How do I split an EPUB file into chapters?

Use Sigil: open the EPUB, see chapter files in the Book Browser under Text/, delete chapters you don't want in each volume, regenerate the TOC (Tools → Table of Contents → Generate), save as a new EPUB. Or use the epubsplit Python package: pip install epubsplit, then epubsplit book.epub --output vol1.epub 1 2 3 4 5.

Why would I need to split an EPUB?

Common reasons: exceeding platform file size limits (KDP 650 MB, Apple Books/Kobo have similar caps), slow loading on older e-readers with very large files, publishing multi-volume series as separate products, or creating chapter sample previews.

Does Calibre have a split EPUB feature?

Calibre doesn't have a one-click split button, but its Edit Book tool lets you delete chapter HTML files manually and rebuild the TOC. The epubsplit Python package (pip install epubsplit) provides a command-line interface for selecting chapter ranges to extract into separate EPUB files.

What happens to images and fonts when I split an EPUB?

Images and fonts must be included in each split output file. Not all tools handle this automatically. After splitting, run EPUBCheck on each output to confirm no broken asset references. In Sigil, check that the Images/ and Fonts/ folders in each split EPUB contain all assets referenced by the included chapters.

How to Split an EPUB into Chapters or Smaller Files

Large EPUBs can be slow to load on some e-readers, and publishing platforms have file size limits. Here's how to split an EPUB into chapters or volumes using Calibre, Sigil, and epubsplit.

Why Split an EPUB?

File size limits: KDP has a 650 MB EPUB upload limit. Apple Books and Kobo have similar caps. Large illustrated or academic texts can exceed these.
Slow loading: Very large files (>50 MB) cause slow navigation on older Kindle and Kobo devices.
Publishing volumes: Serialized fiction or multi-volume nonfiction often ships as separate EPUB files.
Chapter samples: Sharing individual chapters as previews.

Method 1: Sigil (Most Precise)

Open the EPUB in Sigil.
In the Book Browser panel, see all chapter files under Text/.
To create Volume 1: select chapters to keep, right-click unwanted chapters → Delete.
Go to Tools → Table of Contents → Generate Table of Contents to rebuild the TOC.
Save as a new EPUB name (File → Save As).
Reopen the original EPUB, delete Volume 1 chapters, save as Volume 2.

Method 2: epubsplit (Command Line)

The epubsplit Python tool splits EPUBs by chapter from the command line:

# Install
pip install epubsplit

# List chapters in the EPUB
epubsplit book.epub --listfiles

# Extract chapters 1–10 as Volume 1
epubsplit book.epub --output vol1.epub 1 2 3 4 5 6 7 8 9 10

# Extract chapters 11–20 as Volume 2
epubsplit book.epub --output vol2.epub 11 12 13 14 15 16 17 18 19 20

epubsplit correctly updates the OPF manifest, spine, and NCX/NAV TOC for each output file.

Method 3: Calibre's Edit Book

Right-click the EPUB in Calibre → Edit book.
In the file list, identify chapter HTML files (e.g., chapter01.xhtml).
Delete the files not needed in this volume.
Update the OPF spine and manifest, regenerate the TOC.
Save as a new EPUB. Repeat for each volume.

After Splitting: Verify Each Part

Run EPUBCheck on each output file to confirm valid structure.
Open in an e-reader to verify TOC and chapter navigation work.
Check that images and embedded fonts are included in each split file — they may link from only one chapter and not carry over automatically.

Reduce File Size Without Splitting

If the goal is size reduction (not volume separation):

Compress images: In Calibre's editor → Tools → Compress images losslessly. Often reduces size 30–60% for image-heavy books.
Remove embedded fonts: Embedded fonts add several MB; removing them reduces size if the target platform substitutes fonts.

Converting a PDF to EPUB? toolkit.bot detects headings and generates chapter structure automatically.

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