Convert M4B Audiobooks to MP3

By the M4AtoMP3.org editorial team · October 21, 2026 · 10 min read

M4B is the audiobook flavour of M4A: the same MPEG-4 container and AAC audio, plus chapter markers and a flag that tells players to remember where you stopped. It is the standard format for Apple Books and many DRM-free audiobook stores. It is also a format that a surprising number of devices cannot play — older MP3 players, many car stereos, some kids’ audio players and a lot of Android apps. Converting to MP3 solves that, but there is one decision to make first: one big file, or one file per chapter?

First: is your M4B convertible?

• DRM-free M4B — from stores such as Libro.fm, Downpour, publishers that sell direct, LibriVox (public domain), or audiobooks you have recorded or assembled yourself. These convert freely.
• Apple Books purchases — protected with FairPlay DRM. They cannot be converted by legitimate tools.
• Audible .aax/.aaxc — a different, DRM-protected format. Whether you may remove DRM for personal use depends on your country’s law and Audible’s terms. This guide does not cover that.

If a tool reports “protected” or “encrypted”, the file is DRM-protected.

Option A: one long MP3

Simplest, and good for players that resume where you left off (most podcast apps and many audiobook apps do, even with MP3).

With the browser M4A to MP3 converter: rename the file from .m4b to .m4a if the file picker does not show it, then convert at 128 kbps, its lowest setting (use ffmpeg below if you want a smaller 64 kbps mono file). A 10-hour book needs a fair amount of memory; on older phones or Chromebooks, use a desktop browser.

With ffmpeg:

ffmpeg -i book.m4b -map 0:a -ac 1 -c:a libmp3lame -b:a 64k -id3v2_version 3 book.mp3

-ac 1 makes it mono, which is ideal for narration and halves the size at a given quality.

The downside: a single 10-hour MP3 is awkward on devices without resume, and seeking to chapter 23 means scrubbing.

Option B: one MP3 per chapter (recommended)

This works on every device, gives natural navigation, and if a player loses your place you lose at most one chapter.

Step 1: list the chapters

ffprobe -v error -show_chapters -of csv=p=0 book.m4b

Each line shows the chapter number, start and end times and the title.

Step 2: split with a short script

macOS / Linux (bash, needs jq):

n=1
ffprobe -v error -show_chapters -of json book.m4b | jq -r '.chapters[] | "\(.start_time) \(.end_time) \(.tags.title)"' |
while read -r start end title; do
  printf -v num "%03d" "$n"
  ffmpeg -nostdin -n -loglevel error -i book.m4b -ss "$start" -to "$end" -map 0:a -ac 1 -c:a libmp3lame -b:a 64k \
    -metadata track="$n" -metadata title="$title" "$num - ${title//\//-}.mp3"
  n=$((n+1))
done

The zero-padded number (001, 002 …) keeps chapters in order on players that sort by file name, and -nostdin stops ffmpeg from consuming the chapter list.

Windows PowerShell:

$ch = (ffprobe -v error -show_chapters -of json book.m4b | ConvertFrom-Json).chapters
$i = 1
foreach ($c in $ch) {
  $t = ($c.tags.title -replace '[\\/:*?"<>|]', '-')
  ffmpeg -n -loglevel error -i book.m4b -ss $c.start_time -to $c.end_time -map 0:a -ac 1 -c:a libmp3lame -b:a 64k -metadata track=$i -metadata title="$($c.tags.title)" ("{0:D3} - {1}.mp3" -f $i, $t)
  $i++
}

Step 3: set consistent album tags

So players group the chapters as one book, give every file the same Album (the book title) and Artist (author or narrator). Add -metadata album="Book Title" -metadata artist="Author" to the ffmpeg command, or batch-edit afterwards in Mp3tag or Kid3. Copy the cover too — see keeping metadata when converting.

Option C: graphical tools

• fre:ac (Windows, Mac, Linux) — free, reads M4B, can split by chapters and writes tags.
• foobar2000 — loads M4B chapters as separate tracks in a playlist; select them and convert to MP3 with a naming pattern like %tracknumber% - %title%.
• Audacity — can import the M4B (with the FFmpeg library) and export by labels, but you would have to place chapter labels manually.
• AudioBookConverter (open source) — focused on the reverse direction (MP3 → M4B), but useful if you later want to rebuild a single M4B.

Choosing the right bitrate for audiobooks

Narration is far less demanding than music. Recommended settings:

Do not exceed the source bitrate: many M4B audiobooks are 64 kbps AAC, and converting them to 320 kbps MP3 only makes huge files. For more on size trade-offs, see how to reduce MP3 file size.

Playing the MP3s

• Car stereo: copy the chapter folder to a USB stick. Number-prefixed names ensure order. Our car USB guide explains folder and format limits.
• Phone: audiobook apps such as Smart AudioBook Player (Android) or BookPlayer (iOS) treat a folder of MP3s as a single book with resume.
• Kids’ players and old MP3 players: use 64 kbps mono for maximum compatibility and the longest battery life.

Converting back: MP3 to M4B

If you later want a single file with chapters for Apple Books, tools like AudioBookConverter or ffmpeg with a chapter metadata file can merge MP3s into one M4B. Re-encoding MP3 to AAC adds another lossy step; at 64 kbps narration it is rarely audible.

Troubleshooting

The file picker doesn’t show my M4B. Many converters filter by extension. Renaming to .m4a is safe — the format is identical.

Chapter times look wrong. Some M4B files use a separate text track for chapters. ffprobe -show_chapters reads standard chapter atoms; if it returns nothing, try foobar2000 or fre:ac, which read both types.

The first seconds of each chapter are cut off. Put -ss after -i (as in the scripts above) for precise cutting.

Huge memory use in the browser. Ten-hour files can exceed what a phone browser can hold. Use a desktop browser or ffmpeg.

Splitting by length instead of chapters

Some M4B files have no chapter markers, or have a single chapter covering the whole book. In that case, split into fixed-length parts — 15 or 30 minutes works well for car stereos and players that don’t resume. ffmpeg’s segment muxer does it in one command:

ffmpeg -i book.m4b -map 0:a -ac 1 -c:a libmp3lame -b:a 64k -f segment -segment_time 1800 -reset_timestamps 1 "part-%03d.mp3"

-segment_time 1800 makes 30-minute parts named part-000.mp3, part-001.mp3 and so on. Splits happen at frame boundaries, not at sentence breaks, so occasionally a word is cut between two files; it’s a minor annoyance on most players.

Keeping track of where you were

MP3 has no standard “bookmark” field, so your position lives in the player, not the file. If you switch between devices, apps that sync positions — or simply noting the chapter number — avoid hunting. Smart AudioBook Player and BookPlayer remember the position per folder, which works well with one-MP3-per-chapter collections.

Summary

M4B is M4A with chapters. For broad compatibility, split DRM-free audiobooks into one MP3 per chapter using the chapter list, encode narration at 48–64 kbps mono, number the files and give them consistent album and artist tags. For a quick single file, rename to .m4a and use a standard converter. DRM-protected Apple Books and Audible titles are a different matter and are not covered here.

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