What is an EPUB file?

EPUB is a container: a wrapper that holds streams encoded by something else. It is used for handing a finished file over.

The extension is .epub, and the full name is Electronic Publication. Both matter less than what the file can hold, which is what the rest of this page is about.

W3C published it in 2007. The specification is EPUB 3.3.

Age is worth knowing here for one practical reason: the older a format is, the more programs have had time to learn it.

It is published in full, so anyone can implement it from the document rather than by inspection, which is why it turns up in so many programs and why files written twenty years ago still open. A published specification is not the same thing as a royalty-free one: where a format wraps a codec, the patent licensing is a separate question the standard does not settle.

EPUB is not limited to one page, which matters when converting to something that is: one conversion produces one file holding a single page — the first one, unless the page is something the converter lets you pick.

EPUB carries optional encryption.

An encrypted file has to be unlocked before anything can convert it, here or anywhere else — a password is not something a converter can work around, and one that claimed to could not be trusted with the file either.

Calibre, Apple Books and Kobo read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

No browser reads it.

That is the single most common reason to convert it: not that the format is bad, but that the place you want to show the file cannot read it.

EPUB can hold document properties.

Whether any of it survives a conversion depends entirely on the target, and the honest answer is usually "some of it".

EPUB is meant to be opened and changed. Keep the file in this format for as long as the work is going on, and export from it whenever a finished copy is needed.

A PDF is a set of fixed pages. An EPUB is not paginated at all — it is a book’s text and structure, and the reading application decides where pages fall based on the screen, the font and the size the reader chose.

That is why a novel is readable on a phone and a PDF of the same novel is not. It is also why page numbers in an EPUB are approximate or absent: there is no page one until somebody opens the book on a particular device at a particular size, which is disconcerting the first time you try to cite something.

Rename an EPUB to .zip and open it: inside are HTML files, one per chapter or section, a stylesheet, the images, the embedded fonts, and two index documents — a manifest listing everything and a navigation document providing the table of contents.

That is not an analogy. EPUB 3 is HTML5 and CSS, the same technology as a web page, and that choice is why the format is so easy to produce, convert and repair. It also means a badly made EPUB has the same problems as a badly made web page — inline styles fighting the reader’s settings, fixed pixel sizes, and text that will not reflow because somebody laid it out in a table.

Not every book can reflow. A children’s picture book, a cookbook, a comic, a textbook with diagrams beside the text — these depend on the relationship between the picture and the words, and letting the words move breaks the book.

For those, EPUB has a fixed-layout mode where each page is a defined size and everything sits at a coordinate. It works, and it gives up the property that makes EPUB worth having: on a phone, a fixed-layout page designed for a tablet is unreadable without pinching. Choose it because the content genuinely requires it, not because a designer wanted control.

The format has no encryption of its own. What locks a book is a scheme applied on top, and there are two in common use — Adobe’s, which is what most libraries and independent shops issue, and one used by individual retailers.

This is worth understanding because it explains an otherwise confusing situation: an EPUB from one shop opens in any reader, and one from another needs a specific application and an account. Both are EPUB files. Library loans are the most visible case, since the expiry is enforced by the DRM rather than by anything in the book. Removing it is a separate question with legal answers that differ by country, and not something this site does.

Apple Books opens EPUB natively on an iPhone, iPad or Mac. Google Play Books accepts an upload and syncs. Calibre is the desktop standard and manages a library as well as reading it. Thorium, Kobo, Moon+ and a dozen others do it well.

And Kindles now accept EPUB directly through Send to Kindle, which removed the format’s only significant compatibility gap. Amazon converts on their side, so what appears on the device is a Kindle format, but the file you send is an EPUB and no conversion is needed at your end.

Because the text is real text with real structure, a screen reader can navigate an EPUB by heading, read it aloud, and let a reader change font, size, spacing and colours arbitrarily. For dyslexic readers, a large-print requirement or anyone using assistive technology, this is the difference between a book that works and one that does not.

A PDF cannot match it — its accessibility depends on tags that must be added deliberately, and most PDFs have none. The European Accessibility Act is now pushing publishers to ship accessible EPUB by default, which mostly means real headings, alternative text on images, and not defeating reflow with fixed layouts that were not necessary.

Calibre converts to EPUB from almost anything and is the practical tool for a personal library. Sigil edits the innards directly, which is what you want for repairing a broken table of contents or fixing the styling on an EPUB that renders badly. Writing tools like Vellum and Pandoc produce them from manuscripts.

Two habits make the difference between a book that reads well and one that does not. Use real heading structure rather than large bold text, because that is what builds the navigation. And avoid fixed pixel sizes in the stylesheet — a font size in pixels overrides the reader’s own setting, which is precisely the thing an EPUB exists to respect.