Why is my EPUB file so large?

The most common causes: (1) High-resolution images from the original PDF embedded at full size. (2) Rasterized page images — some converters can't extract text and embed each page as an image instead. (3) Large embedded fonts. Extract the .epub as a ZIP to see which files are biggest. A text-only EPUB should be under 1 MB; over 10 MB usually means images or rasterized pages.

How do I reduce the file size of an EPUB?

Use Calibre's Polish books feature (right-click → Polish books) to recompress images and remove unused files. For manual control, extract the .epub as a ZIP, run jpegoptim or optipng on the images folder, then repack. To remove large embedded fonts: Calibre Convert → Look & Feel → check 'Remove all embedded fonts'. Typical reduction: 50–80% for image-heavy files.

What is the normal file size for an EPUB?

Text-only novels and reports: 50–500 KB. Text with a few diagrams or charts: 500 KB–3 MB. Image-heavy textbooks or illustrated guides: 3–20 MB. Over 50 MB almost always indicates rasterized page images or unoptimized high-resolution photos — worth reconverting from the original PDF with a tool that extracts real text.

My EPUB has images instead of text — how do I fix it?

Your PDF converter rasterized the pages instead of extracting text. The fix is to reconvert using a tool with proper text extraction. toolkit.bot/pdf2epub performs layout analysis and extracts real searchable text from PDFs — the output is reflowable text, not page images. Go to toolkit.bot/pdf2epub, upload your PDF, and download a properly-converted EPUB.

Why Is My EPUB So Large? How to Reduce EPUB File Size

A typical text-only EPUB should be under 1 MB. If your EPUB is 20, 50, or 100 MB, something is wrong — usually unoptimized images, large embedded fonts, or inefficient PDF conversion that carried over rasterized content. Here's how to diagnose and fix it.

Why EPUBs Become Large

High-resolution images: The most common cause. If your PDF had full-resolution figures, diagrams, or photos, these are extracted and embedded in the EPUB at original size. A single 300 DPI print-quality image can be 5–15 MB.
Rasterized pages: Some PDF converters can't extract text and instead rasterize each page as an image. The EPUB ends up as a series of images — essentially a scanned PDF repackaged. Huge file, unsearchable, no text reflow.
Embedded fonts: Large custom font families can add 1–5 MB each. An EPUB with 5 embedded fonts may carry 15 MB of font data.
Duplicate images: Bad converters sometimes embed the same image multiple times or include both the original and a thumbnail.

How to Check What's Inside Your EPUB

An EPUB is a ZIP file. Rename it from .epub to .zip and extract it. The largest files in the OEBPS/ or EPUB/ folder are your problem items — typically in images/ or fonts/.

# On Mac/Linux:
cp book.epub book.zip
unzip book.zip -d book-contents
du -sh book-contents/EPUB/images/*  # see which images are large
du -sh book-contents/EPUB/fonts/*   # see font sizes

Method 1: Compress Images with Calibre

Open Calibre, add your .epub.
Right-click → Convert books → Convert individually.
Under Comic Options, check "No processing" is off.
Under Look & Feel → Layout, set image compression to your preferred level.
Alternatively, use the Polish books feature (right-click → Polish books) — it recompresses images and removes unused files without changing content.

Method 2: Compress Images Manually

Extract the EPUB (rename to .zip, unzip). Run images through an optimizer:

# Install: brew install jpegoptim optipng (Mac) or apt-get (Linux)
jpegoptim --max=85 book-contents/EPUB/images/*.jpg
optipng -o5 book-contents/EPUB/images/*.png

# Repack to EPUB
cd book-contents
zip -X ../book-optimized.epub mimetype
zip -rg ../book-optimized.epub . -x mimetype

Method 3: Remove Embedded Fonts

In Calibre's Convert dialog → Look & Feel → check Remove all embedded fonts. This replaces the EPUB's fonts with the reader's system fonts. Appropriate if the custom fonts aren't essential to the reading experience (e.g., body text fonts on a technical document).

What If the EPUB Contains Rasterized Pages?

If your PDF-to-EPUB conversion produced rasterized page images (common with Calibre on complex PDFs), the fix is to reconvert using a better tool. toolkit.bot performs proper text extraction with layout analysis — the output contains real text, not page images. File sizes are typically 95–99% smaller than rasterized conversions.

Go to toolkit.bot/pdf2epub, upload your PDF, and download a properly-converted EPUB with real text.

Target File Sizes

Text-only books (novels, reports): 50–500 KB
Text with a few diagrams: 500 KB–3 MB
Image-heavy books (textbooks, illustrated): 3–20 MB
Over 50 MB: Almost always contains rasterized pages or unoptimized images — worth reconverting

Getting a large, image-heavy EPUB from your PDF? toolkit.bot extracts real text for a clean, lightweight EPUB.

Convert PDF to EPUB →