The One KDP Margin That Changes Depending on How Long Your Book Is
Published 2026-09-14
Most margins are fixed. One isn't.
Amazon KDP's outside, top and bottom margins are simple fixed minimums — 0.25" without bleed, 0.375" with it, regardless of your book's length. The gutter (inside margin, next to the spine) is different: it scales up as your page count grows, from 0.375" for a slim 24-page book all the way to 0.875" for a chunky 700+ page one.
Why length changes the physics
A thicker perfect-bound paperback curves more sharply near the spine when opened — think of how much harder it is to lay a 600-page book flat compared to a 100-page one. Text set too close to the spine on a thick book effectively disappears into that curve and becomes hard to read without cracking the spine. A thin book doesn't have this problem nearly as much, which is why its required gutter is smaller.
The exact thresholds
KDP's published gutter requirements step up at five page-count breakpoints: 0.375" for 24–150 pages, 0.5" for 151–300, 0.625" for 301–500, 0.75" for 501–700, and 0.875" for 701–828. Miss the right tier for your final page count and you risk KDP flagging your interior file, or worse, a physical copy where spine-adjacent text is genuinely hard to read.
One rule easy to get backwards
Because the gutter is the inside margin, it has to be set as your inside margin value in your word processor or layout software — not added on top of your outside margin, and not confused with bleed, which is a completely separate, page-size-changing setting for edge-to-edge color or images.
Get all three numbers at once
Our KDP Bleed, Margin & Gutter Calculator takes your trim size and final page count and returns your document page size (with bleed if needed), the correct gutter for your page count, and your minimum outside margin — all from Amazon's own published formatting rules.