp kindle").< limit) p reduces volume:> Kindle app (iOS/Android/desktop) supports EPUB as of recent updates. Most Kindle e-readers (Paperwhite, Oasis, basic Kindle) do NOT support EPUB—need conversion to MOBI/AZW3 via Calibre or Send to Kindle email (auto-converts). Kindle app = yes EPUB, Kindle device = no EPUB (need conversion). Compress EPUB before converting to reduce file size for both formats (smaller EPUB = smaller converted MOBI/AZW3). Amazon KDP technical limit: ~650MB. Recommended: <50mb critical: experience. files for optimal reader>10MB incur delivery fees ($0.15/MB over 10MB at 70% royalty). Example: 100MB EPUB costs $13.50 delivery fee per sale. Compress to <10mb $13.50 (<100mb (<50mb (savings: 2gb 300mb apple books: delivery eliminate fees google kobo life: limit limit.< maximum p per play preset recommended). sale). to varies with writing> Reduce size 30-70% by optimizing images for e-readers. Choose Lossless (5-15%, preserve all quality), Balanced (40-60%, recommended), E-Reader Optimized (50-70%, perfect for Kindle/Kobo/Nook), or Maximum (60-80%, smallest file). Better compression than Calibre's "Polish books" (50-70% vs 20-40%). Text and formatting always preserved—only images optimized. Files stay private in your browser, never uploaded. Perfect for self-published authors uploading to Amazon KDP (eliminate delivery fees) or Calibre users seeking maximum compression.Frequently Asked Questions
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