MOBI to EPUB: Read Old Kindle Files on Kobo and Other Readers

You copied the-martian.mobi onto your new Kobo, and it isn't in the library. Or it shows up greyed out with unsupported file format. Or you emailed it to Send to Kindle and got a bounce back. Nothing is broken on your device — the format simply aged out.

MOBI was Amazon's format until 2022. Kobo, Apple Books, PocketBook, Tolino and almost every non-Amazon reader never supported it, and Amazon itself now prefers EPUB and converts it internally to AZW3/KFX. So the fix for a shelf full of .mobi files is the same in every direction: convert MOBI to EPUB once, and the files work everywhere.

This guide covers the DRM check you should do first, the one-click online route, the Calibre ebook-convert command for batches, and how to get the finished EPUB onto each device.

Why MOBI stopped working everywhere at once

Two separate things happened, and most people only notice one of them.

• Kindle no longer supports MOBI for new sideloads. Since late 2022, Send to Kindle rejects .mobi and .azw uploads. It accepts EPUB, PDF, DOCX and TXT — and when you send an EPUB, Amazon converts it on their servers into their own AZW3/KFX format for your device. You never see the EPUB again; you see a Kindle book.
• Kobo, Apple Books and the rest never supported MOBI at all. Kobo reads EPUB, KEPUB, PDF, CBZ/CBR and a few text formats. Apple Books reads EPUB and PDF. Dropping a .mobi into either produces silence or an unsupported-format error.

So EPUB is now the only file you need to keep. It is the input format Amazon wants, the native format Kobo wants, and the only ebook format Apple Books will index. Converting your backlog isn't a workaround — it's future-proofing.

Check for DRM before you convert anything

Converters handle the format. They cannot handle encryption. If a MOBI came from your Kindle library with Amazon's DRM attached, every tool will fail on it, usually with a vague "file is corrupt or not supported" message.

Quick ways to tell them apart:

• Filename. Files named .azw, .azw3, .kfx or .prc from a Kindle download are usually protected. A plain .mobi from Humble Bundle, Project Gutenberg, StoryBundle, Smashwords, a publisher's direct store or your own author copy is usually not.
• Size and opening test. Drag the file into Calibre. If it imports with a cover and a readable preview, it's DRM-free. If Calibre refuses it, it's encrypted.
• Source. Anything you bought and downloaded for Kindle is encrypted unless the seller advertised DRM-free.

If a book is DRM-locked, the legitimate route is to read it in the Kindle app, or re-buy/re-download it DRM-free from stores that sell it that way. Stripping DRM is restricted or illegal in many countries, and we don't cover it here. Everything below assumes clean, DRM-free MOBI files — which is most of a typical pre-2022 bundle library.

The fast route: convert MOBI to EPUB online

For a handful of books, or when you're on a tablet, a phone or a work laptop where you can't install software, a browser converter is the shortest path. Upload the file, download the EPUB, side-load it.

Use the MOBI to EPUB converter on File Convert Online: drop in dune-1965.mobi (a typical novel is 400 KB–2 MB), wait a few seconds, and you get dune-1965.epub back at roughly the same size. Cover image, chapter list and table of contents carry over, because both formats store the same underlying HTML and CSS. If you also have PDFs, AZW3 or FB2 files in the pile, the wider ebook converter hub handles those pairings too.

Rule of thumb: online for anything under a dozen books, Calibre below that threshold's other side.

The batch route: Calibre ebook-convert on the command line

Calibre ships a CLI tool called ebook-convert. It's the same engine as the GUI, but scriptable — ideal when you have 200 files from a decade of bundles.

On macOS the binary lives inside the app bundle, so add it to your PATH first:

export PATH="/Applications/calibre.app/Contents/MacOS:$PATH"

A single conversion is one line:

ebook-convert "the-martian.mobi" "the-martian.epub" \
  --output-profile kobo_aura_one \
  --preserve-cover-aspect-ratio \
  --change-justification original

What those flags do: --output-profile tunes default font and image sizing for the target screen (use tablet for Apple Books, kindle_pw3 if you're going back to a Kindle); --preserve-cover-aspect-ratio stops squashed covers; --change-justification original leaves the author's text alignment alone instead of forcing full justification.

To convert a whole folder at once, on macOS or Linux:

for f in *.mobi; do
  ebook-convert "$f" "${f%.mobi}.epub" --output-profile kobo_aura_one
done

And the Windows PowerShell equivalent:

Get-ChildItem *.mobi | ForEach-Object {
  ebook-convert $_.FullName ($_.BaseName + ".epub") --output-profile kobo_aura_one
}

Expect roughly 1–3 seconds per novel. Files that error out with "DRMError" or "Unknown MOBI header" are the encrypted ones — set them aside rather than retrying.

Two extras worth knowing: --embed-all-fonts if the original used a distinctive typeface, and --enable-heuristics for old, badly-formatted bundle files with broken paragraph breaks. Don't enable heuristics by default; it can mangle poetry and code samples.

Getting the EPUB onto your reader

• Kobo: connect by USB, open the KOBOeReader drive, copy EPUBs into the root folder (not a subfolder of a subfolder), eject, and the device indexes them on wake. If a book still doesn't appear, the file is probably still a .mobi you renamed — renaming does not convert. For better pagination you can convert to KEPUB instead, but plain EPUB works fine.
• Apple Books: double-click the EPUB on a Mac, or share it to Books on iPhone/iPad. It syncs via iCloud to your other devices.
• Kindle: email the EPUB to your @kindle.com address or use the Send to Kindle web page. Amazon converts it to AZW3/KFX on delivery — this is the send to kindle epub flow that replaced MOBI entirely.
• PocketBook, Tolino, Boox, Onyx: USB copy into any folder; all of them read EPUB natively.

Clean up your library once, not every time

The practical move is to batch-convert the whole backlog in one evening and keep EPUB as your archive format. Specifically:

• Put every .mobi in one folder and run the loop above, or upload them in groups online.
• Move the files that failed into a drm/ folder — those stay in the Kindle app.
• Spot-check five converted books on the device for cover, TOC and footnotes before deleting anything.
• Keep the originals on a backup drive until you're satisfied; they're small.

After that, buy DRM-free EPUB where you can. One format, every reader, no conversion step ever again.

FAQ

Does converting MOBI to EPUB lose quality? No. Both formats wrap HTML and CSS, so text, chapters, images and the table of contents transfer intact. Complex fixed-layout comics or textbooks are the exception and may need a reflow check.

Can Kindle still read MOBI files already on the device? Yes — books synced before 2022 keep working. You just can't send new .mobi files.

Why does my Kobo say unsupported file format? Because it's genuinely a MOBI, or a .epub that's actually a renamed MOBI. Convert it properly and re-copy.

Will my highlights and bookmarks come across? No. Annotations live in the reader's own database, not in the book file.

AZW3 or EPUB for Kindle? EPUB. Amazon accepts it and does the AZW3 conversion for you.