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EPUB
What is an EPUB file?
The open e-book standard. Reflows to fit the screen, which is what makes it readable on a phone.
What EPUB is
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.
Where EPUB came from
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.
The specification is public
Anyone can implement it without asking permission or paying a licence, which is why it turns up in so many programs and why files written twenty years ago still open.
It can hold several pages in one file
A EPUB file is not limited to one page, which matters when converting to something that is: a target that holds one image produces one file per page, and the numbering it chooses is worth checking before you send it on.
What it does to protect itself
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.
What opens EPUB
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.
Opening it in a browser
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.
What it can carry besides the content
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".
It is a working format
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.
The facts, in one place
| Extension | .epub |
|---|---|
| Media type | application/epub+zip |
| Published by | W3C |
| First published | 2007 |
| Specification | EPUB 3.3 |