Free tool
KDP spine width calculator
TL;DR
Spine width = page count × paper thickness. KDP's multipliers: 0.002252in (B&W white and standard color), 0.0025in (B&W cream), 0.002347in (premium color). Your full 6×9 wrap cover is two 6in panels + the spine + 0.125in bleed on every outside edge. Get the spine wrong and KDP rejects the cover file — run your numbers below.
How the math works
Amazon prints your cover as one continuous sheet: back cover, spine, front cover, plus 0.125in of bleed on the outside edges. The spine is exactly page count × paper thickness, so a 220-page book on white paper needs a 0.495in spine — and if your uploaded cover's spine is sized for a different page count, KDP's preflight rejects it or your text wraps around the fold. Finish your interior first, get the final page count, then size the cover.
Pricing the book next? Use the paperback royalty calculator to see what each sale pays at your page count.
Or skip the geometry entirely. ebookdone generates the full wrap cover with the spine computed from your actual page count — plus the gutter-correct interior PDF, EPUB, and KDP metadata — from $9 per book.
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