Kindle books use the .azw format, which Amazon created specifically for their devices

Amazon's Kindle devices and apps read .azw files, a proprietary format that Amazon developed to work with their ecosystem. The ".azw" stands for "Amazon Whispernet," though the name is mostly historical now. When you buy a book from the Kindle Store, it arrives on your device as an .azw file, encrypted with digital rights management (DRM) so only your registered Amazon account can open it.

Amazon has released several versions of this format over the years. The original .azw was based on the open Mobipocket format but added Amazon's own protections and features. Later versions like .azw3 (also called KF8) support more advanced formatting, including better typography, embedded fonts, and interactive elements. Most Kindle devices and apps now use .azw3 for new releases, though older .azw files still work.

The practical result is that a Kindle book you buy cannot be read on a Kobo reader, an Apple Books app, or most other e-readers without conversion. That lock-in is intentional — it keeps readers invested in Amazon's platform and makes it harder to switch devices later.

Key Takeaways

  • Kindle books use .azw or .azw3 file formats, which are proprietary to Amazon and encrypted with DRM protection.
  • A Kindle book purchased from Amazon's store cannot be opened on other e-readers like Kobo or Nook without converting the file.
  • If you own a Kindle book, you can convert it to other formats using tools like Calibre, though this requires removing the DRM encryption first.
  • Self-published authors can upload .azw files directly to Kindle Direct Publishing, or upload other formats and let Amazon convert them automatically.
  • The .azw3 format supports richer formatting than older .azw, including custom fonts and interactive elements, but both work on modern Kindle devices.

How DRM encryption locks Kindle books to your account

Every Kindle book you purchase is encrypted with digital rights management (DRM), a technology that ties the file to your Amazon account. When you read a book to your Kindle device or app, the device checks that your account is authorized to read it. If you try to open the same .azw file on a different account or a non-Amazon device, it will not open.

This encryption is not a password you can remove yourself. It is baked into the file at the moment Amazon sells it to you. Amazon can remove it on their end if you request it, but they rarely do. The DRM exists to prevent unauthorized sharing and to enforce Amazon's terms of service, which say you own a license to read the book, not the book itself.

The practical effect is that your Kindle library is portable only within Amazon's ecosystem. You can read your books on a Kindle e-reader, a Kindle tablet, the Kindle app on your phone, or the Kindle app on your computer — but nowhere else without additional steps.

Converting Kindle books to other formats if you own them

If you want to read a Kindle book on a different device, you have two options: convert it yourself, or ask Amazon to remove the DRM.

The most common conversion tool is Calibre, a free, open-source e-book manager that runs on Windows, Mac, and Linux. Calibre can convert .azw and .azw3 files to .epub, .pdf, .mobi, and other formats that work on Kobo, Apple Books, or other readers. However, Calibre cannot remove DRM encryption on its own — you will need a separate plugin called DeDRM (Defective by Design Removal of Restrictions on Media), which is legal to use in most countries for books you own, though the legal status varies by jurisdiction.

The simpler route is to contact Amazon directly and ask them to remove the DRM from a specific title. Amazon does not advertise this option, but they will sometimes honor the request if you explain that you want to read the book on a different device. There is no may provide they will agree, and the process is not fast.

A third option is to buy DRM-free e-books from other sources instead. Many independent publishers, academic presses, and self-published authors sell .epub files directly from their websites without any encryption. Project Gutenberg offers over 70,000 free public-domain books in multiple formats, including .epub and .kindle.

What file formats Kindle accepts for self-publishing

If you are publishing a book yourself through Kindle Direct Publishing (KDP), Amazon accepts several file formats and converts them to .azw3 automatically. You can upload a .docx file (Microsoft Word), a .pdf, an .html file, or an .epub file. You can also upload a pre-formatted .azw or .azw3 file if you have already created one.

Most self-published authors use .docx because it is the easiest to format and the most forgiving of small errors. Amazon's conversion process handles basic formatting like bold, italics, headers, and page breaks. If you upload a .pdf, Amazon will convert it, but the process is less reliable for complex layouts — .pdf works better for books with fixed design, like art books or technical manuals with diagrams.

If you want the most control over how your book looks on Kindle, you can create a .azw3 file yourself using tools like Sigil (free, open-source) or Vellum (paid, Mac-only). This route takes more time but gives you pixel-level control over fonts, spacing, and interactive elements. Most self-published authors do not need this level of control and are fine with Amazon's automatic conversion.

Why Amazon created its own format instead of using existing ones

When Amazon launched the Kindle in 2007, the most common e-book format was .mobi (Mobipocket), an open format that many e-readers supported. Amazon licensed the Mobipocket technology and built their own format on top of it, adding DRM encryption and features specific to Kindle devices.

The main reason was control. By creating a proprietary format, Amazon could may support that Kindle books only worked on Kindle devices and apps, which locked readers into their ecosystem. If Amazon had used an open format like .epub, readers could easily move their libraries to a competitor's device. The .azw format made that much harder.

A secondary reason was features. Amazon wanted to support things like Whispersync (which syncs your reading position across devices), X-Ray (which shows character and location information), and Kindle Notes and Highlights (which sync to Amazon's servers). These features required a format that Amazon controlled completely.

The publishing industry has largely moved toward .epub as the standard for e-books, especially after Apple Books and most independent publishers adopted it. But Kindle remains the largest e-book platform by market share, so .azw is still the most common proprietary format in use.

The difference between .azw, .azw3, and other Kindle formats

Amazon has released several versions of the Kindle format, and understanding the differences matters if you are publishing or converting books.

.azw is the original Kindle format, based on Mobipocket. It supports basic formatting like bold, italics, and headers, but has limited support for advanced typography, custom fonts, and interactive elements. Most older Kindle books use this format. Newer Kindle devices can still read .azw files, but Amazon no longer recommends it for new publications.

.azw3 (also called KF8, for Kindle Format 8) is the current standard. It supports advanced formatting, embedded fonts, better control over spacing and alignment, and interactive elements like embedded audio and video. If you are publishing a new book on KDP, Amazon will convert it to .azw3. Most books published in the last five years use this format.

.azw4 is a variant used specifically for Kindle textbooks and fixed-layout books (like children's books with illustrations). It preserves the exact page layout and design, which is important for textbooks and picture books but less important for novels.

.mobi is the older Mobipocket format that Amazon's .azw was based on. You may encounter .mobi files if you are working with older e-books or converting from other sources, but Kindle devices prefer .azw3 now. Calibre can convert between all of these formats.

Frequently Asked Questions

Can I read a Kindle book on my iPad or Android phone?

Yes, if you read the free Kindle app from the App Store or Google Play. The app works the same way as a Kindle device — it reads your .azw files and syncs your reading position across all your devices through your Amazon account. You do not need to convert the file.

What happens if I buy a Kindle book and then switch to a different e-reader?

The book stays in your Amazon account, but you cannot read it on the new device without converting it first. You would need to remove the DRM encryption using a tool like Calibre and the DeDRM plugin, then convert the .azw file to .epub or another format your new device supports. This is legal for books you own in most jurisdictions, but check your local laws.

Can I send a Kindle book to someone else?

No. Kindle books are tied to your Amazon account and cannot be shared or transferred. Amazon's terms of service prohibit it. If someone else wants to read the book, they need to purchase their own copy or borrow it through Kindle Unlimited if they have a subscription.

Do I need to convert my book to .azw before uploading it to Kindle Direct Publishing?

No. You can upload a .docx, .pdf, .html, or .epub file, and Amazon will convert it to .azw3 automatically. Most self-published authors upload a .docx file because it is the easiest to format and the most reliable for conversion.

Is .epub better than .azw?

.epub is more widely supported across different devices and platforms, so it is better if you want your book to be readable on multiple devices. .azw is better if you are publishing specifically for Kindle and want to use Kindle-specific features. For most readers, the difference is invisible — both formats display text, images, and basic formatting the same way.