Fun with Formats: mobi/Kindle

by | Jan 16, 2020

By David Kudler

The majority of people who read ebooks (in the English speaking world, at least) buy them on Amazon. That means, most likely, reading them on a Kindle.

But what “reading them on a Kindle” means varies a lot more than you may realize. And understanding those variations at least a little is important for an ebook publisher.

Mobi/Kindle: the Frankenstein Monster

If you’ve ever looked at the files on your Kindle or downloaded a file from KDP for preview purposes, you may have noticed a bewildering variety of file formats: mobi, azw, azw3, kfx, azk, and more.

In fact, those are all simply Kindle files optimized for different versions of Amazon’s ereaders — pre-2010 Kindles, old iPhones and iPads, Kindle apps for computers, newer Kindles, etc.

When you create a Kindle ebook using Kindle Previewer or download one from KDP, you’ll get a file that ends . mobi. Inside, it contains the two file formats that serve as the basis for that alphabet soup of variations:

  1. mobi — An old-style Palm Pilot Mobipocket file for old Kindles (aka MOBI7) and some older versions of Kindle apps
  2. kf8/kfx — A newer file format based on ePub3 that works on all new Kindles and Kindle Apps

So those “mobi” files are actually a Frankenstein monster, grafting an older (fairly limited) ebook format into Amazon’s variation of the current standard. The older mobi format has much less finesse than the newer kf8/kfx in terms of typography, images, metadata, and all around formatting. However, there are still millions of those older Kindles around, and so Amazon happily embraces and supports them.

The good thing about those Franken-files is they should load on most new and old Kindles and Kindle apps.

The not-so-good thing is that there’s no way (currently) to edit those files directly. So if you spot any content or formatting problems, or have updates you’d like to integrate, you’ll either have to work from the original file (whatever format that may have been in) and go through the conversion process from scratch, or convert from mobi to an editable format like ePub.

From a publishing point of view, there’s probably not a huge amount you need to be worried about with regards to the differences between the two embedded formats — if your book is simple, with few or no images and relatively straightforward formatting.

If you have inset images that text wraps around, drop-caps, tables, indented verse, or other fancy grace notes, you’ll need to add some @media queries that allow you to optimize your ebook’s CSS for both old and new Kindles. (That’s a longer, more technical post; I’ll cover that next time.)

Creating and Editing Kindle Files

There are essentially four ways to create a Kindle file:

  1. Convert using a computer app such as Calibre, Kindle Previewer, or the Kindle Creator app
  2. Export from a writing/publishing app like InDesign, Sigil, Scrivener, Jutoh, etc.
  3. Upload your base document (ie, Word doc or ePub file) to KDP
  4. Use the KindleGen Java applet in your computer’s command line (if you’re geeky like that)

In reality, of course, they all amount to the same thing, since the conversion apps and KDP all use KindleGen. In general, you should use whichever tool best fits into your workflow.

Nonetheless, there are a few distinctions that are worth considering.

First of all, just using the KDP interface is the simplest. It’s something you’re probably already doing — unless you’re forgoing 10% of your largest single retail outlet for the ease of using an aggregator like PublishDrive or Smashwords. It will always be running the latest version of KindleGen, so no updating. I usually recommend to my clients that they create a valid ePub file and upload that — you’ll generally get the results you expect.

One time when I use the Kindle Previewer is when I’m embedding fonts. There’s a somewhat higher incidence of success. So if you’ve got some pretty typography — and you’re sure it’s properly embedded and you have the right to use it — consider converting your ePub3 file to mobi format using the Previewer app, and then uploading that file to KDP. It’s still an iffy proposition, since Amazon doesn’t seem to want custom fonts. Still, it’s worth a shot.

As I have said, you can’t directly edit a Kindle file. However, if you find something in the file you want to change, and you don’t want to go through the whole process of reconverting from the original file(s), there are some options. Both Sigil and Calibre allow you to convert the Franken-file into an ePub file; in Calibre, the function is built in, while in Sigil, you need a plugin. All you’re really doing in either case is opening up the KF8/KFX ePub-variation component of the mobi file in an ePub editor. Make the changes, reconvert to Kindle format, and you’re good to go!

Next time, I’ll talk through the finer points of using @media queries to refine (or rather, simplify) the way that your ebook will display on older Kindles while allowing you to make it pretty on newer ones.
Photo: BigStockPhoto

tbd advanced publishing starter kit


  1. Jemima Pett

    You can now upload an ePub file? That’s news to me… I dint’ see any chance of that in October. Great!

    It also explains why I sometimes get review copies of books I can’t read on my kindle (paperwhite) – they have funny formats like asz. I guess if I read them on my iPad in the Kindle App they’ll load, provided I update the App.

    Good news, thanks!

  2. Michael W. Perry, medical writer

    I’ve had good luck exporting print book content from InDesign to reflowable epub 3 and sending the result to Amazon. That said, it would be great if Amazon and Adobe would make like friends and build Kindle export into ID, perhaps including a few features in kf8/kfx not present in epub3. That would let us check the formatting before sending it off.

    Hopefully, Affinity Publisher, a competitor to InDesign that is far less expensive, will soon add epub export. Its next update is expected to include import from ID’s IDML format. It has Mac, Windows and iPad versions.



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