Audiobook File Specifications by Retailer

ACX's spec is the most detailed we read: MP3 at 192 kbps or higher, constant bit rate, 44.1 kHz, with loudness and noise limits. Google states 128 kbps for mono and 256 kbps for stereo MP3, the part of Apple's specification we read (version 5.3.5, December 2024) states no bit-rate floor, and BookFunnel wants MP3 files. We could not read a spec from a primary page for Voices by INaudio, so this page gives no numbers for it. Kobo's limits appear below with a caveat, because they come from search-result text of its help pages.

Specs actually read, by retailer

The table shows only what each source stated on November 8, 2026. A blank means the page was silent. The summaries come from an extraction tool, so confirm figures on the linked page before you export.

Apple's specification, in the portion read, addresses AI only by listing an "AI Generated by" contributor role, without explanation. Apple's separate formatting guidelines say audiobooks narrated by AI must state this in the title or narrator field and the description. The unread 36,000 or so characters could say more. For rules on who accepts AI audio, see Apple Books, Google Play and Voices by INaudio. A matching file spec is not an acceptance rule.

One export that clears the most stated minimums

A mono, 192 kbps constant-bit-rate MP3 at 44.1 kHz reaches the stated figures at the most outlets on this page. That is our reading of the table, not any retailer's advice.

• It meets ACX's format floor, BookFunnel's 192 kbps minimum and Google's 128 kbps mono minimum, and it is the .mp3 Kobo's help text asks for. Apple accepts MP3 but prefers other formats.
• Stereo is the trap. A 192 kbps stereo MP3 clears ACX and BookFunnel but falls short of Google's stated 256 kbps stereo minimum.
• Keep a lossless master if your tool can produce one. Google's publisher policies and Apple's specification both prefer better sources than MP3, and Apple rejects an MP3 upscaled to WAV, so you cannot get that quality back later.
• Check the route before you process anything. INaudio requires the provider's original, unmodified LPF package for digital voice narration. We read that as leaving no room for mastering those files, though the FAQ does not spell it out, so ask INaudio first.

File sizes against Kobo's limits

Kobo's limits, as shown in search-result text, are the only per-file size caps on this page, so they are worth working out before export. At a constant bit rate, megabytes per minute is kbps x 60 / 8 / 1,000. These are estimates: they treat 1 MB as 1,000,000 bytes, ignore tags, and Kobo's text does not define its units.

At 192 kbps, ACX's 120-minute limit bites before Kobo's file limit does. At 320 kbps, Kobo's bites first. Kobo's help text suggests lowering the bit rate or switching from stereo to mono if files are too large.

Pre-upload file checklist

Use this after the listening pass and before you upload. Each line gives the pass figure and who states it, as read on 2026-11-08. A figure applies only to the outlets named, and ACX requires human narration unless authorized, so for AI audio its numbers serve as a general yardstick. Content problems and metadata belong in proofing an AI audiobook.

Measure RMS, peak and noise floor on every file, not on one chapter, with an audio editor or analyser that reports all three. Check the noise floor again after any gain change, because raising the level raises any noise with it.

Where a tool's export tier meets a spec

Several tools gate bit rate or export format by plan, so check the plan before you plan the spec.

• ElevenLabs. Its pricing page lists 128 kbps on Free and Starter and 192 kbps from Creator up. ElevenLabs' Studio docs list 128 kbps for Creator and 192 kbps from Pro for new projects, so confirm the bit rate of your actual export. A 192 kbps MP3 meets ACX's floor, but ACX requires human narration unless authorized, so that benchmark is useful only as a general yardstick. A 128 kbps file would meet Google's stated minimum only if it is mono, and we have not checked the channel layout. The docs list MP3 up to 192 kbps and PCM on Pro through the API. See the ElevenLabs review.
• Murf. Its help page lists MP3, WAV and FLAC, with a High setting of 48 kHz, 16-bit, and says export needs a paid plan without naming one. It states no MP3 bit rate. See the Murf review.
• Speechify Studio. Its pricing page says MP3 download needs a paid plan. Bit rate is not stated. See the Speechify review.

None of these tool pages says anything about loudness. ACX's RMS, peak and noise-floor limits would need mastering outside the generator. Direct sellers can use selling audiobooks direct.

Who should not rely on this page

If you are about to submit, do not treat this table as the spec. Download the current version from each retailer. Voices by INaudio has no figures here, Kobo's come from search-result text, and Apple's read is incomplete. Back to the AI audiobooks hub for other routes.

Last verified 2026-11-08. Rules and prices change: check the linked primary page before you rely on this.