MOBI vs AZW3: what's the difference?

Published November 4, 2026 · 6 min read

MOBI and AZW3 are both Amazon Kindle formats, but MOBI is the original one from the 2000s, with very limited styling, while AZW3 (also called KF8) is its 2011 successor, built on modern HTML and CSS. If you copy books to a Kindle over USB, use AZW3; if you use Send to Kindle, send EPUB instead of either. Below: how the two compare, how AZW and KFX fit in, and how to convert between them.

MOBI vs AZW3 at a glance

In short, AZW3 does everything MOBI does and much more. The only thing MOBI still has going for it is that the oldest Kindles can open it. For how both compare with the open standard, see EPUB vs MOBI vs AZW3.

What is MOBI?

MOBI started as the format of Mobipocket Reader, a reading app for PDAs and early phones. When Amazon bought Mobipocket in 2005, it used the format for the first Kindle in 2007. Files may end in .mobi or .prc; inside, they are the same thing.

The format is built on HTML from the late 1990s. It understands headings, bold and italic text, basic indents, page breaks and images, but there are no real stylesheets. Anything fancier, such as a custom font, a sidebar or a two-column table, is lost or flattened. Today the old variant is often called MOBI 6, to tell it apart from the newer KF8 content that can live in a file with the same extension (more on that below).

Amazon has been phasing MOBI out for years: its KindleGen tool, which made MOBI files, was discontinued, and Send to Kindle stopped accepting MOBI files in 2022. Kindles still open MOBI files that are already on them.

What is AZW3 (KF8)?

Amazon introduced Kindle Format 8, or KF8, with the Kindle Fire in late 2011, and then brought it to its e-ink Kindles and reading apps through software updates. Files in this format usually end in .azw3.

KF8 is much closer to EPUB than to MOBI. A book is made of HTML chapters, CSS stylesheets, images and, optionally, embedded fonts, so publishers can keep the same design they use in their EPUB editions. It also supports fixed-layout books, where every page is drawn exactly as designed, which is what comics, manga and picture books need.

AZW3 is the best format for copying books to a Kindle over a USB cable: the Kindle shows the cover in the library, applies the book's fonts and styles, and lets you change the font as usual.

Why some MOBI files are really AZW3

The file extension doesn't always tell you which format you have. When Amazon introduced KF8, it needed books that worked on new and old Kindles alike, so KindleGen produced combined files: one .mobi file holding both a MOBI 6 version and a KF8 version of the same book. A new Kindle reads the KF8 part and an old one falls back to the MOBI 6 part.

Calibre, the free ebook manager, can still make all three kinds. Its MOBI output has a MOBI file type setting with the values old (MOBI 6 only, the default at the time of writing), both (combined) and new (KF8 only). Its separate AZW3 output always writes KF8.

So if a .mobi file shows embedded fonts and modern styling on your Kindle, it probably carries a KF8 part inside. Our MOBI and AZW3 to EPUB converter reads all of these, old, new and combined, and picks the richer KF8 version when a file has both.

AZW vs AZW3

Despite the similar names, AZW is not an older AZW3. AZW is the extension Amazon used for Kindle Store books in the first years of the Kindle: in practice, a MOBI file with Amazon's DRM. AZW3 is the newer KF8 format, and on its own it has no DRM.

The .azw extension has also been used loosely over the years, so a file with that name may hold MOBI, KF8 or even KFX content. If it came from the Kindle Store, it is almost certainly protected, and neither our tools nor Calibre will convert it.

KFX vs AZW3

KFX is the format Amazon introduced in 2015 together with its Enhanced Typesetting, which improves hyphenation, kerning and page layout. Many books bought in the Kindle Store today are delivered as KFX, and Send to Kindle may convert your EPUB to a Kindle format behind the scenes.

The difference for you is that KFX is not a format you normally create or keep. Amazon's tools produce it, it usually comes with DRM, and most converters, including ours and Calibre on its own, don't read it. AZW3 is the most capable Kindle format you can make and edit yourself.

Which format should you use?

• Sending books with Send to Kindle (email, app or website): neither. Send an EPUB. At the time of writing, Amazon's list of accepted formats includes EPUB, PDF and Word files, but not MOBI or AZW3. Amazon converts the EPUB for your device and syncs it to your Kindle apps. Step by step: how to put EPUB books on a Kindle.
• Copying books over a USB cable: AZW3. Drop the file in the Kindle's documents folder. It keeps the book's styling and is read by every Kindle released since 2012.
• A first- or second-generation Kindle or a Kindle DX: MOBI, since these models predate KF8.
• Reading anywhere other than a Kindle (Kobo, Apple Books, Google Play Books): EPUB. Those apps don't open MOBI or AZW3.
• Keeping a library for the long term: EPUB as the master copy. You can always make an AZW3 from it in a minute, while going the other way can lose details.

If you just want to look inside a MOBI or AZW3 file before deciding, our online ebook reader opens both in the browser, without installing anything.

How to convert MOBI to AZW3 (or to EPUB)

MOBI or EPUB to AZW3, with Calibre

1. Install Calibre from its official website and open it.
2. Click Add books and choose your MOBI or EPUB files.
3. Select the book and click Convert books.
4. In the top-right corner of the conversion window, set Output format to AZW3 and click OK.
5. When the Jobs counter in the bottom-right corner goes back to 0, connect your Kindle by USB and use Send to device, or right-click the book, choose Save to disk and copy the AZW3 file to the Kindle's documents folder yourself.

MOBI or AZW3 to EPUB, in your browser

To read your Kindle files on any other device, or to send them with Send to Kindle, convert them to EPUB. Our converter does it without uploading the file: choose the MOBI or AZW3 file and the EPUB downloads a few seconds later. The full walkthrough, including Calibre, is in how to convert AZW3 to EPUB.

Frequently asked questions

Is AZW3 better than MOBI?

Yes, for every Kindle made since 2012. AZW3 supports stylesheets, embedded fonts and fixed layouts that MOBI can't handle.

Can I just rename a .mobi file to .azw3?

No. Renaming doesn't change what's inside. A MOBI 6 file stays MOBI 6 with a new name. Convert it with Calibre instead.

Does Kindle Paperwhite support AZW3?

Yes. Every Paperwhite, like every current Kindle, reads AZW3 files copied over USB. It also still reads MOBI files.

Why won't Send to Kindle accept my MOBI or AZW3 file?

Because neither is an accepted format at the time of writing. Convert the file to EPUB and send that. If an EPUB is rejected too, see EPUB won't send to Kindle.

Is AZW3 the same as KF8?

Yes. KF8 (Kindle Format 8) is the name of the format, and .azw3 is the extension its files usually have.

Will converting MOBI to AZW3 improve the formatting?

Not much. A MOBI 6 file has already lost the original styling, so the AZW3 can only keep what is left. If you have the EPUB the MOBI was made from, convert that instead.

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