Book trim sizes explained (in millimetres, not letter names)

Quick answer: A book's trim size is the width × height of a single page after the printer cuts it, measured in millimetres. A "5.5 × 8.5 book" means 140 × 216 mm, and an "A5 book" means 148 × 210 mm — two different books. Always give your printer the trim in millimetres, because letter names like A5 or "6 × 9" hide the exact size and can lead to a mismatched reprint.

Key takeaways

• Trim size = the finished, cut page, not the sheet it was printed on. It is fixed at design time and cannot be changed later without redoing the layout.
• Millimetres are the specification. "A5", "6 × 9", "B-format" are labels; 148 × 210 mm, 152 × 229 mm, 129 × 198 mm are specifications.
• The trim you choose sets your price band, because it decides how many pages fit on a sheet of paper and how much paper each copy uses.
• Pick from the standard sizes below unless you have a specific reason not to — non-standard trims mean custom cutting and a higher price per copy.
• You must match your trim to your genre for the reader to read the book as "normal", and to your page count for the spine to be thick enough for a title.

Contents

• What "trim size" actually means
• The standard trims, in millimetres
• Which trim fits which genre
• Why the exact millimetre matters
• How trim, page count and price are connected
• How to choose your trim
• Common mistakes

What "trim size" actually means

When a book is printed, pages are laid out on large sheets, folded, and then cut (trimmed) down to size. The size they are cut to is the trim size. It is the size of the page your reader holds, and the size every page of the interior is designed against.

Two consequences follow immediately:

1. Trim is decided before layout, not after. Change the trim and the entire interior — running heads, margins, page breaks, index — has to be re-flowed.
2. Trim is a physical dimension, so it is written in millimetres (or inches), never as a letter name alone. "A5" tells the printer a paper convention; "148 × 210 mm" tells the printer exactly what to cut.

The standard trims, in millimetres

These are the sizes you will actually meet. Where a familiar letter name exists, it is a label for the millimetre figure beside it — not a replacement for it.

Read the table as a starting point, not a rule. The point of the table is the middle column — that middle column is what goes on your printer's job sheet.

Which trim fits which genre

Readers carry an expectation of how big a book "should" be, and a mismatched trim feels wrong in the hand even when the words are perfect.

• Fiction and memoir — 129 × 198 mm (B-format) or 138 × 216 mm. These read as "a novel".
• General non-fiction and self-help — 152 × 229 mm (US Trade). The most common self-publishing size, and the widest printer support.
• Illustrated / photographic non-fiction — 156 × 234 mm (Royal) or 189 × 246 mm (Crown Quarto), to give images room.
• Poetry and short fiction — 138 × 216 mm or 129 × 198 mm, to keep the page from looking empty.
• Workbooks and academic — 210 × 297 mm (A4), because the reader will write on the page.
• Children's picture books — usually a square or landscape trim, chosen with the artwork; this is the exception that should be decided with your designer, not from a table.

Why the exact millimetre matters

A letter name and its millimetre figure are close, but a letter name can be rounded differently from one quote to the next. "6 × 9" is 152.4 × 228.6 mm exactly, and is often written as 152 × 229 mm. On a first print run a small difference like that is invisible. On a reprint, or on a series where a later volume must sit beside the first, it is exactly the mismatch that makes one book a millimetre taller than its siblings on the shelf.

This is why the rule is simple: put the trim in millimetres in the first line of every print request and every quote. It is not pedantry: a re-laid interior after a trim was given by letter name is an avoidable cost.

How trim, page count and price are connected

Trim is not only a look. It is part of the arithmetic that decides what a copy costs to produce:

• A larger trim holds more words per page, so the same manuscript becomes fewer pages — but each page uses more paper.
• A smaller trim needs more pages for the same words, which can push the book past a paper or binding threshold and change its price band.
• Thin books at small trims can fall below the minimum spine width for a printed spine title; below a certain page count the spine cannot carry text at all.
• Non-standard trims require custom cutting and are usually more expensive per copy than a standard size, with no benefit the reader can see.

The practical rule: choose a standard trim that fits your genre, then check the resulting page count sits comfortably above your printer's minimum for spine text and below the threshold that changes your price band.

How to choose your trim

1. Start from genre. Pick the standard trim your readers already expect.
2. Check the page count with your manuscript in that trim — the two are chosen together, not separately.
3. Write the size in millimetres, width × height, exactly as it will be cut.
4. Use the same millimetre figure everywhere — interior file, cover file, printer job sheet, quote request. One number, one place it came from.
5. Match the whole series. If this is book two, the trim is book one's trim. Decide a series trim once, at book one.

Common mistakes

1. Sending "A5" or "6 × 9" to a printer with no millimetre figure. Two printers read the same letter name two different ways.
2. Designing the interior first and choosing the trim later. Every page must then be re-flowed.
3. Mixing trims across a series. The books no longer line up on a shelf, and the covers are no longer interchangeable.
4. Choosing a trim that leaves too few pages for a spine title. The spine comes back blank.
5. Choosing a non-standard trim for looks alone. The reader rarely notices; the price per copy usually rises.
6. Forgetting bleed. Artwork meant to reach the edge must extend past the trim line; the printed page still ends at the trim.

Your trim size and your page count lock together, and both feed into the production price of a print run. If you would like your book laid out at a standard trim that suits its genre, see our self-publishing packages.

Related guides

Keep going: how to write a book, the types of book editing, book editing services and proofreading services. For a dictionary definition, see trim (Cambridge Dictionary).

Disclosure: Harmony Publishing is a self-publishing company and links to its own services where they match a step.