In depth
Everything you need to know about audiobook chapter length
Audiobook chapter length is set by the book, not by the format. A chapter's running time is its word count divided by the narration pace, about 155 words a minute (9,300 words per finished hour, ACX's figure for most narrators). So a 3,000-word chapter runs about 19 minutes and a 6,000-word chapter about 39. The estimator above gives you the average for the whole book: enter the words and the number of chapters.
Two questions follow: does any chapter break a store's file limit, and are the chapters comfortable to listen to? This guide covers both, and how FullCastBook handles chapters when it builds the audio.
Average audiobook chapter length
There's no official average, because chapters follow the writing. A 90,000-word novel in 30 chapters averages 3,000 words, about 19 minutes each. The same book in 60 short chapters averages under 10 minutes, and a non-fiction book in 12 long chapters averages over 48.
So how long should an audiobook chapter be? Long enough to hold a scene, short enough to fit a sitting: a commute, a walk, a chore. Long chapters aren't a problem for listeners, since players remember the position inside a chapter. The real trouble is at the extremes: a chapter longer than a store allows in one file, or one so short that its title takes longer than its text.
Store limits per file
As of October 2026, ACX requires each file to contain only one chapter or section and to run no longer than 120 minutes, with opening and closing credits in separate files. Spotify for Authors also wants a separate audio file for every chapter, no longer than 2 hours, as MP3 at 192 kbps or higher, WAV or FLAC. Kobo Writing Life sets a size limit rather than a time limit: 200 MB per MP3 file and 2 GB for the whole audiobook.
Google Play's spec for delivered audiobooks accepts the book as one file or as numbered files, and states no length cap on the page we read. In practice, two hours is the audiobook file length limit to plan for. At 155 words a minute, a two-hour file holds about 18,600 words.
Very long chapters: where to split
If a chapter runs over two hours, or simply drags as one block, split it at a natural break: a scene change, a time jump, a switch of point of view. Never mid-sentence or mid-conversation. ACX asks for a secondary header for continuity when a section is split, so the second file opens with something like "Chapter 12, Part 2".
In FullCastBook, you can do this before generating anything. In the import screen, open the chapter with Edit or split, put the cursor where the new part should start and choose Split here. Both parts keep their place in the book, and each becomes its own marker and its own file.
Very short chapters: merge or keep
Short chapters are a style choice, and fast-paced fiction often uses them on purpose, so keep them unless they cause trouble. In audio, a one-paragraph interlude announced with its own title can feel abrupt, and dozens of tiny files make a per-chapter ZIP harder to manage.
To merge, use Merge with the next chapter in the import screen. The merged text still contains the second chapter's heading, so edit it out if you don't want it read aloud.
Front matter, prologues and epilogues
Copyright pages, contents lists and dedications usually stay out of the audio; in the import screen, untick them before you create the project. A prologue or an epilogue is a chapter like any other and gets its own file and marker.
Stores expect short spoken credits instead of printed front matter. ACX's opening credit must state the title, author and narrator, and its closing credit should signal the end; ACX suggests finishing with "The End".
Chapter markers in an M4B vs files in a ZIP
Chapters reach the listener in two ways. In an M4B the whole book is one file, and each chapter is a marker the player lists and skips to, so chapter length doesn't affect file handling. In a ZIP of MP3s every chapter is its own file, which is what per-chapter uploads like ACX's and Spotify's expect, and where the two-hour limit applies. FullCastBook exports both from one project, plus a single MP3 of the whole book; see the M4B audiobook maker.
When text has no headings, the importer can't know where chapters fall, so a long text is cut at paragraph breaks into parts of about 20,000 characters, roughly 3,400 words or 22 minutes each. Treat those as placeholders: add real headings to the source, or split and rename the parts in the import screen. For whole-book hours, use the audiobook length calculator, and the step-by-step guide covers the rest of production.
