Definition
Gutter
The gutter is the extra inside margin next to a book's spine, added because binding swallows part of the page. Required gutter grows with page count — on KDP from 0.375" (≤150 pages) to 0.875" (828 pages).
Thicker books curve more sharply at the spine, so more of the inner page disappears into the binding. That's why gutter requirements are a table by page count rather than one number, and why the gutter can only be finalized after the book is typeset and its true page count known. Professional workflows render, count, then apply the bracket — by hand in InDesign, automatically in tools like Ceeryo.
Related tools & guides
Measure yours
Drop an interior PDF into the free KDP checker: it detects the trim, measures the inside margin on every page against the gutter your page count needs, and names the pages that fall short.