Kindle uses a proprietary format called AZW, which Amazon developed to lock books to their ecosystem

When you buy a book on Amazon and read it on a Kindle device or app, you are reading a file in AZW format (or one of its variants: AZW3, KF8). Amazon created this format specifically so that books purchased through their store work only on Kindle devices and Kindle reading apps — not on other e-readers like Kobo or Nook. The format is based on the open standard EPUB but wrapped in encryption that ties each book to your Amazon account.

This matters because it means you cannot easily move a Kindle book to another device or share it with someone else, even if you own it outright. You are not buying a file you control; you are buying a license to read that file on Amazon's platforms. If Amazon removes a book from their store or closes your account, your access to those books can disappear.

Key Takeaways

  • Kindle books are stored in AZW or AZW3 format, which only works on Amazon devices and apps, not on other e-readers.
  • Amazon encrypts Kindle books so they are tied to your account, preventing you from transferring them to other devices or lending them freely.
  • If you want to read books on multiple devices from different manufacturers, you will need to buy from stores that use the open EPUB format instead.
  • Some publishers offer DRM-free EPUB versions of their books that work on any e-reader, though these are less common than Kindle versions.

How AZW differs from EPUB, the standard format most other e-readers use

EPUB is the open standard format used by most other e-readers: Kobo, Nook, Apple Books, and many library lending systems all use EPUB. A book in EPUB format is a compressed file containing text, images, and formatting instructions. Because EPUB is open, any device maker can build a reader for it, and you can move an EPUB file between devices freely.

AZW is Amazon's proprietary variant. It is based on the same underlying technology as EPUB but wrapped in Digital Rights Management (DRM) encryption. That encryption layer is what prevents you from opening an AZW file on a non-Amazon device. Even if you read the file to your computer, you cannot straightforward rename it or convert it without removing the DRM — which is illegal under US copyright law, even if you own the book.

In practice, this means a book you buy on Kindle is locked to Amazon's ecosystem in a way that a book in EPUB format is not. You can read EPUB books on your phone, your tablet, your e-reader, or your computer, all at the same time, with the same file. With Kindle books, you are reading through Amazon's apps or devices, and Amazon controls what you can do with the file.

Why Amazon chose a proprietary format instead of using EPUB

Amazon built AZW to create what is called vendor lock-in. When your books only work on Kindle devices and apps, you are less likely to switch to a competitor. If you have spent years building a Kindle library, buying a Kobo e-reader means leaving those books behind — or buying them again. This lock-in is profitable for Amazon because it keeps customers on their platform.

From a publisher and author perspective, the DRM encryption also prevents unauthorized copying and sharing. Publishers see this as protecting their intellectual property, though it also means readers have fewer rights than they would with a physical book (which you can lend, resell, or give away).

Amazon has also used AZW variants to add features that EPUB did not originally support, like enhanced typesetting and interactive elements. However, most of these features are now available in the newer EPUB3 standard, so the technical argument for a proprietary format has weakened over time.

What happens when you read a Kindle book to your device

When you buy a book on Amazon and read it to a Kindle device or the Kindle app on your phone, you are downloading an AZW file encrypted with a key tied to your Amazon account. The Kindle device or app has the decryption key built in, so it can read the file. Your computer does not have that key, so if you try to open the AZW file in a text editor or another program, you will see gibberish.

Amazon stores a copy of every book you buy in your Amazon library in the cloud. If you delete the book from your device, you can re-read it later. If you lose your device, you can read your books to a new one using the same Amazon account. This cloud backup is convenient, but it also means Amazon knows exactly which books you own and can theoretically revoke access if they choose to.

The Kindle app is available for phones, tablets, computers, and web browsers, so you can read your Kindle books on almost any device — but only through Amazon's app, not through a third-party reader.

How to read books on multiple devices if you do not want to be locked into Kindle

If you want the freedom to read the same book on a Kindle, a Kobo, your phone, and your computer without restriction, you have a few options. Some publishers and independent authors sell DRM-free EPUB files directly through their websites or through stores like Smashwords or Draft2Digital. These files work on any device and are yours to keep and move around.

Project Gutenberg and other digital libraries offer thousands of public-domain books in EPUB format for free. These are older books — anything published before 1928 in the United States — but they include classics, reference works, and historical texts. You can read them and read them on any e-reader.

Some libraries lend books in EPUB format through systems like OverDrive or Libby. These books are encrypted too, but the encryption is temporary — it expires when the loan period ends. You can read them on any device that supports the library's lending system.

If you already own Kindle books and want to read them elsewhere, you can convert them using tools like Calibre, but this requires removing the DRM encryption, which is a legal gray area. A safer approach is to check whether the publisher offers a DRM-free version or to buy future books in a format that is not locked to one platform.

The difference between AZW, AZW3, and KF8

Amazon has released several versions of its format over the years. AZW was the original format, based on the older MOBI standard. KF8 (Kindle Format 8) was introduced in 2011 to support more advanced formatting, images, and interactive features. AZW3 is the current standard, which is essentially KF8 with a different file extension.

For most readers, the difference does not matter. Your Kindle device or app handles all three formats automatically. The main difference is that older Kindle devices may not support AZW3 as well as they support AZW, so very old e-readers might have trouble with newer books. If you are buying a Kindle book today, it will almost certainly be in AZW3 format.

Frequently Asked Questions

Can I convert a Kindle book to EPUB so I can read it on another device?

Technically, yes, using software like Calibre, but it requires removing the DRM encryption that protects the file. In the United States, removing DRM is illegal under the Digital Millennium Copyright Act, even if you own the book. Some countries have exceptions for personal use, but the legal status is unclear. A safer option is to buy DRM-free EPUB versions from the publisher or a different store.

If I buy a Kindle book, do I own it or am I just renting it?

You are licensing it, not owning it. Amazon's terms of service say you have the right to read the book on Kindle devices and apps, but Amazon can remove books from your library if they are pulled from the store or if your account is closed. With a physical book or a DRM-free EPUB file, you own the object itself and can do what you want with it.

Why do some books cost more on Kindle than in paperback?

Publishers set the prices independently for different formats. Sometimes a Kindle version costs less, sometimes more. There is no rule. Prices also vary by region and change over time. If a Kindle book seems expensive, check the paperback price — you might find it is cheaper to buy the physical copy.

Can I lend a Kindle book to a friend?

Some Kindle books have a lending feature that lets you lend the book to another person for 14 days, but not all publishers allow it. Even when lending is available, only one person can read the book at a time, and the borrower must have a Kindle device or app. It is much more restrictive than lending a physical book.

What happens to my Kindle books if Amazon goes out of business?

If Amazon shut down the Kindle service, you would lose access to your encrypted books because only Amazon's servers hold the decryption keys. This is a real risk with any DRM-protected content. With DRM-free EPUB files, you would keep them forever because they do not depend on any company's servers.