MOBI vs EPUB: what changed

Short answer: EPUB is the living standard, MOBI is the retired one. Convert your MOBI files to EPUB, keep the originals until you have checked the results, and stop acquiring new ones.

Where MOBI came from

MOBI began as the Mobipocket format in the late 1990s, built on the HTML of that era. Amazon bought Mobipocket in 2005 and made it the basis of the Kindle. That lineage explains its limitations: it inherited a markup language from before CSS was widely used, so its typography options are thin and modern layout is largely beyond it.

Amazon then moved on twice. AZW3 (also called KF8) added much better CSS support, and KFX, the current format, added better typography and tighter control. Meanwhile AZW is essentially MOBI with Amazon’s DRM wrapped around it.

In 2022 Amazon stopped accepting MOBI files through Send to Kindle, which was the practical end of the format. A lot of carefully assembled libraries became awkward that day.

What you actually lose with MOBI

In practice, the visible differences in a plain novel are small: both will show you the text. The differences appear in poetry, drop caps, footnotes, tables and anything where the author’s formatting mattered. MOBI flattens more of it.

Converting a MOBI library

Calibre does this well and free on a desktop. A sensible procedure:

1. Copy the folder first. Convert the copy, never the only copy you have.
2. Convert in batches by publisher or source. Files from the same origin tend to convert the same way, so you can spot-check one and trust the batch.
3. Check three things per book: the table of contents, one chapter opening, and a footnote or a poem if the book has any. That catches almost every conversion failure.
4. Fix metadata while you’re there. Author, title, series. See organising a digital library.
5. Keep the MOBI originals for a while. Delete them when you’ve actually read one of the converted books without irritation.

Conversion works on files without DRM. A protected book stays protected, and Calibre will decline to convert it; that is a legal boundary rather than a technical one, covered in ebook DRM explained.

Do you need to convert at all?

Not urgently. Plenty of Android readers still open MOBI happily, Lectern among them (MOBI and KF8), so an old library remains readable today. The argument for converting is about the next ten years rather than this week: format support only shrinks, and EPUB is where the tooling, the standards work and the accessibility features now live.

The one case for keeping MOBI is a legacy Kindle device that predates the newer formats and reads MOBI over USB. If that’s your reason, keep them, and keep EPUB copies as well.

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