Why can't the browser convert EPUB to AZW3 directly?

AZW3 is a proprietary Amazon format that requires Calibre's open-source conversion engine. Replicating that engine in JavaScript would require hundreds of megabytes of code and would still not match Calibre's quality. Calibre is free, trusted, and runs on Windows, macOS, and Linux.

Is my EPUB file uploaded anywhere?

No. The file is read entirely inside your browser using the File API. No data is sent to any server.

What is the difference between AZW3 and MOBI?

AZW3 (KF8) is the modern Kindle format with CSS3 layout support, fixed-layout and nested tables. MOBI is the older format still supported by all Kindles. For new conversions, AZW3 is preferred.

Does Calibre support all EPUB files?

Calibre converts DRM-free EPUB files. If your EPUB has Adobe Digital Editions DRM, you must remove the DRM first (check the laws in your jurisdiction). DRM-free files from Project Gutenberg, Smashwords, or direct publisher stores convert without issues.

How do I install Calibre?

Download the free installer from calibre-ebook.com. It is available for Windows, macOS, and Linux and requires no account or licence.

Can I send the AZW3 directly to my Kindle?

AZW3 files must be sideloaded: connect your Kindle via USB and copy the file into the documents folder. Amazon's Send to Kindle service does not support .azw3 files. If you want wireless delivery, you can send the original EPUB directly via Send to Kindle instead.

EPUB to AZW3 Calibre Guide & Metadata Viewer

Drop an EPUB file to inspect its metadata and get step-by-step instructions for converting it to AZW3 (Kindle format) using Calibre.

Select Your EPUB File

Drag and drop an EPUB file here or click to browse — stays in your browser, never uploaded

How to Convert EPUB to AZW3 with Calibre

1

Install Calibre (free)

Download the free installer from calibre-ebook.com and install it on Windows, macOS, or Linux. No account required.
2

Add your EPUB

Open Calibre and click Add books (top-left). Select your .epub file. It appears in your library list.
3

Choose AZW3 output format

Select the book in the list and click Convert books. In the top-right dropdown labeled Output format, choose AZW3.
4

Run the conversion

Click OK. Calibre converts the file in seconds. A spinning icon in the bottom-right corner shows progress.
5

Locate the AZW3 file

Click the book title, then click Click to open next to the AZW3 format to open the containing folder.
6

Transfer to Kindle via USB

Connect your Kindle via USB and copy the .azw3 file into the documents folder. (Amazon's Send to Kindle does not support AZW3 files; they must be sideloaded via USB).

EPUB vs AZW3 — Quick Comparison

Feature	EPUB	AZW3 (KF8)
Standard	Open (IDPF / W3C)	Proprietary (Amazon)
Device support	Kobo, Apple Books, Google Play, most readers	Kindle devices and Kindle apps
CSS support	CSS 2.1 / CSS 3	CSS 3 (richer on newer Kindles)
Fixed layout	Yes (EPUB 3)	Yes
Converter	—	Calibre (free)

Done

Summary

EPUB and AZW3 are the two dominant ebook formats. EPUB is the open standard supported by most e-readers, while AZW3 (also called KF8) is Amazon's enhanced Kindle format with richer layout support. This tool reads your EPUB file entirely in the browser, extracts metadata such as title, author, and file size, then walks you through converting it to AZW3 using Calibre — the free, open-source ebook management suite.

How it works

Drop or select your EPUB file — it is read entirely in your browser, nothing is uploaded.
The tool parses the EPUB container and displays the title, author, publisher, language, and file size.
Review the metadata to confirm you have the right file before converting.
Follow the on-page Calibre instructions to convert the file to AZW3 on your computer.
Transfer the converted AZW3 file to your Kindle via USB cable into the documents folder.

Use cases

Convert freely available Project Gutenberg EPUBs for reading on a Kindle device.
Batch-convert your personal ebook library to Kindle-compatible format.
Check EPUB metadata (title, author, language) before converting a large file.
Verify a downloaded EPUB is valid before running Calibre on it.
Prepare self-published manuscripts in EPUB for Kindle Direct Publishing review.
Convert DRM-free EPUBs purchased from non-Amazon stores to read on Kindle.

Frequently Asked Questions

Last updated: 2026-05-28 · Reviewed by Nham Vu