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?

Yes. Connect your Kindle via USB and copy the AZW3 to the Documents folder, or use Amazon's Send to Kindle desktop app to send it wirelessly.

EPUB to AZW3 Converter

Name: EPUB to AZW3 Converter
Availability: InStock
Author: Nham Vu

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

Send to Kindle

Connect your Kindle via USB and copy the .azw3 file into the Documents folder, or use the Send to Kindle desktop app.

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

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

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 or the Send to Kindle service.

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: 2026-06-09 · Reviewed by Nham Vu