What does the ACX checker measure?

It measures the three numbers Audible cares about most: peak level (must be at or below −3 dBFS), RMS loudness (must sit between −23 and −18 dBFS), and noise floor (must be at or below −60 dBFS). It also confirms your file isn't silent and shows its length.

Does my audiobook file get uploaded?

No. The analysis runs entirely in your browser — your chapter never leaves your device. Nothing is stored or sent anywhere.

If everything passes, am I guaranteed to pass ACX review?

These are the core measurable specs and the most common reasons files get rejected, so passing them puts you in great shape. ACX also requires each file under 120 minutes and exported as 192 kbps+ MP3, plus clean room tone and consistent narration — set those in your export.

Free tool

Pass ACX on the first try.

Drop a finished chapter and instantly see whether your peak, loudness and noise floor meet Audible's specs — with a plain-English fix for anything that's off. Runs entirely in your browser; your audio stays on your device.

Try it

Check a chapter against ACX specs

The fastest way to catch a rejection before you submit — no plugins, no signup.

Drop your chapter here

MP3, WAV, M4A or FLAC — up to 25 MB. We'll measure it against the Audible spec.

How it works

Your audio never leaves your device

The checker decodes your file and measures peak, RMS loudness and the quietest half-second of room tone — all on your machine, no server round trip.

Step 1

Drop the chapter

MP3, WAV, M4A or FLAC — up to 25 MB.

Step 2

We measure it

Peak, RMS loudness and noise floor are computed in your browser.

Step 3

Read the checklist

Each spec shows pass or fail with the number and a fix tip.

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Related free tools

Keep going — every one of these runs free in your browser, no signup.

Audiobook Length CalculatorTurn a word or page count into finished runtime, royalties and PFH.Audio Format ConverterConvert between MP3, WAV, OGG, M4A and FLAC — right in your browser.Video to Audio ExtractorPull the audio track out of an MP4, MOV or WEBM as MP3 or WAV.Audio to WAVConvert MP3, OGG, M4A or FLAC to 16-bit PCM WAV.

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Producing a whole audiobook?

Open AudioPod's Audiobook Studio to narrate, master to ACX specs, add chapter credits, and export an ACX-spec package — all on one free account.

Open Audiobook Studio