Ebook file formats, plainly
In your first week of publishing an ebook you will meet six or seven abbreviations, and they arrive with no hierarchy: EPUB, MOBI, AZW, KFX, KPF, PDF, and a few more depending on which article you landed on. Set out as a list, they look like a menu you are expected to order from.
They are not a menu. They do different jobs, and only one of those jobs is yours.
The one you make
EPUB. The open ebook standard, maintained by an industry body rather than owned by any one company. It is the file you build, the file you keep, and for most authors the only ebook file they will ever create. A reflowable book of words — a novel, a memoir, most nonfiction — is an EPUB, and that is the end of the format decision.
Everything else on the list falls into one of three piles, plus one format that does not belong in this family at all. Sorting them by role rather than by extension is what makes the soup stop being soup.
Formats a store makes out of your file
Several of those abbreviations name a store's own delivery format: the shape a book takes inside that company's apps and devices, built around its hardware, its syncing, its type rendering.
You do not create these. You upload an EPUB, the store's pipeline produces one, and you generally never see the result as a file on your computer. You are not asked to manage it, and you should not want to.
A delivery format is an internal engineering decision, and a company revises those whenever it suits. What stays stable is the file you handed over.
It settles a support question too. A file converted by the store is the file that store supports. If something renders oddly in their app, that is their conversion of your file, and the trail leads back to something in your file you can inspect and fix. Hand them a hand-built version of their own internal format instead, and you have quietly taken on the job of maintaining it.
Formats that have been retired
Some abbreviations name formats that are simply over. MOBI is the famous one — Amazon's old format, long superseded — and it still appears in advice telling you to keep one beside your EPUB. That advice is as old as the format.
The instruction outlives the format by years, which is why this pile is worth recognizing. An article from 2014 does not announce its age; it just tells you, confidently, to make a file nobody wants any more. The tell is the instruction itself. If a guide tells you to produce a specific company's file rather than a standard one, it is describing a world that has moved.
Packaging formats from a store's own tool
The third pile confuses people most, because these files look like output. Some stores publish their own authoring or preview tool, and it saves your work in a package of its own — an intermediate, a working state.
That is not a master. You typically cannot upload it anywhere else, and you often cannot get a clean manuscript back out of it. Treat it like any application's project file: fine inside that tool, useless as the thing your book lives in.
Your master is the manuscript. The EPUB is generated from it, every time, and never hand-edited afterward.
PDF, which is a different kind of thing
PDF is not an ebook format in the sense this page means. It is fixed-page: every line break and margin decided in advance, which is what you want for print and what a phone reader cannot use. Why a PDF is not an ebook covers the distinction.
In the Interior Formatter. One manuscript produces both editions — a print-ready PDF and an EPUB 3 that validates with zero errors and zero warnings under epubcheck — so the ebook you upload and the book you print came out of the same source. Start your ebook → · about the Interior Formatter
Three ways this goes wrong
Hunting for a separate file per store. Authors build a folder of near-identical exports, each tweaked for something they noticed once, and the copies drift apart within a month. Kindle, EPUB and the format alphabet makes that case in full.
Converting a converted file. The expensive one. Every conversion reads structure and writes structure, and what it cannot recognize it flattens: a heading becomes a bold paragraph, a footnote becomes loose text, the navigation gets rebuilt from guesswork. Do it twice and the flattening compounds. Nothing warns you, because each conversion succeeded on its own terms. Convert from the manuscript, never from an export.
Using a tool that produces an old format. Some converters still default to output built for a retired era. The file may open on your machine and still be the wrong thing to hand a store.
What to do instead
Make one EPUB from your manuscript. Validate it — What makes an EPUB pass validation explains what the checker looks for, and zero errors with zero warnings is a reachable bar. Then upload that same file everywhere and let each store run its own conversion.
Two exceptions are real: a book where the page is the design wants fixed layout, a variety of EPUB rather than a different family, and your cover is a separate file at separate proportions (Ebook cover size, and why it is not your print front).
One last caution. Which store accepts which format, and what each converts to internally, changes without announcement — including since this page was written. Treat any list of extensions, this one included, as background, and the store's own help page as the answer.
Make your ebook
The tools are free to use: bring your manuscript once and get both a print PDF and an EPUB out of the same source, so the two editions cannot drift apart. You only pay when you want the files without the preview watermark.
Next: What happens to your file after you upload it · More: Kindle, EPUB and the format alphabet · What makes an EPUB pass validation · Why your ebook is enormous, and what to do