Audiobook Spec Checker: ACX, Spotify, Google Play, Kobo

Where it runs

Works in a current Chrome, Edge, Firefox or Safari, on Windows, Mac, Linux and Chromebook computers, Android phones, iPhones and iPads. Nothing to install, no account.

The first file loads the audio engine (ffmpeg, about 31 MB) once, and your browser keeps it. Very long books are easiest on a computer, which has more memory than a phone.

4 simple steps.

Drop your files

Every chapter of the book, or the whole folder: MP3, M4B, M4A, WAV or FLAC.

Measured once

Each file is decoded on your device and measured the way the ACX checker does it.

Judged five ways

ACX, Spotify/Findaway, Google Play, Kobo and Apple Books, each with its own rules and sources.

Fix what fails

Open any file to see what fails and why, with a fix for each.

Audiobook delivery specs by platform

Platform	Format	Levels	Other
ACX (Audible, Amazon, Apple)	MP3, 192 kbps+ CBR, 44.1 kHz	RMS −23 to −18 dB, peak ≤ −3 dB, noise ≤ −60 dB	Room tone 1–5 s at each end; files ≤ 120 min; all mono or all stereo
Voices by INaudio (formerly Findaway Voices)	192 kbps+ CBR, 44.1 kHz	As ACX	Room tone 0.5–1 s at the start, 1–5 s at the end; files ≤ 120 min
Google Play Books	MP3 or AAC ≥ 128 kbps mono / 256 kbps stereo; FLAC or WAV 16-bit ≥ 44.1 kHz	No rule	Book 5 min to 100 h
Kobo	MP3 only (re-encoded to about 64 kbps)	No published rule; ACX levels are safe	Files ≤ 200 MB, book ≤ 2 GB
Apple Books	Through Apple’s distribution partners: their rules apply (the shared format above).

Rules checked on 23 October 2026. Every platform also reviews by ear and checks content (credits, consistency, edits), which no meter can. Failing on levels or format? ACX mastering produces files at ACX’s levels and format, which the other platforms accept too; for Voices by INaudio, keep the room tone at the start to about a second, which suits ACX as well. audiobook.tools is not affiliated with any of these stores.

Questions, answered.

If my files pass ACX, will they pass everywhere?

Nearly always. ACX’s rules are among the strictest, and the distributors that reach Spotify and Apple use the same format: 192 kbps constant-bitrate MP3 at 44.1 kHz with the same loudness, peak and noise limits. Kobo takes any MP3 and re-encodes it to about 64 kbps; Google Play also takes AAC, FLAC and WAV and sets no loudness rule.

How do I publish to Apple Books?

Apple takes audiobooks through its preferred distribution partners rather than direct uploads, so the partner’s delivery rules apply. The Apple column uses the format those distributors share.

Why does Kobo check file size?

Kobo Writing Life caps each audio file at 200 MB and a whole book at 2 GB. A 192 kbps mono MP3 is about 86 MB an hour, so most chapters fit easily.

Where do the rules come from?

Each platform shows its source and the date we checked it. Where a store’s own page couldn’t be read, the tool says so and uses the figures published guides quote. Stores change their rules, so check their pages before publishing.

Are my files uploaded?

No. Every file is measured in your browser and never leaves your device.