Amazon's e-book formats: AZW3, KFX and MOBI

Kindle books are not ePub files. Amazon has used its own formats throughout, and it has changed them more than once.

Which one you are holding matters, because two of the three are effectively historical and one of those has quietly stopped working in the place people expect it to.

MOBI, the one that is finished

MOBI came from Mobipocket, a company Amazon bought in 2005, and it was the Kindle format for years. Files carry a .mobi or .prc extension.

It is over. Amazon ended MOBI support in Send to Kindle, phasing it out through late 2023, so you can no longer email a MOBI file to your own Kindle.

A MOBI file already on a device keeps working. What you cannot do is get a new one there by the usual route, and that is why old advice to convert it to MOBI first now sends people down a path that ends in nothing.

AZW3, the one most people mean

AZW3, also called KF8, arrived with the Kindle Fire in 2011. It is built on HTML5 and CSS3, which makes it far closer to ePub than MOBI ever was, and it supports the styling, fonts and layout that modern books expect.

If you convert a book for a Kindle yourself, AZW3 is usually the format to choose. Calibre writes it, and Kindles read it.

KFX, the current one

KFX is Amazon’s newer format and the one most books from the Kindle store now arrive in. It brought typographic improvements such as better hyphenation and justification, along with features like Page Flip.

It is also the most closed of the three. Calibre needs a plugin to handle it, and support arrives later than it does for the older formats.

What this means in practice

You have an ePub and want it on a Kindle. Email it to your Send to Kindle address. Amazon converts it on the way. This has worked since 2022 and is the supported route.

You have a MOBI file. Convert it to ePub or AZW3 with Calibre. Do not try to send it.

You are converting a book yourself. AZW3 is the safe target.

You bought the book from Amazon. It is KFX, it carries DRM, and it is meant to stay inside Amazon’s apps.

The wider point

None of this is about file formats being difficult. It is about one shop using formats nobody else uses, which is a commercial decision rather than a technical one.

Everywhere else, ePub is the answer, and the reasons it became the standard are the ordinary ones: it is open, it is maintained as a web standard, and anyone may implement it.