How spine width is calculated
Real pages × the thickness of the paper you picked. The Path A, B, or C spec says it; Path D draws the jacket with that number.
The spine is the strip where the title fits (or does not) on a shelf. It is not a Canva decoration. It is physics: how many leaves, and how thick they are. Amazon publishes different formulae for white, cream, and colour stock. Ingram uses its own papers. One extra millimetre and the title eats the front; one less and a stripe of colour shows on the edge.
I do not use a blog calculator with “250 pages × 0.0025.” I use the count of the PDF after layout. Word said 220; after the contents, the images, and closing a signature of 4, it was 228. That 228, with the paper I ticked (cream, white, or heavyweight on Path A/B; album papers on Path C, which are thicker and grow the spine faster), is what enters the math. On this site each paper has a thickness per page; the program multiplies and shows the spine on the spec sheet, next to the trim.
That spec travels to Path D. There the cover PDF is one piece: back + spine + front. If the spine is very thin, sometimes only the title fits, or not even that: the mold already allows for it. Do not force a blurb onto a 3 mm spine. If you change paper or add signature pages, generate the interior again and return to D; do not stretch the old jacket in Photoshop.
Hardcover does not use the paperback spine: you add board (here, 0.06 inches on the hardcover wrap, plus hinge and wrap that glues inside). Reusing the paperback PDF is the mistake KDP hardcover guides are full of. On Path D you pick paperback, paperback with flaps, hardcover, or hardcover with jacket; the mold changes.
Ingram and Amazon can give you an official template if you give them pages and paper. Fine to check against. The file I upload is the Path D file, with my interior’s pages and paper, not a generic 6×9 template from a forum.
