Glossary
What is an EPUB file?
EPUB is the open standard file format for ebooks. Inside, it is a zip archive of HTML and CSS — which is why the text reflows to fit any screen and any font size the reader chooses.
What is actually in the file
Rename a .epub to .zip and open it, and you will find web pages: one HTML file per chapter, a stylesheet, the images, and a manifest listing the reading order. An ebook reader is, for practical purposes, a small browser.
That is the whole reason ebook text reflows. There are no pages in the file. The reader makes pages at display time from the screen size and the font settings, which is why an ebook has no fixed page count and why “page 74” means nothing in one.
Who reads EPUB
- Apple Books, Kobo, Nook, Google Play Books — all natively.
- Most library systems, through OverDrive and Libby.
- Kindle, since 2022. KDP takes EPUB and converts it on upload. The old MOBI format was retired for new uploads.
EPUB or PDF?
A PDF has fixed pages, which is exactly right for printing and exactly wrong for a phone — reading one means pinching and panning across a page designed for paper. EPUB is right for anything read on a screen. Most books that sell well are published as both, and they are two different files rather than one converted at the last minute.
If you are converting one to the other, read ebook to PDF first — the direction matters more than the tool.
Related terms
- Reflowable textReflowable text re-wraps itself to fit whatever screen and font size the reader is using. It is how nearly every ebook works, and it is why an ebook has no fixed page count.
- KDPKDP is Amazon’s free self-publishing platform. You upload a manuscript and a cover, set a price, and the book is on sale in the Kindle store — and, if you want, as a print-on-demand paperback — usually within 72 hours.
- Trim sizeTrim size is the finished width and height of a printed book after the pages have been cut. It is chosen before the interior is laid out, because changing it later re-flows every page and changes the page count.
- DRMDRM is a lock applied to an ebook file that ties it to one store account, so it cannot be copied to another device or app. It is chosen at upload, and on most stores the choice cannot be reversed for that edition.
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