5.06" × 7.81" cover size for a 100-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 100 pages on white stock that comes to 0.225".

The full-wrap cover is two 5.060" covers plus that spine, which is why the no-bleed width is 10.345". 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 100-page 5.06" × 7.81" paperback with a black-ink interior costs $2.30 per copy to print — this trim is in KDP's standard band. That sets a price floor of $4.60: below it, a sale would earn a negative royalty and KDP will not accept the price.

At 100 pages this book is inside KDP's flat-fee band. Every black-ink paperback from 24 to 110 pages costs the same $2.30 to print on Amazon.com, whatever its length — so the next 10 pages cost you nothing per copy. Trimming the interior to save money does not work here; you are already at the floor.

Designing a cover for a 100-page 5.06" × 7.81"

The spine is 0.225" at 100 pages, but the type does not get all of it: KDP wants 0.063" clear on each side, so the usable height is 0.100". 7pt is barely above KDP's 7pt floor. Set the title alone, in a condensed face, and expect it to be read off a shelf only by someone already looking for it.

5.06" × 7.81" is a 1:1.54 page — the classic book proportion, which is what most cover templates assume. The full wrap at 100 pages is 10.595" × 8.060" with bleed, of which the spine takes 2.1% 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 5.06" × 7.81" in paperback only.

Worth knowing before you commit to the size: the same 100-page book at 6" × 9" costs exactly the same $2.30 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 5.06" × 7.81" against 6" × 9" is a design decision, not a margin one.

What 5.06" × 7.81" is for: Matching the classic mass-market paperback footprint — genre fiction aiming to sit alongside traditionally published paperbacks, and series where shelf consistency with existing mass-market titles matters.