What trim size means
Trim size is the width and height of the finished page after the printed sheet has been cut — trimmed — to size. A 6 × 9 inch book is six inches across and nine inches tall when you hold it.
It is quoted width first, always. 6 × 9 and 9 × 6 are not the same book.
Everything physical about the book follows from it. The trim size and the type size decide how many words fit on a page; the word count and the words-per-page decide the page count; the page count and the paper decide the spine width; and the spine width decides the dimensions of the cover artwork. Change the trim size and that entire chain recalculates.
The sizes you will actually choose between
There are dozens available. In practice most books land in a handful, and 6 × 9 inches is the most commonly used trim size for self-published print books — it is the default on most print-on-demand platforms and the one readers' hands are most used to.
- 5 × 8 in — compact, pocketable. Pushes the page count up, which thickens the spine and can flatter a short book.
- 5.5 × 8.5 in — a half-inch bigger in both directions. A common choice for literary fiction and memoir.
- 6 × 9 in — the workhorse. Novels, general non-fiction, business, almost anything text-only.
- 7 × 10 in — room for diagrams, tables and wide code samples. Textbooks and technical books.
- 8.5 × 11 in — letter size. Workbooks, manuals, anything to be written in.
- 8.5 × 8.5 in — square. Children's picture books and photography.
How size changes the feel of the book
A smaller trim with the same word count produces more pages and a thicker spine. The book looks and feels more substantial, costs slightly more to print because of the page count, and is cheaper to post.
A larger trim produces fewer, denser pages. The book is lighter and thinner, and if the page count drops too far the spine becomes hard to print on.
There is a point at which a large page becomes tiring to read: the eye has to travel too far across a long line and loses its place on the return. This is why a 300-page novel is 6 × 9 and not 8.5 × 11 — not a rule, but a reason.
Genre conventions are conventions
Walk into a bookshop and the sizes cluster by section: thrillers at one size, cookbooks at another, poetry smaller than both. Readers register this without noticing, and a book at an unexpected size reads as unfamiliar before anyone has opened it.
That is worth respecting, and it is worth being clear about what it is. No platform publishes a table mapping genres to trim sizes, and no one will reject your file for choosing differently. It is industry convention and reader habit, not a specification. If you have a reason to break it, break it on purpose.
Spine width, and how to calculate it
The spine is as thick as the stack of pages plus the cover. For a perfect-bound paperback on a print-on-demand service, the page count and the paper type give you the number directly.
The standard multipliers used by print-on-demand printers are per page, and your page count means physical pages — a 215-leaf book is 430 pages.
- Cream or natural-shade paper — page count × 0.0025 inches.
- White paper — page count × 0.002252 inches.
- So a 430-page book on cream paper has a spine of 430 × 0.0025 = 1.075 inches.
- Hardcover and offset printing use different figures. Ask your printer for theirs rather than reusing these.
Bleed, margins and the numbers on the file
Bleed is extra image area beyond the trim line, so that when the cutter lands a fraction off there is still ink to the edge rather than a white sliver. The standard allowance is 0.125 inches on each outside edge.
You need bleed only if something — a photograph, a block of colour, a rule — runs to the edge of the page. A text-only interior does not need it.
Margins are the other direction. The inside margin, the gutter, has to be wider than the outside ones because part of it disappears into the binding, and the thicker the book the more it swallows. A 400-page book needs a visibly wider gutter than a 120-page one or the text runs into the spine.
Page count limits
Print-on-demand has both a floor and a ceiling, and they are not optional. A black-and-white paperback typically runs from 24 pages at the low end to around 828 at the high end, with the ceiling varying by trim size and paper — a large trim on thick paper hits the binding limit sooner.
Colour interiors have a higher minimum, commonly around 72 pages.
If your book exceeds the ceiling, the answer is two volumes, a smaller type size, or a larger trim — in that order of honesty.