5.25" × 8" cover size for a 400-page book

How this spine width is worked out

A paperback spine is the stacked thickness of its pages, so the width is the page count multiplied by a per-page paper thickness. At 400 pages on white stock that comes to 0.901".

The full-wrap cover is two 5.250" covers plus that spine, which is why the no-bleed width is 11.401". KDP's standard 0.125" bleed then adds a quarter inch to both dimensions, giving the file size you actually upload.

What this book costs to print

On Amazon.com, a 400-page 5.25" × 8" paperback with a black-ink interior costs $5.80 per copy to print — this trim is in KDP's standard band. That sets a price floor of $9.99: below it, a sale would earn a negative royalty and KDP will not accept the price.

At 400 pages the per-page charge applies: KDP's black-ink paperback rate above 110 pages is $1.00 plus $0.012 a page, which is how this book reaches $5.80 per copy. Each further 50 pages adds about $0.60.

Designing a cover for a 400-page 5.25" × 8"

The spine is 0.901" at 400 pages, but the type does not get all of it: KDP wants 0.063" clear on each side, so the usable height is 0.776". 55pt is a designable spine in its own right: title, author, imprint, and enough height that a rule or a repeated motif reads rather than crowds. Switch to cream and the same 400 pages measure 1.000" — 0.099" wider, which is more than the clearance itself, so a cover built on the white figure will not fit.

5.25" × 8" is a 1:1.52 page — the classic book proportion, which is what most cover templates assume. The full wrap at 400 pages is 11.651" × 8.250" with bleed, of which the spine takes 7.7% of the width. KDP's barcode block is a fixed 2in × 1.2in whatever the trim, so on this size it claims 6% of the back cover — keep anything that needs reading out of it. KDP offers 5.25" × 8" in paperback only.

Worth knowing before you commit to the size: the same 400-page book at 6" × 9" costs exactly the same $5.80 to print. KDP prices paperbacks by trim CATEGORY and page count, not by area, and both of these are in its standard-size band — so at this length 5.25" × 8" against 6" × 9" is a design decision, not a margin one.

What 5.25" × 8" is for: A slightly roomier alternative to 5 × 8 — fiction that needs marginally wider lines than 5 × 8 allows, and memoirs and essay collections.