sylvain durand

Sideload books to a Kindle with Calibre

I recently had the opportunity to play for a few days with an Amazon Kindle Paperwhite (2021). If the reader itself is of excellent quality, the counterpart is its very closed ecosystem, turned around the Amazon store and a proprietary format with DRM.

Fortunately, the fantastic Calibre software allows you to use your Kindle with your own library of books and take full advantage of your e-reader.

While everything works “out of the box”, over time I have discovered a number of tricks that allow me to have books that are as well formatted as those in the official store, a tidy library, and good quality covers.

This article will show you how to:

Converting ebooks to the KFX format

Why using the KFX format?

Since the launch of the first Kindle, several successive ebook formats have been used by Amazon for its Kindle:

Below you can see three examples between mobi and kfx. In mobi, the first line on the second paragraph is too widely spaced, and there is no ligature on “firm”. You may notice that the fifth line of the last paragraph is not justified at all!

Differences between mobi and kfx format

KFX output with Calibre

KFX is, unfortunately, a closed language, not documented, and which requires Kindle Previewer 3, available under Windows or MacOS only.

Once this software is installed, we can just use Calibre with the KFX Output plugin. To do so, in Calibre, go to Preferences → Plugins, click on the Get new plugins button and get KFX Output. To export to KFX by default, go to Preferences → Behavior and select it as the preferred output format.

Now all you have to do is convert your books! As you will see, the conversion is quite slow: on a 2021 MacBook Pro 14, the conversion of a small novel takes 48 seconds, ten times longer than for mobi or azw3.

Importing metadata from the Kindle store

Amazon’s Kindle store offers overall very good quality metadata (author, title, series, summary…) and covers, which can be easily downloaded when adding a book manually.


Let’s start by getting the Kindle High-res Covers plugin: as before, in Preferences → Plugins, click on the Get new plugins button and get Kindle hig-res covers.

Then, in Preferences → Metadata download, make sure both and Kindle high-res cover are checked (and uncheck the others if necessary). Then, click on, then Configure selected source: select your prefered country and choose “Amazon servers” in Server to get data from.


Now, to get the metadata of a book, you only need to find its Kindle ID. It is visible on the Amazon website, when you visit the page displaying the Kindle edition of the book, under the name “ASIN”. It is a sequence of ten characters beginning with a B (otherwise, you are not on the page of a kindle ebook but of a paper book).

In Calibre, just select your book and click on Edit metadata. Delete all the existing fields, and in the Ids field, put amazon:BXXXXXXXXX (to be replaced by the one you found). Then click on Download metadata: Calibre will propose you metadata and covers.

Cover on lock screen

Since 2021, the Kindle can finally display the current book cover when the e-reader is turned off. This option is activated on the tablet. To have a nice rendering, it is necessary that the cover has the same ratio as the reader.

The Prettify cover plugin for Calibre simply allows you to resize the covers, with several options depending on the format of the cover.

After installation, you just have to remember to specify the resolution of the e-reader (1236×1648 for the Kindle Paperwhite 2021, 1072×1448 for the Kindle Paperwhite 2018).

Unfortunately, it appears that the Kindle leaves a margin around the screen, of a size I could not determine, so the cover will not be displayed in exactly the right resolution.