How to Reduce EPUB File Size

EPUB files are ZIP archives. Most are larger than they need to be — not because of text (that's tiny) but because of images and how the ZIP was packed. Here's where the bloat actually comes from and how to fix it.

Where the Size Comes From

• Unoptimized images — The biggest factor by far. A raw PNG from any decent camera is 8–12MB. A cover image should be under 500KB. Most EPUBs contain several images that have never been compressed.
• Wrong image format — PNG is lossless and large. For photos and gradients, JPEG is 5–10x smaller with no perceptible difference. PNG is only appropriate for images with transparency or hard edges (logos, diagrams with text).
• Embedded fonts — Each font file adds 30KB–2MB. A "regular + bold + italic + bold-italic" family multiplies that by four. Full CJK fonts can hit 10MB each.
• Poor ZIP compression — EPUBs are ZIPs. Some generators use compression level 0 (store-only). Re-zipping at maximum compression saves 10–20% with zero content changes.
• Orphaned files — Deleted chapters, old cover revisions, and editor temp files often stay in the ZIP. They're invisible in readers but add to the file size.

Method 1: Epublys Compressor

1. Open the Compress EPUB tool
2. Upload your EPUB (up to 10MB free)
3. Pick a compression level:
   • Low — Re-ZIP only. Fast, 10–20% savings, no image changes
   • Medium — Image optimization + re-ZIP. 30–50% savings, visually identical
   • Maximum — Aggressive image compression + font subsetting + re-ZIP. 50–70% savings, minor quality reduction at very high zoom
4. Download and run the EPUB validator to confirm it's still well-formed

Method 2: Manual (Maximum Control)

Rename book.epub to book.zip, unzip it, then:

• Compress images with ImageOptim (Mac), Squoosh (web), or convert -quality 75 via ImageMagick
• Subset fonts: pyftsubset font.ttf --text-file=book_text.txt --output-file=font-subset.ttf — a subsetted font contains only the glyphs actually used in the book
• Delete any files not listed in the OPF manifest
• Repackage: zip -X -r book-compressed.epub mimetype META-INF OEBPS

The mimetype file must be first and uncompressed — that's required by the EPUB spec. The -X flag strips extra file attributes that bloat the archive.

Image-Specific Notes

• Cover images: most readers cap display at 1200×1800px. Anything larger is wasted resolution — resize down before embedding.
• Interior illustrations: 800px wide at 96dpi is enough for any e-reader screen. High-res only matters if you're also producing a print edition.
• WEBP: supported in EPUB 3 by Kobo and Apple Books, gives 25–35% better compression than JPEG. Avoid it if your readers use older devices.

The EPUB validator also reports orphaned files (in the ZIP but not in the manifest) and unused manifest items — both are safe to delete.

• Merge EPUB files — merge first, then compress the combined file
• Convert EPUB to PDF — a smaller source EPUB produces a smaller PDF
• Validate your EPUB after compression