Audiobook File Formats Explained (MP3, M4B, AAC, and More)
Audiobook formats sound more complicated than they are. You really only need to know a handful, what each is good for, and which one to hand a retailer. Here is the whole landscape in plain language.
The Formats That Matter
- MP3 — the universal format. Plays everywhere; the safe default, usually one file per chapter.
- M4B — an audiobook container (AAC inside) with chapters and bookmarking in one file.
- AAC / M4A — the audio codec inside M4B; efficient, good quality per megabyte.
- WAV — uncompressed and large; a mastering format, not a delivery one.
- AAX — Audible's proprietary, DRM-protected format; you do not create these, Audible does.
What Retailers Want
Most retail delivery (ACX included) is built around chaptered audio to a technical spec — commonly MP3 at 192kbps CBR and 44.1kHz. Deliver one cleanly named file per chapter and you are safe. See ACX requirements for the exact numbers.
The Two Numbers to Know
- Bitrate (e.g., 192kbps) — how much audio data per second; higher is cleaner and larger.
- Sample rate (e.g., 44.1kHz) — how often the audio is sampled; 44.1kHz is the audiobook standard.
The short answer
For delivery, use MP3 (192kbps, 44.1kHz) per chapter — universal and accepted everywhere. Keep M4B for tidy personal listening. AudioBookByMe outputs the right specs automatically; deeper dive in M4B vs MP3.
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Free: ACX Audio Requirements Checklist
The complete spec sheet for publishing on Audible, Apple Books, and more — plus tips to speed up your audiobook production.
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