Optimize EPUB files — locally, in your browser
E-books are ZIP packages, and most ship under-compressed. This tool rebuilds the package with stronger compression — entry bytes untouched, DRM never touched.
What this tool does
The optimizer re-compresses every entry in the EPUB container and rewrites the package with the mimetype stored first and uncompressed, exactly as the OCF spec requires. Your chapters, styles and images are byte-identical — only the wrapping gets tighter.
DRM-protected books are detected via the encryption manifest and rejected with a clear message. DRM removal is never attempted, at all, ever.
How to optimize an EPUB without uploading it
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Add the file. Drop your .epub into the home-page tool.
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Optimize. The container is re-packed with stronger compression, byte-lossless.
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Download locally. The smaller EPUB is saved straight back to your disk.
Supported operations
This list is generated from CompressKit's capability registry — only implemented and tested operations appear here.
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Optimize the file package
Lossless re-compression of the container; entry order and mimetype preserved.
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Inspect file details
OCF container + OPF metadata: title, author, spine size.
Frequently asked questions
- Is my e-book uploaded anywhere?
- No. The package is rebuilt in your browser’s worker. Nothing leaves your device.
- Does it remove DRM?
- No — never. DRM-protected EPUBs are detected and rejected with a clear message. The tool only ever processes DRM-free books.
- Will my reader still open the optimized file?
- Yes — the container is rebuilt to spec (mimetype first, uncompressed) and entry bytes are identical. Savings come purely from better compression.
Ready to try it?
The EPUB optimizer tool runs on this site with no upload step — your file is processed by your own browser and saved back to your device.
Open the EPUB optimizer tool