How This Spine Width Was Calculated
KDP uses a fixed PPI (Pages Per Inch) constant for each paper type to determine spine thickness. For white standard paper, the PPI constant is 444. The spine width formula is straightforward:
Spine Width = Page Count ÷ PPI
For a 400-page book on white standard paper: 400 ÷ 444 = 0.9009 inches (22.88 mm).
Spine Text Guidelines for This Width
With a spine of 0.9009 inches (22.88 mm), there is space for spine text. KDP recommends a safety margin of 0.0625 inches (1.6 mm) on each side to account for manufacturing tolerance. This leaves 0.776 inches (19.7 mm) of safe text area.
Want to Calculate a Different Page Count?
Use the KDP Spine Width Calculator to enter any page count, paper type, and binding type for an instant result. The calculator also handles hardcover case-laminate editions, which require an additional 0.063 inch board allowance.