M4B vs MP3 for Audiobooks: Which Format Should You Export?

Export one M4B when people will listen in an audiobook app, and export chapter files when a store will ingest the book. M4B packs chapter markers and bookmarks into one file, with resume built in, but as of October 2026 none of the four store specs we checked lists M4B as an accepted upload.

That mismatch is the whole story. M4B is the best format for listening and an awkward one for delivering. The M4B audiobook export covers both, plus MP3 and WAV, without spending a single point.

M4B vs MP3 for audiobooks: what actually differs

An M4B is AAC audio inside an MPEG-4 container, built for spoken word. Format references describe its selling point as chapter breaks plus bookmarks and resume playback, so a listener can leave at minute 212 and come back to minute 212. An MP3 is just audio. Whatever chapter structure it has comes from separate files or from markers a player may or may not read.

The single MP3 sits in the middle. VocalLab can write chapter markers into it, but support for those markers varies from player to player, so test the file on the device your listeners use before you rely on it.

File size: the arithmetic

Size is bitrate times duration, nothing more. A 90,000-word novel runs about 8 hours 49 minutes, or 31,765 seconds. M4B exports offer AAC from 32 to 128 kbps with 64 as the default, and MP3 exports run from 128 to 320 kbps. Multiply kilobits per second by 125 to get bytes per second, then by the seconds in the book.

Figures are decimal megabytes and ignore tag overhead. The practical read: for speech, an M4B at the default 64 kbps is about a third the size of a 192 kbps MP3 of the same book. One caveat on bitrate. You can only export at or below the quality the book was generated at, so a book generated at a lower quality cannot be exported higher.

What audiobook stores accept, as of October 2026

We read each store's own specification page in October 2026. The pattern is consistent: stores want audio split by chapter or track, and none of the four lists M4B. Specs change, so treat this as a snapshot and check the linked page before you upload.

Kobo is missing from the table on purpose. Its publishing pages would not load for us, and we would rather leave a gap than guess. Also note what the table does not say: a file that meets a spec is not the same as a book a store accepts. Whether a store takes AI narration is a separate policy, and ACX in particular has its own rules on that.

One more reason chapter files travel better: Apple's spec caps a WAV track at 2 GB. Depending on sample rate and channel count, a lossless file of a whole nine-hour book can pass that limit, while a lossless file per chapter stays comfortably below it.

M4B chapter markers and bookmarks in practice

VocalLab reads M4B chapter markers from the encoded audio instead of estimating them from the script, so the marker sits where you hear the chapter begin, even late in a long book. An M4B export is for whole books with several chapters. A single chapter always exports as one file.

Two practical limits. The M4B does not embed cover art, so add it in your player or a tagging tool. And loudness mastering, if you switch it on, is measured once over the whole book, so chapters keep their relative levels.

Playback is where M4B earns its keep. Apple's guide says Books on a Mac imports MP3 audiobooks through File, Import. What each player does with the file after that varies, so load a two-chapter test export into the exact app your listeners use.

A calm British narrator is a fine voice for that test file, since all you need to hear is where each chapter starts.

Settings to check before you export

Three choices change the file you get, and all three are easier to settle before the export than after. None of them costs points, so a second export is cheap, though it is still a second pass over a long book.

1. 1

   Fix the chapter titles

   Marker names come from your chapter titles. If the editor says Chapter 14 and the book calls it Low Tide, rename it first, or every player will show the wrong name.

2. 2

   Pick the bitrate for the destination

   M4B offers AAC at four steps from 32 to 128 kbps. MP3 offers four steps from 128 to 320 kbps. Store specs set their own floor, ACX asks for 192 kbps or higher, so match the spec rather than the default.

3. 3

   Decide on loudness mastering

   It is optional. When on, one pass is measured over the whole book and chapters keep their relative levels, so a quiet chapter is not lifted to match a loud one.

Then listen to the first 30 seconds of chapter 1, a chapter boundary in the middle, and the last 30 seconds of the final chapter. Those three spots catch almost every packaging problem, and they take a minute.

Which format to export for your situation

• Selling or giving the book directly

  Export M4B with chapter markers. One file, resume built in, about 254 MB at the default bitrate for a nine-hour book.

• Uploading to a store

  Export a ZIP of chapter files, or WAV if the store prefers lossless. Check its spec first, then the store's policy on AI narration.

• Embedding on a website

  A single MP3 or a ZIP of chapter MP3s plays in almost any browser player, and you can link each chapter separately.

• Keeping a master

  WAV is lossless. Keep one, then derive M4B and MP3 files from it whenever a platform asks.

There is one real trade-off in choosing M4B alone: it is awkward to edit. Suppose you catch a mispronounced surname in chapter 9 after launch. With a chapter ZIP you regenerate that chapter, swap one file and re-upload it. With a single M4B you re-export the whole book and replace a 254 MB file. Neither is painful, but if you expect corrections, keep the chapter files as your working copy and treat the M4B as the release build. Think of it like software: chapter files are the source tree, and the M4B is the shipped binary.

Because exporting costs no points, there is no reason to pick one. Generate the book once, then package it as an M4B for listeners and a chapter ZIP for any store that asks. Starting from a manuscript? The free audiobook maker page covers the steps before export.

For the longer walk from ebook to finished files, see the ebook to audiobook guide.

Frequently asked questions