Convert EPUB to AZW3

If you're sending a book to a Kindle, you probably don't need to convert to AZW3 anymore. Since 2022, Amazon's Send to Kindle service accepts EPUB directly and handles conversion on their end. Manual AZW3 conversion is now mostly for: sideloading via USB to older Kindles, archival, or when you need specific control over the output that Amazon's converter doesn't give you.

Path A: Send to Kindle (recommended for 99% of cases)

1. Find your personal Send-to-Kindle email at Amazon → Manage Your Content and Devices → Preferences → Personal Document Settings. It looks like yourname@kindle.com.
2. Attach the EPUB to an email sent from an approved email address (in the same settings page) to your Kindle address.
3. Amazon converts EPUB → KF8 on their side. The book appears on your Kindle within a few minutes under "Personal Documents".

There's also a web uploader and Send-to-Kindle apps for desktop and mobile. All do the same thing — EPUB in, KF8 out, delivered to your Kindle library.

Before sending, validate the EPUB and fix the metadata. Broken manifest entries cause Amazon's converter to fail silently (you get a generic "we couldn't process this" email). The metadata is what shows in your Kindle library.

Path B: Calibre to AZW3 (for control or sideloading)

1. Add your EPUB to Calibre.
2. Convert books → output format AZW3.
3. Under AZW3 Output, you can set:
   • Share not sync — for books going onto a Kindle not linked to your Amazon account
   • Enable heuristic processing — cleans up source files with messy paragraph breaks; leave off if your EPUB is clean
   • Font size multiplier, margins, page-break settings
4. Copy the resulting AZW3 to your Kindle's documents folder via USB.

What changes in the conversion

• CSS gets subsetted. KF8 supports a subset of EPUB 3 CSS — complex selectors, transforms, and some display properties get dropped. If your book uses heavy CSS, expect visual drift.
• Fonts may be re-embedded or replaced. Amazon strips fonts not on their whitelist. Calibre preserves them but adds size.
• Drop caps and SVG decorations often need the EPUB to be designed Kindle-first to survive. Test on an actual Kindle before publishing.
• Fixed-layout EPUBs (kids' books, manga) should convert to the fixed-layout AZW3 variant, not reflowable KF8. Calibre handles this if the EPUB is properly tagged.

When Send-to-Kindle rejects your EPUB

The most common rejection causes (in order):

1. EPUB validation failures — missing mimetype, broken references. Run the validator first.
2. File size over 50 MB. Compress images with the EPUB compressor, or split into volumes.
3. DRM-locked files. Send-to-Kindle refuses DRM-bearing EPUBs that aren't purchased from Amazon.

Related

• Convert AZW3 to EPUB — the reverse direction
• Send EPUB to Kindle — detailed walkthrough
• EPUB to Kindle format — format overview
• Validate EPUB — pre-flight before sending