Spine width Defined.
Definition
Spine width is the thickness of a printed book's spine, which the cover file must include between the back and front covers. KDP calculates it from the page count and the paper: on white paper each page adds 0.002252 in, and KDP prints spine text only on books with more than 79 pages.
Sources checked 20 October 2026. Figures: KDP's paperback royalty rates, trim classes and spine figures, as of October 2026.
How it works
A KDP paperback cover is one PDF: back cover, spine and front cover side by side, with bleed all round. Its width is bleed, plus the back cover's width, plus the spine, plus the front cover's width, plus bleed; its height is the trim height plus bleed at the top and bottom. Cream paper is thicker than white, so the same page count makes a wider spine. Spine text must sit inside the spine's safe area, with clearance from each fold, because every book varies slightly when it is bound.
An example
A 200-page, 6 x 9 inch paperback on white paper has a spine of 0.45 in. With the trim on each side plus bleed, the full cover file is 12.7 in wide and 9.25 in tall. At that thickness the spine can carry the title and author, set small enough to clear both folds.
Common mistakes
Designing the cover before the page count is final: every added page widens the spine and shifts the front cover. Using the white-paper figure for a cream-paper book. And putting text on the spine of a short book: KDP rejects a cover with spine text when the book has too few pages. Its help page words the threshold two ways, so design to the stricter: more than 79 pages.
