The short answer
- AZW3 is a binary ebook container that represents Kindle Format 8 content with HTML and CSS, but its unpublished specification and interoperability remain controlled by Amazon.
- Open an AZW3 file with an Amazon Kindle app or Calibre, both listed in the IANA registration. On a computer, use the reader's Open or Add Books command and select the .azw3 file. On a phone or tablet, import the file into an installed Kindle-compatible reading app using the device's file picker or share menu. If it fails, confirm the .azw3 extension and application/vnd.amazon.mobi8-ebook type, then update the reader; a generic MOBI association may not identify KF8 correctly.
- AZW3 is rated low risk because ordinary KF8 content has a limited active-execution surface: script is reserved for Amazon use, while iframe, form, input, canvas, audio, and video are unsupported. This is not a guarantee of safety—IANA says the media type's security issues have not been assessed. Open untrusted files only in an updated, preferably isolated reader.
AZW3 (Kindle Format 8) File
AZW3 is the registered extension for Kindle Format 8 (KF8). IANA registers it as application/vnd.amazon.mobi8-ebook, with Amazon.com, Inc. as author and change controller. KF8 can represent semantic HTML, images, embedded SVG, style sheets, fonts, backgrounds, borders, and positioning on KF8-enabled devices and apps. It is not an open standard, and its specification has not been published.
AZW3 conversion may change or remove content and layout. KF8 does not support audio, video, form, iframe, input, or canvas, and omits some CSS selectors, pseudo-elements, structural pseudo-classes, and properties such as max-width. A source that depends on those features cannot be assumed to convert losslessly. KF8 and legacy Mobi styling can also differ through @media amzn-kf8 and @media amzn-mobi rules.
How to open it
Open an AZW3 file with an Amazon Kindle app or Calibre, both listed in the IANA registration. On a computer, use the reader's Open or Add Books command and select the .azw3 file. On a phone or tablet, import the file into an installed Kindle-compatible reading app using the device's file picker or share menu. If it fails, confirm the .azw3 extension and application/vnd.amazon.mobi8-ebook type, then update the reader; a generic MOBI association may not identify KF8 correctly.
Open an AZW3 file with an Amazon Kindle app or Calibre, both listed in the IANA registration. On a computer, use the reader's Open or Add Books command and select the .azw3 file. On a phone or tablet, import the file into an installed Kindle-compatible reading app using the device's file picker or share menu. If it fails, confirm the .azw3 extension and application/vnd.amazon.mobi8-ebook type, then update the reader; a generic MOBI association may not identify KF8 correctly.
AZW3 conversion may change or remove content and layout. KF8 does not support audio, video, form, iframe, input, or canvas, and omits some CSS selectors, pseudo-elements, structural pseudo-classes, and properties such as max-width. A source that depends on those features cannot be assumed to convert losslessly. KF8 and legacy Mobi styling can also differ through @media amzn-kf8 and @media amzn-mobi rules.
File safety
AZW3 is rated low risk because ordinary KF8 content has a limited active-execution surface: script is reserved for Amazon use, while iframe, form, input, canvas, audio, and video are unsupported. This is not a guarantee of safety—IANA says the media type's security issues have not been assessed. Open untrusted files only in an updated, preferably isolated reader.
An unknown sender, a name ending in two extensions, or contents that do not match the extension — check before you open it.
Converting it
AZW3 conversion may change or remove content and layout. KF8 does not support audio, video, form, iframe, input, or canvas, and omits some CSS selectors, pseudo-elements, structural pseudo-classes, and properties such as max-width. A source that depends on those features cannot be assumed to convert losslessly. KF8 and legacy Mobi styling can also differ through @media amzn-kf8 and @media amzn-mobi rules.
Convert a copy, never the original, then compare quality, metadata, and layout against what you started with.
Common questions
What is an AZW3 file used for?
An AZW3 file contains a Kindle Format 8 ebook represented with HTML and CSS. KF8 supports features such as semantic HTML, images, embedded SVG, style sheets, embedded fonts, backgrounds, borders, and positioning on enabled Kindle devices and apps.
How do I open an AZW3 file?
Use an Amazon Kindle app or Calibre, which IANA lists as opening software for this media type. Choose the app's Open or Add Books command and select the .azw3 file.
How do I open an AZW3 file on Windows?
Install a current Kindle app or Calibre for Windows, then use Open or Add Books to select the AZW3 file. Do not rely on a generic MOBI file association because it may not identify KF8 correctly.
How do I open an AZW3 file on Mac?
Use a current Kindle app or Calibre on the Mac and import the file through Open or Add Books. If macOS has no association for .azw3, choose the reader manually from Open With.
How do I open an AZW3 file on a phone?
Install a Kindle-compatible reading app, then select the AZW3 file in the phone's file picker or share menu and import it into the app. Actual rendering depends on whether that device and app enable KF8 features.
Why will my AZW3 file not open?
Check that the extension is .azw3, use the registered type application/vnd.amazon.mobi8-ebook, and try an updated Kindle app or Calibre. Renaming another file to .azw3 does not make it KF8, and family-level BOOKMOBI or TEXtREAd markers do not uniquely prove that a file is AZW3.
What is the difference between AZW3 and MOBI?
AZW3 identifies Kindle Format 8 and uses application/vnd.amazon.mobi8-ebook. It should not be replaced with the MOBI-family type application/x-mobipocket-ebook; KF8 and legacy Mobi can also receive different CSS through @media amzn-kf8 and @media amzn-mobi.
Can I convert an AZW3 ebook without losing its layout?
Lossless conversion cannot be assumed. Content relying on unsupported elements such as audio, video, form, iframe, input, or canvas—or unsupported CSS features such as max-width—may be removed or laid out differently.
Is an AZW3 file safe to open?
Its ordinary active-content surface is limited because several interactive elements are unsupported and script is reserved for Amazon use. However, IANA says security issues have not been assessed, so use an updated, preferably isolated reader for untrusted files.
Can a file signature prove that an ebook is AZW3?
Not by itself. PRONOM's BOOKMOBI or TEXtREAd plus kindle: signatures cover the broader Kindle/Mobipocket family and are also associated with AZW, MOBI, and AMR, so no exclusive AZW3 signature is provided here.
Related formats
Sources
Media Type Registration: application/vnd.amazon.mobi8-ebook · IANA
HTML and CSS Tags Supported in Kindle Format 8 · Amazon Kindle Direct Publishing
Media Queries · Amazon Kindle Direct Publishing
Amazon Kindle ebook File — Signatures · The National Archives PRONOM
Publishing on Kindle: Guidelines for Publishers · Amazon.com