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Spine width

Printing & file preparation

Also called: spine thickness

How thick the book's spine is — page count multiplied by a per-page paper thickness, plus a fixed board offset on hardcovers. It determines where the fold lines sit on your cover file.

A paperback spine is just the stacked thickness of the paper, so its width is the page count times a per-page constant that depends on the paper you chose. White and cream stock differ, and colour stock differs again. Hardcovers add a fixed allowance for the board on top of that.

Spine width matters because it positions the two folds on a full-wrap cover. If your calculated spine is wrong by a tenth of an inch, spine text sits off-centre and cover artwork wraps onto the wrong face — a mistake that is invisible in the design file and obvious in the printed book.

KDP only prints spine text on books over 79 pages, but legibility is a separate and stricter question. KDP also requires clear space on each side of spine text, which eats into a thin spine fast: an 80-page black-and-white interior leaves roughly four points of type height once that clearance is taken. Around 150 pages is a more realistic floor for spine text you actually want read.

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Related terms

  • Trim size

    The finished width and height of a printed book after the pages are cut. KDP publishes a catalog of standard trim sizes, and the one you pick affects both layout and printing cost.

  • Full-wrap cover

    A single file containing back cover, spine, and front cover as one continuous image. It is the format KDP requires for print books, as opposed to a front-cover-only image.

  • Interior paper

    The stock your pages print on — white, cream, or groundwood for black ink, and colour stock for colour interiors. It changes both the printing cost and the spine width.

  • Print on demand

    Manufacturing a book only after someone orders it. There is no print run, no inventory, and no minimum quantity — and the per-copy cost is the same whether you sell one or a thousand.