Trim size Defined.
Definition
Trim size is the finished width and height of a printed book's pages after the paper is cut. You choose it when you set up a paperback or hardcover on KDP, and it decides the page size of your interior file, how many pages the book runs to, its printing cost class and, through the page count, its spine width.
Sources checked 20 October 2026. Figures: KDP's paperback royalty rates, trim classes and spine figures, as of October 2026; KDP's Amazon.com paperback printing costs, as of 19 October 2026.
How it works
KDP publishes a list of trim sizes, each with a minimum and maximum page count by ink and paper; its page calls 6 x 9 inches the most common paperback trim in the US. You set the trim as the page size in your writing or layout software before setting margins, because the margins depend on the page count and on whether anything bleeds. KDP prices print in two classes, and any trim wider than 6.12 in or taller than 9 in is a large trim with higher printing costs. Every printed book has at least 24 pages.
An example
A manuscript that fills 180 pages at 6 x 9 inches fills more at 5 x 8, because each smaller page holds fewer words. More pages mean a wider spine, and past 110 pages they also mean a higher printing cost, since KDP's flat fee covers only 24 to 110 pages. Running the word count through a converter before choosing the trim avoids redoing the cover later.
Common mistakes
Formatting the interior at US Letter or A4 and then choosing 6 x 9 on KDP, so the file is rejected or the text sits wrongly on the page. Picking a large trim without checking how much more it costs to print. And changing the trim after the cover is designed: the page count changes, so the spine width and the whole cover wrap change with it.
