Free online MOBI to AZW3 converter

When a Kindle-oriented sideloaded copy needs CSS, embedded-font, or navigation features absent from legacy MOBI, convert it to AZW3. Amazon's Send to Kindle service no longer accepts MOBI uploads and expects EPUB rather than AZW3.

About MOBI

MOBI is a legacy ebook format derived from Mobipocket. Amazon used MOBI and its older Kindle variants for early Kindle books and personal documents, so .mobi files remain common in older libraries and downloads.

MOBI was designed for basic reflowable text and has limited support for modern HTML and CSS. A file with the .mobi extension may contain classic Mobipocket data, an older Kindle format, or a hybrid file with both older and KF8-compatible content; the extension alone does not identify every internal feature.

Commercial Kindle books may carry DRM. DRM-protected files cannot be converted by normal ebook converters; use an authorized DRM-free copy or the retailer's supported reading software.

About AZW3

AZW3, also called Kindle Format 8 (KF8), is an Amazon Kindle format that stores richer XHTML and CSS content than legacy MOBI. It supports improved styling, embedded fonts, SVG, better image handling, navigation, and other layout features that old MOBI cannot reliably preserve.

AZW3 is primarily useful for direct USB sideloading to compatible Kindle devices. It is not the same as Amazon's current cloud-delivery format, and Send to Kindle does not accept AZW3 as an upload format; use EPUB for Send to Kindle.

For a long-term master copy or use on non-Kindle readers, EPUB is generally more interoperable than AZW3.

Convert a MOBI file to AZW3

For a non-protected file, upload the .mobi file to 101convert.com, select AZW3 as the output format, start the conversion, and download the .azw3 result. Do not upload private books, annotations, or files you are not permitted to process through a web service.

Calibre is the most practical desktop option when metadata, cover art, output settings, or USB transfer require inspection.

1. Install Calibre and choose Add books to import the .mobi file.
2. Select the title and choose Convert books.
3. Set Output format to AZW3 in the format selector at the upper right.
4. Check the title, author, language, and cover under Metadata; correct them before converting if necessary.
5. Leave styling options unchanged unless the source has visible problems. Use Look & feel to adjust font scaling, margins, or spacing only after checking the source.
6. Click OK, then open the result in Calibre's viewer and check the table of contents, chapter breaks, images, links, and paragraph formatting.
7. Use Save to disk to export the file, or connect a compatible Kindle by USB and choose Send to device.

Calibre also provides a command-line converter:

ebook-convert input.mobi output.azw3

Use ebook-convert --help to view device profiles and conversion options. Device profiles can affect page dimensions and image scaling, but they do not repair poor source markup.

Quality and compatibility limits

Conversion repackages and translates the source content; it cannot create missing structure, restore low-resolution images, or turn flattened formatting into editable semantic markup. Old MOBI files may contain obsolete HTML, fixed spacing, weak chapter detection, or incomplete tables of contents.

AZW3 can represent richer formatting, but a converted reflowable book does not automatically become a well-designed fixed-layout ebook. Inspect books with tables, footnotes, image-heavy pages, right-to-left text, or complex CSS on the target device.

Kindle model, firmware, and transfer method affect sideloaded-book behavior. Test the resulting file on the intended Kindle, keep the original .mobi until testing is complete, and use EPUB with Send to Kindle when cloud delivery is required.