Kindle books use a proprietary format called AZW, which Amazon controls

Amazon's Kindle devices and apps read books in AZW format, a file type that Amazon created and owns. This format is based on an older standard called Mobipocket, but Amazon modified it to add digital rights management (DRM) — a lock that prevents you from copying or sharing the book file. When you buy a book from Amazon's Kindle Store, you are buying a license to read it on Amazon's ecosystem, not owning a file you can move freely between devices.

AZW files are compressed, which means they take up less storage space than a printed book's worth of text. They also contain formatting information that tells your Kindle device how to display the text, adjust font sizes, and show images. This is why the same book can look slightly different on a Kindle Paperwhite than on a Kindle app running on your phone — each device interprets the formatting instructions differently.

Amazon also uses variations like AZW3 (for newer Kindle devices with better formatting support) and KFX (the newest format, used on current Kindle devices and apps). All three are proprietary to Amazon, meaning no other e-reader manufacturer uses them.

Key Takeaways

  • Kindle books use AZW, AZW3, or KFX format, all owned by Amazon and locked with digital rights management so you cannot share or transfer them.
  • You can read Kindle books on any device that has the Kindle app installed — phones, tablets, computers, and e-readers — but the file itself stays in Amazon's system.
  • If you want to read the same book on a non-Kindle e-reader like a Kobo or Nook, you will need to buy it in a different format, usually EPUB.
  • Some publishers offer DRM-free Kindle books, but most do not, so check the product page before buying if you want the option to move the file later.

How Kindle's lock prevents you from moving books to other devices

The DRM on AZW files is tied to your Amazon account. When you read a Kindle book, Amazon's servers check that your account owns a license to that book, then send you a file that only works when you are logged into that account. If you try to open the AZW file on a device that is not registered to your Amazon account, it will not open.

This lock also prevents you from converting the file to another format. Tools exist that can strip the DRM, but doing so violates the Digital Millennium Copyright Act (DMCA) in the United States, even if you own the book. This means that if you buy a Kindle book and later want to read it on a Kobo e-reader or a different platform, you cannot legally move the file — you would have to buy the book again in EPUB format.

The lock does not prevent you from reading the book on multiple devices at once. You can have the Kindle app open on your phone, your tablet, and your computer simultaneously, and Amazon will sync your reading progress across all of them. But the file itself never leaves Amazon's control.

EPUB is the open standard that most other e-readers use

EPUB (Electronic Publication) is an open format that any e-reader manufacturer can use. Kobo, Apple Books, Google Play Books, and most independent e-reader apps all support EPUB. Unlike AZW, EPUB is not owned by any single company — it is maintained by an international standards organization called the International Digital Publishing Forum.

EPUB files can contain DRM, but they do not have to. Many publishers add DRM to EPUB files sold through retailers like Apple Books or Google Play, but some publishers and independent authors sell DRM-free EPUB files directly. This means you can legally move a DRM-free EPUB between devices, convert it to other formats, or back it up on your computer.

EPUB is also more flexible than AZW for text formatting. Readers can adjust fonts, spacing, and margins more freely in EPUB files, which is why many people prefer reading EPUB on e-readers. However, EPUB is less reliable for books with complex layouts, such as graphic novels or heavily illustrated children's books — AZW and KFX handle those better.

PDF and other formats you might encounter

PDF is a fixed-layout format, meaning the text and images are locked in place exactly as the publisher designed them. PDFs work on almost any device, but they do not reflow text the way EPUB or AZW do. If you buy a PDF of a book and open it on a small phone screen, you will have to zoom and scroll to read it, rather than having the text automatically adjust to fit the screen.

Some publishers sell books in PDF format directly to readers, and some textbooks are only available as PDFs. Amazon also offers a service called Kindle Create that lets authors convert PDFs to AZW format, though the conversion is not always perfect. If you receive a PDF from a publisher or author, you can usually read it on any device, but it will not have the same reading experience as an EPUB or AZW file designed for e-readers.

Older e-readers sometimes supported formats like Mobi (the predecessor to AZW) or TXT (plain text). These formats are rarely used now, but if you have old e-books in these formats, they may not work on current devices. Converting them to EPUB or AZW is usually possible through free tools, but the quality of the conversion depends on how the original file was formatted.

Why Amazon chose a proprietary format instead of EPUB

Amazon created AZW instead of using EPUB because it gave them control over the reading experience and the ability to add features that EPUB did not support at the time. When Kindle launched in 2007, EPUB was less mature, and Amazon wanted to may support that books would display consistently across all Kindle devices. By owning the format, Amazon could update it without waiting for a standards committee to approve changes.

The proprietary format also reinforces Amazon's lock-in strategy. If you buy a Kindle book, you are more likely to keep buying from Amazon because moving your library to another e-reader would require repurchasing books. This is a common business practice in tech — Apple uses its own formats for music and video, Microsoft uses its own formats for Office documents — but it does mean that Kindle books are less portable than books in open formats.

Over time, EPUB has improved, and many publishers now prefer it because it works across multiple platforms. However, Amazon has not switched to EPUB, and there is no sign they plan to. If you want to build a library that is not tied to Amazon, buying EPUB books from other retailers is the better choice.

How to check what format a book is in before you buy

On the Kindle Store website or app, the product page for each book will say "Kindle Edition" or "Kindle eBook" if it is in AZW or AZW3 format. It will not always specify which version, but current Kindle devices automatically handle both. If you are buying from another retailer like Kobo or Apple Books, the page will say "EPUB" or show an EPUB icon.

If you are buying directly from a publisher or independent author, check the file format before you complete the purchase. Some publishers list it clearly; others require you to contact them. If you want a DRM-free book, search for the title plus "DRM-free" — sites like Smashwords and Draft2Digital specialize in DRM-free e-books and will tell you the format upfront.

You can also use a tool like Calibre (a free, open-source e-book manager) to convert between formats after you buy, but remember that converting a DRM-protected file is illegal in most countries. Calibre works well for converting DRM-free books, and it can also help you organize your library across multiple devices and retailers.

Frequently Asked Questions

Can I read a Kindle book on a Kobo or other e-reader?

No, not without removing the DRM protection, which is illegal in most countries. If you want to read the same book on multiple e-reader brands, you need to buy it in EPUB format from a retailer that supports Kobo, Apple Books, or Google Play. Some books are available in both Kindle and EPUB versions, but you will have to purchase each one separately.

What happens if Amazon shuts down or removes a book from my library?

Amazon can remotely delete books from your Kindle library if a publisher requests it or if there is a licensing dispute. This happened in 2009 when Amazon deleted copies of George Orwell's 1984 from users' devices. Because the file is locked to your account, you cannot back it up or access it if Amazon removes it. With DRM-free EPUB books, you own the file and can keep it forever.

Is AZW better than EPUB for reading?

Neither is objectively better — it depends on the book and your device. AZW and KFX handle complex layouts and images better, which is why graphic novels and illustrated books often look better on Kindle. EPUB offers more flexibility for adjusting text size and spacing, which many readers prefer for novels. If you read mostly text-heavy books, the difference is minimal.

Can I convert my Kindle books to EPUB?

Technically, tools exist to do this, but removing the DRM protection violates the DMCA in the United States and similar laws in other countries, even if you own the book. If you want EPUB versions of books you have already bought in Kindle format, you would need to repurchase them from a retailer that sells EPUB.

Do all Kindle devices support AZW3 and KFX?

Newer Kindle devices and apps support both AZW3 and KFX, but older Kindle devices (more than five years old) may only support AZW. Amazon automatically converts books to the format your device supports, so you do not have to worry about this. If you use an older device, Amazon will send you the AZW version instead of the newer formats.