Glossary → Spine width
Spine width.
the thickness of the book block, which sets the middle panel of the cover file
The width of the spine panel on a flat cover, set by page count and paper. KDP builds the cover width as bleed, back cover, spine, front cover and bleed, so the cover file is as wide as the spine makes it. Lulu gives formulas for softcover perfect binding: pages divided by 444, plus 0.06 in, or pages divided by 460 plus 0.06 in for 460 PPI paper used for magazines and comics (pages per inch). Hardcover casewrap and linen with dust jacket use a table instead, from 0.25 in at 24 to 84 pages to 2.125 in at 800 pages or more. Saddle stitch and coil binding have no spine. Because the number depends on the final page count, the cover cannot be finished before the interior is. Change the page count after the cover is built and the spine panel and the art on it no longer match the book.
Where this matters: KDP spine width, bleed and cover size › Spine width and spine text: KDP and other book printers
Related terms: PPI (pages per inch)