Trim size is the finished dimensions of a printed book — the size it is after the pages have been cut. It is one of the first decisions in laying out an interior and one of the hardest to change later, because almost everything else depends on it.
The sizes people actually use
Amazon KDP offers a long list. Four of them account for most books:
| Size | Where you see it |
|---|---|
| 5″ × 8″ | Novellas, poetry, short story collections. Feels small and personal in the hand. |
| 5.25″ × 8″ | A slightly roomier version of the same feeling. |
| 5.5″ × 8.5″ | Literary fiction, memoir, trade paperbacks. |
| 6″ × 9″ | The default. Most novels and most non-fiction. |
Larger sizes — 7″ × 10″, 8.5″ × 11″ — exist for workbooks, textbooks and anything with diagrams, and look odd for prose.
If you have no strong feeling, choose 6″ × 9″. It is the size a reader's hand expects for a novel, it is available in every marketplace, and nobody has ever picked up a 6×9 paperback and thought about its dimensions.
Look at your own shelf
Better than any table: take five books off your shelf that are the same kind of thing you have written, and measure them. Genre conventions are real and readers notice them without being able to name them. A thriller in the proportions of a poetry chapbook reads as self-published before anyone opens it.
This is the most useful ten minutes you can spend on the decision.
What the choice actually changes
Your page count. A smaller page holds fewer words, so the same manuscript runs longer. A 90,000-word novel is roughly 300 pages at 6×9 and well over 400 at 5×8. That is not a small difference.
Your printing cost. Print-on-demand pricing is driven largely by page count, so a smaller trim size costs more per copy for the same book. Choosing 5×8 because it feels more literary is a legitimate decision that also makes each copy more expensive to make and squeezes your royalty.
Your margins. More pages means a thicker book, and a thicker book needs a wider inside margin — see book margins and gutters. So trim size influences the page count, which influences the gutter, which influences the text block, which influences the page count again.
Your spine. Page count times paper thickness. A thin book cannot carry spine text at all: KDP will not print a title on a spine below 79 pages. The spine calculator gives you the width for a given page count and paper.
Standard sizes, and why to stay in them
KDP distinguishes between sizes it prints for every marketplace and sizes with limited distribution. Choosing an unusual trim can quietly restrict where your book can be sold or which retailers can order it.
There is rarely a good reason to leave the standard list for a book that is mostly words. If your book genuinely needs an unusual shape — a square format for photographs, a landscape workbook — that is a real requirement, and worth checking availability before you lay anything out.
Cream or white paper
Not a trim size decision, but it is made at the same moment and it changes the object.
Cream is slightly warmer to read and is conventional for fiction. White is crisper and conventional for non-fiction and anything with images. Cream is also physically thicker: the same 300-page book is 0.676″ on white and 0.750″ on cream, which is enough to matter on a cover designed for the other one.
Decide before you lay out, not after
The reason to settle this early is that changing it later means redoing the work. A new trim size means a new page count, which means a new gutter, which means a new layout — and if you have already had a cover designed, a new spine width for that too.
If you are unsure, work out the numbers for two candidates before committing. The margin and gutter calculator will estimate a page count from your word count for any trim size, which is enough to compare them honestly.
Once you have chosen, our print book formatter takes a .docx and produces the interior at that trim — mirrored margins, a gutter sized to the finished page count, and the page count worked out by laying the book out rather than estimating it.