Audiobook File Inspector: chapters, tags, bitrate

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 file

Drag any audiobook file onto the page: MP3, M4B, M4A, FLAC, WAV, OGG or Opus.

Read the details

Format, bitrate, chapters, tags and cover art appear instantly, read straight from the file.

Watch the terrain

The whole book is measured on your device and drawn as a 3D loudness landscape.

Check the levels

RMS, peak and noise floor are shown against ACX targets so you know where you stand.

What’s inside an audiobook file?

An audiobook file is more than audio. It carries a container (such as MP4/M4B or MPEG), an audio codec (AAC, MP3, FLAC), technical settings like bitrate and sample rate, and often metadata: title, author, narrator, cover art and, in M4B files, chapter markers. When an app shows the wrong cover, no chapters or the wrong order, the answer is almost always in this metadata.

Formats at a glance

Format	Codec	Chapters	Typical use
M4B	AAC	Yes, embedded	Single-file audiobooks for Apple Books, BookPlayer, Audiobookshelf
M4A	AAC	Sometimes	Music and audio; can be renamed to M4B
MP3	MP3	Rarely	One file per chapter; ACX uploads
FLAC / WAV	Lossless / PCM	No	Masters and editing

Reading the loudness terrain

The terrain is the loudness of your book over time. Its length is the full duration, and its height is the RMS loudness of each moment. Quiet passages and room tone are low valleys; dramatic scenes rise as peaks. Columns that turn red contain peaks above −3 dBFS, the ACX limit. The teal beacons standing behind the landscape mark where each chapter begins.

Checking against ACX

For ACX (Audible) submissions, each file needs an overall RMS between −23 dB and −18 dB, peak values no higher than −3 dB and a noise floor no higher than −60 dB. The gauges above show your file against those targets. A dedicated ACX checker that tests every requirement for a whole book at once is next on our shelf.

Questions, answered.

Is my audiobook uploaded?

No. The file is read and measured inside your browser with WebAssembly. It never leaves your device, which is also why there is no size limit or upload wait.

What can the inspector tell me?

The container and codec, bitrate and whether it is constant or variable, sample rate, channels, duration and size, every embedded chapter with its start time, all tags such as title, author and narrator, the cover art and its dimensions, and the loudness of the whole book.

Why does my MP3 show no chapters?

Most MP3 files do not store chapters. Audiobooks sold as single files usually use M4B, which stores chapter markers inside the file. Multi-file MP3 audiobooks usually treat each file as a chapter.

What do RMS, peak and noise floor mean?

RMS is the average loudness of the recording. Peak is the loudest single moment. Noise floor is the level of the quietest half-second, which shows how much background noise there is. ACX asks for RMS between −23 and −18 dB, peaks no higher than −3 dB and a noise floor of −60 dB or lower.

How long does measuring take?

Reading tags and chapters is instant. Measuring loudness decodes the whole book on your computer, so a long audiobook can take a few minutes. The first time, the audio engine (about 31 MB) is downloaded once and then cached.

Can it open Audible AAX files?

No. AAX and AAXC files are DRM-protected and cannot be inspected or converted here. The inspector recognises them and tells you so.