Audiobook File Formats: What Every App and Store Uses

The short answer

Audiobooks you own as files are MP3 or M4B. Store audiobooks mostly stay inside each store’s app: Audible’s are protected, Spotify and Libby keep downloads in their apps, Kobo’s are MP3 in its apps.^1,6,8,12 Libro.fm and LibriVox give you DRM-free files.^10,11 For your own library, M4B is the best format; for selling, each store sets its own.

Best for your own books: M4B
Plays everywhere: MP3
Audible: Protected, app only
DRM-free sources: Libro.fm, LibriVox

What format each store and app gives you

Audiobook formats by store and app (for listeners)
	What you get	DRM-free file	Plays outside the app
Audible	Protected downloads, standard or high quality	No	Approved MP3 players via AudibleSync (Windows)
Apple Books	Audiobooks in Books; MP3 and Audible books can be imported on a Mac	–	–
Google Play Books	Export from the web library, if the publisher allows	If allowed	If allowed
Kobo	MP3, in Kobo’s apps and some eReaders	No	No
Spotify	Whole-book downloads in the app	No	No
Libby	Streams and downloads in the app; MP3 for some library cards	No	MP3 cards only, during the loan
Libro.fm	DRM-free downloads	Yes	Yes
LibriVox	MP3 (64 or 128 kbps) and, for most books, M4B	Yes	Yes

Audiobook formats by store and app (for listeners). From each service’s own help pages, checked 22 October 2026. Audible’s help pages no longer name its file formats. – means the service doesn’t say.

Audible

Audible’s help pages now describe downloads only as “standard-quality files or high-quality files”, for its apps.¹ On Windows, AudibleSync downloads titles to transfer to supported MP3 players and accessibility devices; they can’t be played on the computer itself.² Third-party sources describe Audible’s files as AAX, an encrypted M4B, mostly at 64 kbps.³ Producers deliver MP3s to ACX, which Audible re-encodes for sale.¹⁴ This site doesn’t remove DRM. How to listen to Audible books on each device, and the DRM-free alternatives, are in Audible AAX files.

Apple, Google, Kobo, Spotify and Libby

Apple Books: on a Mac you can import “mp3 audiobooks, Audible audiobooks and many EPUB books”, then sync to an iPhone with Finder.¹³
Google Play Books: on a computer you can export an audiobook from your library, if the publisher allows (check “Export option” on the book’s page); files are “DRM-free if allowed by the publisher.” Google doesn’t say which file format.^4,5
Kobo: “Audiobooks from Kobo are in MP3 format,” playing in the Kobo apps for Android and iOS and on some eReaders, not in the desktop app.^6,7
Spotify: you can download a whole audiobook for offline listening in the app, not single chapters.¹²
Libby: “Audiobooks in Libby can’t be transferred to MP3 players or iPods.” Library cards with MP3 permission can download MP3 files, to be deleted when the loan ends.^8,9

Where to get DRM-free audiobook files

Libro.fm sells DRM-free downloads you can play “on any device”.¹⁰ LibriVox offers its public-domain recordings as 64 and 128 kbps MP3s, ZIPs of whole books and, for most books, M4B files with chapters.¹¹ Files you make yourself, from your own recordings or CDs you own, are DRM-free too.

Audiobook Web PlayerPlay DRM-free MP3s and M4Bs from any source in your browser, with chapters and your place remembered. Free · runs in your browser · nothing uploadedOpen the player

What stores accept from authors

Audio file formats each store accepts from publishers
	Accepted files	Key rule
ACX (Audible)	MP3	192 kbps or higher CBR, 44.1 kHz, one chapter per file
Google Play Books	MP3, AAC, FLAC, WAV, ZIP, LPF	MP3 ≥ 128 kbps mono or ≥ 256 kbps stereo
Spotify for Authors	MP3, WAV or FLAC	Square cover, 3000 × 3000 px recommended
Kobo Writing Life	MP3 only	Up to 200 MB a file, 2 GB in total; compressed to about 64 kbps

Audio file formats each store accepts from publishers. From each store’s own requirements, checked 22 October 2026. See our full store requirements page.

ACX is the strictest: MP3 at 192 kbps or higher, constant bit rate, 44.1 kHz.¹⁴ Google takes several formats, with minimum bitrates.¹⁵ Spotify takes MP3, WAV or FLAC.¹⁶ Kobo takes only MP3 and compresses most audio to 64 kbps on arrival, “so any higher fidelity is not necessary.”¹⁷ The store requirements page has every rule in one place.

Platform Spec CheckerCheck your files against ACX, Spotify, Google Play and Kobo at once. Free · runs in your browser · nothing uploadedOpen the tool

M4A, AAC, Opus and FLAC

Beyond MP3 and M4B, four other formats turn up in audiobook folders. What each is, and when it’s the right choice:

Format	What it is	Chapters	Plays on	Use it for
M4A	The same MPEG-4 file as M4B, with a music extension²⁵	Yes	Almost everything	A copy of an M4B for players that skip .m4b
AAC (.aac)	The codec inside M4A and M4B, as a bare stream	No (no tags either)	Most players; Android lists raw AAC²⁰	Nothing: wrap it in M4B
Opus (.opus, .ogg)	An open codec that keeps speech clear at low bitrates	Yes, as comments	Android 5.0+ (in .ogg, .mp4, .mkv); Apple added Ogg Opus to Safari in iOS 18.4 and macOS 15.4; Audiobookshelf, Voice^20,21,24,18	Your own server or Android
FLAC	Lossless compression: every bit of the master	Rarely read	Android, desktop players²⁰	Masters and archives
AAX, AAXC	Audible’s protected formats	Yes	Audible’s apps and approved devices	Listening in Audible; see Audible AAX files

An MP4 file can carry Opus (there is a published specification for it),²² but ffmpeg’s M4A and M4B writer only accepts AAC, ALAC and a few others,²³ and Apple doesn’t say whether Books plays Opus at all. So an “Opus M4B” is a curiosity: for an audiobook to play everywhere, keep AAC in the M4B and use Opus only where you know the player handles it. The audio to M4B converter turns M4A, AAC, FLAC, WAV, OGG and Opus files into one chaptered AAC M4B.

The best format for your own audiobooks

Listening: M4B with AAC audio. One file per book, chapters, cover; Audiobookshelf’s docs call it “convenient if you want to have 1 file per book”.¹⁸ AAC also beats MP3 at the same size.¹⁹
Old devices and car stereos: MP3, one file per chapter. “MP3 is supported by pretty much everything.”¹⁸
Masters and archives: WAV or FLAC, lossless, kept for re-exporting.

More detail in M4B vs MP3 and what is an M4B file.

Any Audio to M4BConvert M4A, AAC, FLAC, WAV, OGG or Opus audiobooks into one chaptered M4B. Free · runs in your browser · nothing uploadedOpen the tool

Everything else, answered.

What format are audiobooks?

Usually MP3 or M4B. Stores use their own, often protected, versions: Audible’s downloads are protected and play only in its apps, Kobo’s are MP3 in its apps, and Libby and Spotify stream or download inside their apps. DRM-free audiobooks, such as Libro.fm’s and LibriVox’s, come as MP3 or M4B files.

What format does Audible use?

Audible’s help pages no longer name a format; they offer standard- and high-quality downloads for its apps. Third-party sources describe its files as AAX, an encrypted version of M4B. They play only in Audible’s apps and on approved devices.

Are Audible books MP3 files?

No. Producers upload MP3s to ACX, but Audible re-encodes them into its own protected formats for listeners. On Windows, AudibleSync can transfer titles to supported MP3 players and accessibility devices.

What format does Apple Books use for audiobooks?

Apple’s Mac guide says you can import MP3 audiobooks and Audible audiobooks into Books. Books on iPhone gets audiobooks you’ve imported on a Mac through Finder sync.

What format does Kobo use?

MP3: Kobo says “Audiobooks from Kobo are in MP3 format.” They play in the Kobo apps for Android and iOS and on some Kobo eReaders over Bluetooth, not in the Kobo desktop app.

Can I download Google Play audiobooks?

Sometimes: on a computer you can export an audiobook from your Play Books library if the publisher allows it; the book’s details page shows whether export is available. Google says downloads are DRM-free if the publisher allows.

Can I put Libby audiobooks on an MP3 player?

Generally no. Libby says its audiobooks can’t be transferred to MP3 players. Some library cards have permission to download MP3 files, which must be deleted when the loan ends.

What is the best audiobook format?

For your own listening, M4B with AAC audio: one file, chapters, cover and good quality at small sizes. For the widest compatibility, MP3. For selling, whatever each store asks for, such as 192 kbps CBR MP3 for ACX.

Is M4A the same as M4B?

Inside, yes: both are MPEG-4 audio files, usually AAC, and both can hold chapters. The .m4b extension tells players it’s an audiobook, so they remember your place; rename a copy to .m4a for a player that won’t open .m4b.

Is Opus good for audiobooks?

For size, yes: it handles speech well at low bitrates, and Android, Audiobookshelf and Voice play it. But it isn’t as widely supported as AAC or MP3, and ffmpeg won’t write Opus into an .m4b file. Use Opus for your own server or Android, AAC M4B for everything else.

What is the standard audiobook file extension?

.m4b is the audiobook-specific extension (MPEG-4); .mp3 is the most common general one. AAX and AAXC are Audible’s protected formats.