A trim size is a book's finished dimensions after printing and cutting. It is chosen before you typeset, because it determines the text block, the margins and ultimately the page count. For novels, a small number of sizes account for nearly everything on the shelf.
There is no canonical list. Each printer publishes its own, and they differ.
The Sizes You Will Actually Meet
For fiction, in the US market:
- 5 x 8 in: compact, common for literary fiction and poetry
- 5.25 x 8 in: slightly wider, and the most restricted on binding
- 5.5 x 8.5 in: a widely used general fiction size
- 6 x 9 in: the default for trade paperbacks, and the most common single answer
In UK and European markets, the equivalents are metric:
- 129 x 198 mm (B format) and 135 x 216 mm (Royal), the standard UK paperback sizes
- A5, 148 x 210 mm
Larger sizes exist for non-fiction, textbooks and illustrated books, but a novel outside this range starts to look unusual on a shelf.
For working through the decision itself, see how to choose a trim size for a novel.
The Ordering Trap
American and British sources state dimensions in opposite orders, and nothing warns you.
US printers give width by height: a 6 x 9 book is 6 wide and 9 tall. The Chartered Institute of Editing and Proofreading records the British convention in its glossary entry for format: "the size of a book (height x width, in millimetres)."
So a book described as 198 x 129 and one described as 5.08 x 7.8 are the same object. If a number looks like a portrait book with the proportions of a landscape one, you are probably reading the other convention.
Why There Is No Standard List
Trim sizes come from manufacturing rather than from typography, so the authority is whoever is printing your book.
Wiley states this outright to its own authors: "Wiley publishes books in various dimensions. For a list of acceptable trim sizes, check with your Wiley contact before you start composing your book." Its internal design specification exists, but it is sent to the typesetter rather than published.
The professional bodies do not fill the gap. CIEP defines trim size, "the size of a publication after it has been printed and cut to its intended dimensions", but publishes no list and no measurements. The Book Industry Study Group and BIC cover metadata, barcoding and supply chain rather than page design.
The practical consequence: check your printer's list first, then design to it. Choosing a size your printer does not offer means reflowing the whole book.
Trim Size Drives Everything Downstream
Changing trim size is not a late adjustment. It changes:
- Page count, because a smaller page holds fewer words
- The gutter, which widens in bands as page count grows
- Spine width, which is calculated from page count
- Print cost, which is driven by page count
- Where chapters fall, and therefore how many blank pages you accumulate
A novel that runs 300 pages at 6 x 9 will run appreciably longer at 5 x 8. That extra length can push you into a wider gutter band, which narrows the text block, which adds pages again.
Decide the trim size first, then typeset.
Doing This in Deckle
Deckle offers six trim sizes: 5 x 8 in, 5.5 x 8.5 in, 6 x 9 in, 6.14 x 9.21 in, 13.5 x 21.5 cm and A5 (14.8 x 21 cm).
That covers the common US fiction sizes, KDP's 6.14 x 9.21 option, and two European sizes. All are stated width by height in the US convention.
The choice feeds straight into the export: the page canvas, the mirrored margins, the bleed allowance on three edges, and the estimated page count all derive from it. Because it drives the page count, it is worth settling before you start fine-tuning the interior.
Common Mistakes
- Choosing a size your printer does not offer. Check the list first; there is no universal set.
- Reading a UK size as a US one. Height by width versus width by height. Check which is larger.
- Changing trim size after typesetting. It reflows the book and shifts every chapter opening.
- Picking a size because you like the number. Genre convention matters more; an unusual size reads as self-published.
- Forgetting the knock-on effects. A smaller page means more pages, which can mean a wider gutter and a thicker spine.
FAQ
Q: What is the most common trim size for a novel? A: 6 x 9 inches in the US trade market. 5.5 x 8.5 and 5 x 8 are also common, the latter for literary fiction.
Q: Is trim size the same as page size? A: Effectively, yes, once the book is cut. Before cutting, the sheet is larger because it carries bleed.
Q: Can I use a custom size? A: Only if your printer offers it. Print-on-demand services publish a fixed list.
Q: Does trim size affect an ebook? A: No. Reflowable files have no fixed page dimensions.
Q: Which comes first in the numbers? A: Width in US usage, height in British usage. The larger number is the height either way.