EPUB File Size Analyzer

This tool answers one question: where is the weight in your EPUB? Because an EPUB is a ZIP archive of HTML chapters, stylesheets, images, fonts and metadata, the file you ship is the sum of those parts — and almost always one or two parts dominate. Authors and self-publishers use the analyzer before submitting to Kindle Direct Publishing, Apple Books or Kobo, where oversized files raise delivery fees or get flagged in upload. KDP, for context, suggests staying under roughly 50 MB pre-conversion, so seeing your real breakdown helps you decide whether trimming is even necessary.

It also fits production and conversion work. If you assemble EPUBs from a template or a Word-to-EPUB pipeline, the analyzer shows whether a cover scan, embedded font family, or duplicated stylesheet has quietly inflated every build. For editors and admins preparing a file for a store, sharing platform, or long-term archive, it gives a plain inventory of what is inside before the file leaves your hands.

It is a diagnostic, not a cleanup tool. Treat it as the step that tells you what to fix — which image to re-export, which font to drop — so that a later compression or rebuild pass is targeted rather than guesswork.

Drop or select an EPUB and everything runs in your browser. The tool unzips the archive locally, walks every file entry, and classifies each by extension into images (JPG, PNG, GIF, WebP, SVG), HTML/XHTML, stylesheets, fonts (TTF, OTF, WOFF, WOFF2), audio, video, or other. Nothing is uploaded, and nothing remains once you close the tab.

You get three headline numbers — ZIP size on disk, total uncompressed size, and file count — followed by a by-type bar chart with each category's size, file count, and share of the total. Below that is a largest-files list you can sort by size, name, or type, with per-file sizes and percentages, so a single heavy chapter image is easy to spot. A plain-text report listing every file by size is available to download for your records.

Two honest limits: it reads .epub only, and it is strictly read-only — it does not compress, edit, or repackage anything. It also reports uncompressed byte sizes per file, which is the right number for deciding what to shrink, but means the per-file figures will not add up exactly to the smaller ZIP-on-disk size.

The most common surprise is the gap between uncompressed and ZIP size. Text, HTML, and CSS compress heavily inside the archive, so a category that looks large in the breakdown may add almost nothing to the file you actually ship. Images, fonts, and audio are usually already compressed and barely shrink further — so when the goal is a smaller download, weigh categories by how compressible they are, not only by their raw share of the bar chart.

Classification is by file extension, so an oddly named or extensionless asset lands in Other rather than its true category, and a misnamed image counted as Other will understate your image total. DRM-protected files are detected through META-INF/encryption.xml and reported as unanalyzable, since their contents are encrypted. The largest-files view also shows the top entries rather than the full archive, which is fine for finding offenders but is not a complete manifest.

Finally, the tool flags nothing as a problem on its own. A large fonts total may be entirely intentional for a design-heavy title; an unused font is something only you can confirm by checking what your CSS actually references.

Start with the headline numbers: confirm the file count matches what you expect and that the ZIP size lines up with the file on disk. Then scan the by-type bars for anything disproportionate — fonts eating a large share often means a full family was embedded when one weight was needed, and a dominant image total points you toward re-exporting or resizing the heaviest few files shown in the largest-files list.

Cross-check the top entries by name. A 4 MB cover, a stray full-resolution photo, or a duplicated stylesheet is usually obvious once sorted by size. Note that this view is read-only: to act on what you find, move to a compression or rebuild step, then re-run the analyzer to confirm the weight actually dropped. Keep the downloaded report as a before-and-after record.

For files headed to an official store, a client, or any legal or contractual delivery, treat this as guidance rather than proof. Open the finished EPUB in a real reader or validator after any change, and verify the file size and rendering against the destination's own requirements before you submit.