The short answer
ACX is the strictest store: 192 kbps or higher constant-bit-rate MP3 at 44.1 kHz, RMS −23 to −18 dB, peaks no higher than −3 dB, noise floor −60 dB or lower, one chapter per file.1 Files that pass ACX meet Google Play, Spotify and Kobo too.2,3,4 A square cover of 2400 × 2400 px or larger works everywhere.
- Safest audio for every store
- 192k CBR MP3
- Levels (ACX)
- −23 to −18 dB RMS
- Cover for every store
- 2400 px square
- Strictest
- ACX
Audio requirements by store
| ACX (Audible) | Google Play Books | Spotify for Authors | Kobo Writing Life | |
|---|---|---|---|---|
| File types | MP3 | MP3, AAC, FLAC, WAV, ZIP, LPF | MP3, WAV, FLAC | MP3 only |
| Bitrate | 192 kbps or higher | MP3/AAC ≥ 128 kbps mono, ≥ 256 kbps stereo | – | Any; re-encoded to about 64 kbps |
| Constant bitrate | Required | Preferred | – | – |
| Sample rate | 44.1 kHz | 44.1 kHz or more (FLAC, WAV) | – | – |
| Loudness (RMS) | −23 to −18 dB | – | – | – |
| Peaks | ≤ −3 dB | – | – | – |
| Noise floor | ≤ −60 dB | – | – | – |
| Room tone | 1–5 s at each end; gaps ≤ 5 s | – | – | – |
| Files | One chapter each, ≤ 120 min | – | – | ≤ 200 MB each, ≤ 2 GB total |
| Book length | – | 5 min to 100 h | – | – |
| Channels | All mono or all stereo | – | – | – |
- ACX also requires the section header spoken in each file, opening and closing credits as separate files, standard letters and numbers in file names, and a retail sample of 5 minutes or less.1
- Google Play accepts a supplementary PDF under 100 MB.2 Auto-narrated books it creates must also be sold on Google Play wherever they’re sold, at no higher price.9
- Voices by INaudio (formerly Findaway Voices) requires constant bit rate files; its full requirements page needs an account to read, and published guides quote ACX’s format and levels.5
- Apple Books takes audiobooks only through its preferred partners, whose delivery rules apply.6
- Kobo asks for a unique ISBN if you supply one (not the print ISBN), and assigns one if you don’t.4
Cover requirements by store
| Minimum size (square) | File types | File size | Colour | |
|---|---|---|---|---|
| ACX (Audible, Amazon, Apple) | 2,400 × 2,400 px or larger | JPG, PNG, TIFF | Under 8 MB | RGB |
| Spotify / Findaway Voices | 2,400 × 2,400 px or larger | JPG, PNG, TIFF | – | RGB recommended |
| Google Play Books | 1,024 × 1,024 px or larger | JPG, PNG | – | RGB recommended |
| Kobo | 600 × 600 px or larger | JPG, PNG | Under 5 MB | RGB recommended |
ACX asks for a square cover filling the 1:1 frame, at least 2400 × 2400 px and 72 dpi, in RGB (24-bit), as JPG, PNG or TIF under 8 MB, showing the title and author, with no borders.7 Spotify for Authors recommends 3000 × 3000 px, PNG or JPEG.3 Google Play’s minimum is 1,024 px and its maximum 7,200 px, square recommended.2 Kobo asks for at least 600 × 600 px, square recommended, up to 5 MB.4 A 2400 or 3000 px square RGB JPEG under 5 MB meets all of them.
Cover Checker & ResizerCheck your cover against every store and save a square 2400 px RGB JPEG that passes them all.Check my coverStore rules beyond the files
| ACX | Google Play | Spotify for Authors | Kobo | |
|---|---|---|---|---|
| Exclusivity | Optional | No | No | No |
| AI narration | Only if authorised | Yes | With a label | Credited as Synthesized Voice |
| Needs an ebook | A Kindle ebook | For auto-narration | No | No |
See how to publish an audiobook for royalties and timelines, ACX requirements explained for the audio rules in depth, and AI audiobooks for each store’s AI rules.