Which file formats a Kindle actually accepts

Amazon has changed the rules twice in the last few years and most of the advice online still describes the old ones. Here’s where it stands now.

What you can send today

Send to Kindle — the web uploader, the desktop app, the browser extension and the email address — accepts:

• EPUB (.epub)
• PDF (.pdf)
• Microsoft Word (.doc, .docx)
• Plain text and rich text (.txt, .rtf)
• HTML (.htm, .html)
• Images (.jpg, .jpeg, .png, .gif, .bmp)

Anything else has to be converted before it will go across.

MOBI is finished

Amazon stopped accepting MOBI for new documents at the end of 2022. This catches people out, because MOBI was the Kindle format for fifteen years and half the converters online still push it at you as the obvious pick.

Anything already in your library keeps working. But convert a PDF to MOBI today, try to send it across, and it bounces. That’s far and away the most common reason a conversion “doesn’t work”. The file is fine. Amazon just won’t take it any more.

EPUB is the format to use now. Amazon converts it to their own internal format on receipt, which is exactly what they used to do with MOBI.

AZW3, KFX and the formats you never see

You will run into these names when reading about Kindle internals:

• AZW3 (KF8) — the format Amazon converted your books into for older devices.
• KFX — the current one, with better typography and layout.

Amazon makes both, on their end, from whatever you send them. You don’t need to produce either, and on a modern account you generally can’t sideload them to any useful effect. Send EPUB and let Amazon handle it. Their converter knows their hardware better than anything you’ll run at home.

Why send EPUB rather than the PDF itself

A Kindle will happily display a PDF, so if that’s all you’ve tried you might wonder what the fuss is about. The trouble is the fixed page. On a six-inch screen it gets shrunk until the type is too small to read, leaving you pinching and panning, or turning the thing sideways to read two paragraphs at a time.

An EPUB reflows. The text rearranges itself around whatever screen and font size you’ve picked, so the Kindle can show it the way it shows a book you bought from them. Which is the thing the device is actually good at.

Converting to EPUB first is what buys you that, and it’s why the quality of the conversion matters. It has to rebuild paragraphs and chapters the PDF never recorded in the first place.

Sending a PDF anyway

Sometimes you should. If the layout is the content, reflowing it wrecks it. Sheet music, knitting patterns, circuit diagrams, annotated maps: send those as PDFs and read them in landscape.

Anything that’s basically prose, convert it.