Book Sizes and Trim Sizes Explained: US, UK and Print-on-Demand Standards
What trim size means, the standard book sizes in the US and the UK, and how to pick the right one before your files are built. With a size table you can actually use.
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A book's trim size is its finished width and height after printing and cutting. The most common size for a novel in the US is 6 × 9 in (152 × 229 mm), while UK fiction typically uses B-format, 129 × 198 mm. Print-on-demand platforms such as Amazon KDP and IngramSpark support both, so the real question is what suits your genre and page count.
Trim size is the first decision in book production, because everything else depends on it: page count, spine width, cover dimensions and, ultimately, the printing cost per copy. Choose it before the interior is typeset, not after.
Standard book sizes at a glance
| Name | Size | Typical use |
|---|---|---|
| A-format (UK) | 110 × 178 mm | mass-market paperbacks |
| B-format (UK) | 129 × 198 mm | standard UK fiction |
| 5 × 8 in (US) | 127 × 203 mm | shorter novels, novellas |
| 5.5 × 8.5 in (US) | 140 × 216 mm | trade paperbacks, memoir |
| 6 × 9 in (US) | 152 × 229 mm | most common POD size, fiction and non-fiction |
| Royal (UK) | 156 × 234 mm | hardbacks, serious non-fiction |
| 8.5 × 11 in / A4 | 216 × 279 mm | workbooks, manuals, children's books |
A fuller reference, including paper stocks, binding types and hardcover options, is maintained on the Librum book specs page, and each term is defined in plain language in the publishing glossary.
How trim size changes your page count and cost
The same 80,000-word manuscript can be a 380-page book at 5 × 8 in or a 300-page book at 6 × 9 in. Fewer pages means a thinner spine and a lower print cost per copy, which matters enormously in print-on-demand, where the platform deducts printing from every sale. You can test this yourself with the free page count calculator and spine width calculator on librum.house.
Picking a size: three practical rules
First, match your genre's shelf. Readers expect a thriller at B-format or 5.5 × 8.5 in, and a business book at 6 × 9 in. Second, decide print territory: if you sell mainly through Amazon US, US sizes are natural; UK-first authors usually choose B-format. Third, keep print and ebook in sync: the ePUB does not have a fixed size, but chapter structure and front matter should mirror the print edition.
Not sure which size fits your book? Librum prepares press-ready files for any trim size and any print-on-demand printer, with a fixed quote within 24 hours. See the print preparation service
Frequently asked questions
What is the most common book size?
For print-on-demand, 6 × 9 in (152 × 229 mm) is the most widely used trim size in the US, and B-format (129 × 198 mm) is the standard for fiction paperbacks in the UK. Both are supported by Amazon KDP and IngramSpark, so availability is not a concern for either choice.
Does trim size affect printing cost?
Yes, indirectly but significantly. A larger trim size fits more words per page, which lowers the page count, thins the spine and reduces the per-copy printing charge that platforms deduct from each sale. Over hundreds of sold copies the difference adds up to real money.
Can I change trim size after the book is typeset?
Technically yes, practically it means re-typesetting the whole interior: new page breaks, new spine width, and a new cover file with different dimensions. It is far cheaper to decide the trim size first, before the layout work begins, than to reflow a finished book.
Next in this series: How long should a book be? Word counts by genre.
