Transcribe a long M4B file your own audiobook, back as text.
A DRM-free M4B audiobook at 64 to 128 kbps takes about 29 to 58 MB per hour, so the 10-hour limit per file arrives long before the 5 GB one, and transcription runs at about 2 minutes per audio hour. The first 20 minutes are free, and an 8-hour narration of 480 minutes fits on the $6 pack of 500 minutes.
M4B, worked out
An audiobook is small for its length. Length, not size, is the line to watch.
Size per hour, calculated
M4B is AAC audio in the same MPEG-4 container as M4A, with chapter marks added. At 64 kbps an hour is 64,000 bits a second times 3,600 seconds, about 29 MB; at 128 kbps it is about 58 MB. A 10-hour book at 64 kbps is under 300 MB.
Ten hours per file, then two files
Many audiobooks run longer than ten hours. A 14-hour narration goes in as two files, for example the first seven hours and the rest, each with its own transcript and timestamps from zero. Name the parts so the order is obvious later.
DRM-free files, and only those
Audiobooks bought from a store are usually copy-protected, and those are not files to upload. This page is for M4B files you are free to work with: your own narration, a recorded lecture series you produced, or a book file its author sent you to check. upload page
If the upload refuses the M4B
If the upload refuses the M4B, convert it to MP3 first; the spoken words survive the conversion. A command such as ffmpeg -i book.m4b -vn -b:a 64k book.mp3 does it in one step and keeps a 10-hour file near 288 MB.
Why long M4B files get stuck elsewhere
A whole audiobook is far longer than most tools expect a file to be.
✗ Clip-sized limits
One hour of M4B at 128 kbps is already more than twice the OpenAI Whisper API's documented 25 MB upload limit.
- Files cut into dozens of pieces
- Length caps measured in minutes
- Chapter files with timestamps reset
- No overview of the whole book
- Unclear handling of the .m4b extension
- Typing out narration by hand
✓ Whole chapters of narration in one file
Up to 10 hours goes in at once and comes back with a brief, chapters and the full text.
- About 2 minutes per hour of narration
- Chapters from what is said
- Names, dates and numbers listed
- DOCX for an editor, SRT for captions
- An 8-hour book on the $6 pack
- Speech transcribed, music beds left out
Sample
A self-narrated memoir, five hours in.
This sample is illustrative, written for this page rather than taken from any real book or narrator. Times show where each line sits in the M4B file.
Export
One transcript. Five clean formats.
Every pack exports all five — no format is held back for a higher tier.
Plain text
De-ummed paragraphs. Ready to paste.
SRT captions
Timed to the speech. Every video editor reads this.
WebVTT
HTML5 player + YouTube uploads.
Show notes
Formatted with chapters and pull-quotes.
Machine-readable
Timestamped text for your own tools.
Pricing
Honest pricing, no surprises.
A pack from $6 (500 minutes) opens the full transcript. After that, no subscription and no auto-renew — every tier is a one-time minute pack, and credits never expire. Buy once, use the minutes whenever you need them.
Casual pack
$6/500 min
8 hours of audio — $0.72 an hour. One-time — credits never expire.
- 500 minutes of audio
- All export formats
- Unlimited AI chat per transcript
- Executive summary & key quote
Best value
$12/2,000 min
33 hours of audio — $0.36 an hour. Credits never expire.
- 2,000 minutes of audio
- Priority queue
- All export formats
- Unlimited AI chat per transcript
Bulk pack
$24/5,000 min
83 hours of audio — $0.29 an hour. Credits never expire.
- 5,000 minutes of audio
- Priority queue
- API access for batch pipelines
- All export formats
FAQ
Long M4B files, the questions people ask.
How long does an M4B audiobook take to transcribe?
About 2 minutes per hour of audio, so an 8-hour narration takes roughly 16 minutes. Because M4B files are small, the upload is quick too.
What does an audiobook cost to transcribe?
An 8-hour book is 480 minutes on the $6 pack of 500. A 14-hour book is 840 minutes, which needs the $12 pack of 2,000. Six books averaging 10 hours total 3,600 minutes on the $24 pack of 5,000. First-time visitors pay nothing for the first 20 minutes.
How big is an M4B per hour?
About 29 MB at 64 kbps and 58 MB at 128 kbps. Even a 10-hour M4B at 128 kbps is under 600 MB, inside the 2 GB that uploads before a pack and far under the 5 GB per-file limit.
My audiobook is longer than 10 hours. How should I split it?
Cut it into two files at a chapter boundary and upload each. With ffmpeg, -t 7:00:00 keeps the first seven hours and -ss 7:00:00 starts the second part there; add -c copy to cut without re-encoding.
Which languages can the narration be in?
Any of 99, detected automatically, so a book read in Spanish, Swedish or Japanese needs no setting. Background music and sound effects are not transcribed; the narration is.
What exports come back, and what happens to the file?
The narration downloads as TXT, DOCX, SRT, VTT or JSON. M4B files are transcribed on our own servers and are never used for training.
Related
Other long formats
Upload the M4B. Your narration, back as text in minutes.
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