5×8" · 150-Page Book.

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 150-page book on white standard paper: 150 ÷ 444 = 0.3378 inches (8.58 mm).

Spine Text Guidelines for This Width

With a spine of 0.3378 inches (8.58 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 0.213 inches (5.4 mm) of safe text area.

With a spine under 0.5 inches, the available text area is limited. Consider using only the title in a small font size, or using only the author's last name. Text that is too large may drift onto the cover panels after binding.

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.