1Drop your EPUB eBooks into the box above, or pick them from your device — there is nothing to install and no account to create.
2Choose how hard to squeeze: a quality or bitrate target, and optionally smaller dimensions if size matters more to you than detail.
3Start the job and let the workers re-pack the file; your original is never modified in place.
4Compare the new size against the original, then download the compressed EPUB file.
Compress EPUB FAQ
What do I get back, and can I control the trade-off?
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A single compressed file per input, at a quality or bitrate level you choose rather than a fixed default. If the result is not small enough, changing the dimensions or the duration will do more than pushing the quality slider further down.
How does Compress EPUB actually make the file smaller?
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Concretely, an eBook is a zip of XHTML and images, and the text is already tiny — so essentially all of the saving comes from recompressing the embedded artwork and dropping font subsets the book never uses, with the markup left untouched. Nothing about it depends on you installing software: the job runs server-side and hands back a finished file.
Is there anything specific to EPUB I should know before compressing?
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Yes — the book is a zip of XHTML with a manifest, so the text costs almost nothing and every decision that matters is about the embedded images and font subsets. It is worth knowing before you overwrite an original you might need again.
What is the most common mistake when compressing EPUB eBooks?
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Compressing the same file repeatedly. Each pass on a lossy format is a fresh generation of loss and the savings shrink while the damage accumulates. Go from the original to your target in one step, and keep the original.
Which eBook formats can I upload to Compress EPUB?
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EPUB is read natively, and MOBI, AZW3, FB2 and CBZ are accepted and normalised into the same internal representation first. The container is identified from its header, so a book renamed by a sync tool still opens.
Is there a file size limit on Compress EPUB?
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Yes: free accounts process eBooks up to 25 MB each, which is orders of magnitude above any normal novel; the OPF manifest, the spine order and the embedded fonts are handled by the same EPUB toolchain a desktop reader uses. A text-only book is a few hundred kilobytes, so the only files that approach the cap are illustrated ones — comics, cookbooks and textbooks with full-page artwork.
Will Compress EPUB damage the reflow or the table of contents?
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No. An eBook is XHTML plus a manifest, not a set of fixed pages, so the structure is edited as structure: the spine order, the NCX or NAV table of contents and the internal anchors are rewritten consistently rather than re-rendered.
Can I process several EPUB eBooks at once?
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Yes. A batch is queued in parallel with the same settings, which is the case that matters here — people arrive with a whole library rather than one book.
Why does a EPUB site host Compress EPUB?
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EPUB.to is built around the open eBook format — XHTML in a zip, reflowable by design, and readable on hardware nobody has thought about the layout for. A book is a zip full of markup, images and fonts, so almost every job people bring to an eBook site is really a job on one of those things. Compress EPUB runs on the same upload and the same account as the conversions because that is where it is needed.
What should I do with the result once Compress EPUB is finished?
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The converter on this site moves books between EPUB, MOBI, AZW3, PDF and DOCX, so a title ends up on whatever hardware is actually going to read it. Doing that after Compress EPUB means the conversion is made from the version you settled on, not from the one you were still fixing.
Is Compress EPUB here the same tool the sibling sites run?
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The engines are shared — the same toolchain, the same workers, the same limits. What a EPUB site adds is a view on reflow: which of these operations a reflowable book genuinely supports, and which ones only make sense on the fixed-layout media inside it. It also starts from one fact about the format this site is named after: the book is a zip of XHTML with a manifest, so the text costs almost nothing and every decision that matters is about the embedded images and font subsets.
Do I need an account, and does anything get kept?
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No account, and nothing is kept: uploads are deleted from the workers shortly after the job finishes, nothing is read and nothing is indexed. Free accounts exist for history and batch size, not for access.
