Choose the Right Trim Size
Your trim size dictates the physical dimensions of your printed book. Choosing the right size is crucial—it affects your page count, your printing costs, and how your book feels in a reader's hands.
Standard KDP Trim Sizes
| Trim Size (Inches) | Trim Size (mm) | Best For |
|---|---|---|
| 5" x 8" | 127 x 203 mm | Novellas, short memoirs, literary fiction. |
| 5.5" x 8.5" | 140 x 216 mm | Trade paperbacks, standard fiction, narrative non-fiction. |
| 6" x 9" | 152 x 229 mm | Industry Standard. Thrillers, sci-fi, fantasy, business books. |
| 7" x 10" | 178 x 254 mm | Workbooks, manuals, academic texts. |
| 8" x 10" | 203 x 254 mm | Children's books, art books, large manuals. |
| 8.5" x 11" | 216 x 279 mm | Textbooks, sheet music, coloring books. |
How Trim Size Affects Cost
Because KDP charges a flat per-page printing fee (after a certain threshold), a larger trim size usually results in a lower page count, which can reduce your printing costs and increase your royalties.
However, if you choose a trim size that is too large for your genre, your book may look amateurish or feel flimsy (like a magazine rather than a novel).
Not sure which to choose?
publishing.co.uk defaults to the industry standard 6x9, but you can select any size at checkout.
Format my book →How to Use This Guide
Review the table above to identify which trim size best matches your book's genre and intended audience. The most important factors to consider are genre conventions (readers expect certain sizes for certain types of books), your page count (which affects printing costs), and the physical feel you want your book to have in a reader's hands.
If you are publishing fiction, the 5.5 x 8.5 inch or 6 x 9 inch sizes are the safest choices. For non-fiction, 6 x 9 inches is the industry standard and works well for most business, self-help, and reference books. Children's books and heavily illustrated titles typically require larger formats such as 8 x 10 or 8.5 x 11 inches.
Once you have selected your trim size, use our KDP Margin Calculator to determine the correct margins for your page count, and our Word Count to Page Count Estimator to see how many pages your manuscript will produce at that size.
Why Trim Size Matters for KDP
Your trim size decision has cascading effects on every other aspect of your book's production. It determines your page count, which in turn determines your margin requirements, your spine width, your printing cost per copy, and ultimately your retail price and royalty.
Choosing a trim size that is too small for your word count will result in a very thick book with high printing costs, potentially making it impossible to price competitively while maintaining a reasonable royalty. Conversely, choosing a trim size that is too large will produce a thin, flimsy-feeling book that readers may perceive as low quality or overpriced.
In the UK market specifically, readers tend to expect trade paperback sizes (around 5.5 x 8.5 or 6 x 9 inches) for most adult fiction and non-fiction. Mass market paperback sizes (4.25 x 6.87 inches) are available on KDP but are less common for self-published titles and can create formatting challenges with narrow margins.
Common Trim Size Mistakes
Choosing a non-standard size. While KDP supports custom trim sizes, using a non-standard size can increase printing costs and create complications with cover design templates. Stick to the standard sizes listed above unless you have a specific reason to deviate.
Selecting a size based solely on cost. While a larger trim size reduces page count and printing costs, it must also be appropriate for your genre. A 7 x 10 inch romance novel will look and feel wrong to readers, regardless of the cost savings.
Forgetting to update your cover template. Your cover dimensions are directly tied to your trim size and page count. If you change your trim size, you must recalculate your cover dimensions, including the spine width. Use our Spine Width Calculator to get the correct spine measurement.
Not considering the reading experience. Larger trim sizes mean wider lines of text, which can be harder to read. If you choose a large trim size, consider using a two-column layout or increasing your margins to keep the line length comfortable (ideally 60-75 characters per line).
For a detailed walkthrough of the entire formatting process, including how trim size affects your margins and layout, see our guide on KDP paperback formatting.
