Convert M4B to MP3

What happens to the chapters

Chapters are where M4B and MP3 differ most. Octoolo copies the book's chapter marks into the MP3 as ID3 chapter tags, but most music players, phones and car stereos don't read those, so in practice the MP3 plays as one long track. Some podcast and audiobook apps do show them.

If you want separate parts, for a player that only skips between tracks, you can cut sections out of the MP3 with the MP3 cutter and export each one, though for a book with dozens of chapters that is slow going. Octoolo has no button that splits a book at its chapters automatically.

The cover picture, the book's title and the author's name are copied into the MP3's tags, so the file still looks like a book, not an anonymous recording, in players that show artwork.

Sizes for long books

A single narrator rarely needs two channels, so mono halves the size at no real cost. With As it is, a book recorded in mono at 64 kbps also comes out at 64 kbps; a stereo one is raised to at least 128 kbps, MP3's floor for stereo in Octoolo, which is why some MP3s end up bigger than their M4B.

Keep stereo for full-cast productions with music and effects, where Balanced is a good middle. The book's sample rate is kept, up to the 48 kHz an MP3 can hold, so voices sound the way they did in the M4B.

Which audiobooks can be converted

Audiobooks bought from Audible or Apple Books are copy-protected. They can't be read by FFmpeg, the engine Octoolo converts with, so they won't convert, and Octoolo has no way around that. Audible's downloads come in their own protected formats anyway.

What does work: M4B files without copy protection, such as books you made yourself from CDs, books sold without protection, and public-domain recordings offered as M4B. If an M4B plays in VLC on your PC, Octoolo can almost certainly convert it.

Going the other way, from a set of MP3 chapters to one file, is a job for the audio joiner: the chapters play one after another on a single track and export as one MP3.

Big files, patience and disk space

A 10-hour audiobook means 10 hours of sound to encode, so it takes longer than a song. The row shows the progress, the speed and the time left, and the stop button on the row ends it if you need the computer for something heavy.

Make sure there is room for the result, up to about 600 MB at 128 kbps. Because nothing is uploaded, there's no cap on the size and no transfer of several hundred megabytes before the work even starts. Several books can go in one batch; Octoolo works on as many as four at once and starts the rest as those finish.

Some long titles come split across several M4B files. Add all the parts together: each becomes its own MP3 under its own name, so “Part 1” and “Part 2” stay in order and sort correctly on a phone or USB stick, with the same cover and author on each.