Average level (RMS)
−23 to −18 dBFS
−600 dBFS
How loud the chapter is overall. Almost every rejection starts here.
ACX chapter check
Check and fix an audiobook chapter against ACX requirements
This measures the numbers ACX checks, tells you which fail and by how much, then fixes them and exports a compliant MP3. The tools on this site never upload your audio: every check and fix runs in this tab.
Check a chapter
No file to hand? Try a sample chapter:
Seven seconds each, synthesized by this site rather than recorded, so nobody’s voice is in them.
Ready.
- Fixes it, not just checks it
- Room tone checked
- ACX · Spotify · INaudio · Author’s Republic
- Whole-book batch
- Nothing uploaded
Everything here, in the order you would use it
Eleven tools, one browser tab
Each one does a different job and runs entirely on this device. No account needed, nothing uploaded, no advertising, and nothing to buy.
- Before you record Room test Ten seconds with your microphone tells you whether the space can hold a book at all. Gives a verdict on your room
- Before you record Microphone test Which microphone the browser hears, at what rate, and whether your level is right. Gives level, peak, noise and a playback
- Before you record Book calculator Open the EPUB, PDF, DOCX or TXT itself and see what the job actually is. Gives finished hours, a budget and a narration script
- Per chapter · you are here Chapter checker and fixer The numbers measured against ACX, Spotify, INaudio or Author’s Republic, then corrected. Gives a compliant 192 kbps MP3, re-measured before you get it
- Per chapter Room tone checker Seconds of room at the head and tail against every platform, then topped up or trimmed. Gives an MP3 with both ends in range, re-measured
- Per chapter Remove background noise Learns the hum or hiss from your own quiet passage and subtracts it from the whole file. Gives a cleaner MP3 and the before and after
- Per chapter Trim audio Type a start and end in seconds and keep only that part, at the file’s own rate. Gives the trimmed MP3 beside the original
- The whole book Batch checker Every chapter against ACX and against the book’s own median, so one loud chapter cannot hide. Gives one ZIP of fixed chapters, a CSV and a JSON report
- The whole book Audio joiner Parts in the order you choose, mismatched rates and channels reconciled first. Gives one MP3, or an M4B with a chapter per part
- A different job Podcast loudness LUFS rather than RMS: the ruler podcasts and streaming use, with each platform’s own target. Gives a normalized MP3, measured again after encoding
- A different job Audio converter WAV, AIFF, MP3 or M4A in; MP3 or uncompressed WAV out, keeping the source rate. Gives the converted file and what the encode cost
The five it rejects for
What it checks, before you give it anything
Each scale below shows the whole range the number can land in, with the band ACX accepts marked on it. The buttons play clips this page synthesizes, not recordings, each scaled so its measured level is exactly the figure on the button.
Peak
−3 dBFS or lower
−600 dBFS
The loudest single moment. Usually a plosive, and it is what makes the level fix non-trivial.
Noise floor
−60 dBFS or lower
−900 dBFS
The room between your words. This is the one that is sometimes not fixable, and we say so.
Room tone at both ends
1 to 5 seconds
0 s10 s
And when it is missing we build a second of it from your own quietest half second, not from digital silence.
Encoding
44.1 kHz · 192 kbps · CBR · MP3
Read from the file's own frame headers, every frame, because “constant” is a claim about all of them.
The trap
Why raising the volume is not the fix
Gain moves all three numbers by the same amount. That is why a file comes back rejected for a different reason than last time.
Above a dashed line is a rejection. Only the level lands where it should; the peak clips and the floor rises with it.
So it corrects in the order the numbers allow
A chapter recorded at −27 dB needs about six decibels. This corrects the floor first — against the level it has to reach before the gain — and then limits the peaks on the signal that actually gets encoded.
Then it re-opens the MP3 it just wrote, decodes it and measures it again. If the file it produced does not pass, it says so and offers no download.
Straight answers
Questions narrators actually ask
What are the ACX audio requirements?
ACX requires RMS between −23 and −18 dB, peaks no higher than −3 dB, a noise floor of −60 dB RMS or lower, one to five seconds of room tone at each end, and a 192 kbps or higher constant-bitrate MP3 at 44.1 kHz, with each file under 120 minutes. This page measures every one of them from your file's own bytes.
Is my audio uploaded when I check it?
No. The tools on this site never upload your audio: decoding, measurement and the fix all run inside your browser tab. That is why a manuscript under NDA can be checked here at all. The separate, opt-in developer API processes only the files its key holders send to it.
If every check passes, will ACX accept my audiobook?
Not necessarily. Automated analysis proves the measurable specs; ACX's human Quality Control also judges pronunciation, extraneous sounds and consistency that no meter measures, and ACX's own Audio Lab FAQ says passing analysis does not guarantee approval.
Why does my DAW show a different RMS than ACX?
ACX's own FAQ notes that RMS can be measured in multiple ways, so DAW meters legitimately disagree with each other. This tool measures whole-file RMS in dBFS the way the requirements describe, then re-measures its own encoded output the same way.
Sources: the ACX audio submission requirements and the ACX Audio Lab FAQ.