How KDP calculates the spine
Spine width is page count times paper thickness. KDP uses 0.0025 in per page for cream paper, 0.002252 in for white paper and standard colour, and 0.002347 in for premium colour.
Cover size with bleed
The full cover width is bleed + back + spine + front + bleed, with 0.125 in bleed on each outer edge. The height is the trim height plus twice 0.125 in.
Gutter by page count
The thicker the book, the wider the inside margin: 0.375 in up to 150 pages, 0.5 in up to 300, 0.625 in up to 500, 0.75 in up to 700 and 0.875 in up to 828 pages.
Papyrus Peak does this automatically: interior and cover adapt to every change in your book. Try for free →