2026-06-14 · all guides

Book Margins Guide: Inside, Outside, and the KDP Gutter Numbers

Why book margins are not equal on all sides

A word processor page has the same margin on the left and right. A book page cannot, because a book is bound. Part of the inner edge of every page curves into the spine, so text placed too close to the binding disappears into the fold. Book layouts therefore use mirrored margins: the inside margin, the one against the spine, is larger than the outside margin, and the extra inner space is called the gutter. On a left-hand page the gutter is on the right; on a right-hand page it is on the left.

This is the first thing that separates a real book interior from a printed document. If you take a manuscript with uniform one-inch margins and send it to print, the text block sits in the wrong place on every spread. Setting mirrored margins with a proper gutter is a five-minute change that instantly makes the interior read as a book.

The KDP gutter requirements, by page count

The thicker the book, the deeper the pages curve into the binding, so the required gutter grows with page count. KDP publishes exact minimums for the inside margin: 0.375 inches for books up to 150 pages, 0.5 inches for 151 to 300 pages, 0.625 inches for 301 to 500 pages, and 0.75 inches for 501 to 700 pages. These are enforced at upload; a file that violates them is rejected by the previewer, not quietly printed wrong.

Note the chicken-and-egg problem: the gutter depends on page count, but changing margins changes page count. Format your interior, check the resulting page total, and confirm you are still inside the right bracket. A book that lands at 148 pages with a 0.375 inch gutter is fine, but if edits push it to 155 pages you must widen the gutter to 0.5 inches and repaginate.

Outside, top, and bottom: the numbers nobody publishes

KDP requires at least 0.25 inches on the outside, top, and bottom for a book without bleed, but the legal minimum is not a good design target. Text running 0.25 inches from the page edge looks crowded and risks being visually clipped by trim variance. Comfortable trade values are 0.5 to 0.75 inches on the outside edge and 0.75 to 1 inch on the top and bottom, with the bottom margin often slightly larger than the top so the text block sits a touch high on the page, which reads as balanced.

The margins also have to make room for furniture. Running heads live in the top margin and page numbers usually live in the bottom margin, and both need clear separation from the body text. If you use them, keep roughly 0.3 inches between the body text and either element, which is another reason the 0.25 inch minimum is unworkable in practice.

A worked example and an easier path

Take a 220-page nonfiction book at 6 by 9 inches. Page count puts it in the 151 to 300 bracket, so the inside margin must be at least 0.5 inches; set it to 0.75 for comfort. Set the outside margin to 0.5 inches, the top to 0.75, and the bottom to 0.85 to hold a centered page number. With an 11 point serif body on 14.5 point leading, that produces a text block around 4.75 inches wide, which yields a comfortable 60 to 70 characters per line.

If you would rather not manage mirrored margins, brackets, and repagination by hand, this is exactly the kind of mechanical work a generator should do. ebookdone computes the correct gutter for the final page count automatically and outputs a KDP-ready PDF, so the margin math never becomes your problem. Either way, run the KDP previewer before publishing; it is the authoritative check.

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