Trim size refers to the final dimensions of a book’s printed pages. It’s one of the smallest choices in book production, but it has significant implications for page count, reader comfort, spine width, printing costs, cover design, and even how a book is perceived before it’s picked up. A workbook, a pocket-sized novel, an image-rich manual, and a reference book rarely feel appropriate in the same trim size.
Beginning authors sometimes “guess” their trim size by finding a favorite book that looks similar to their project, copying that trim size, and not thinking it through further. For example, a long, detailed non-fiction manuscript might end up being too small and cramped for that content. A short booklet might look too skimpy and awkward if a larger size were used. Some books that include tables, charts, activities, and illustrations are more comfortable when given extra width to the page. In the first place, the trim size should be dictated by use, not habit or guesswork.
Start with the reader’s anticipated use for the book. Will they be carrying it in a briefcase? Using it next to a laptop? Laying it flat on a table for long work sessions? Holding it in their hands? A shorter trim size could be better for long reading sessions, as novels and collections of essays fit well in these sizes. Larger formats work well when there are diagrams, exercises, generous margin size, and step-by-step directions that need to fit on each page. It’s not a question of what size looks nice, but of what works best for the particular contents.
Also, consider your projected page count. The same manuscript will yield different page counts in different sizes. A smaller page will hold less text than a larger page, therefore adding to your page count. A larger trim size might reduce the total number of pages, but it can end up making pages that look too wide if the margins, font size, or line spacing were not taken into consideration when selecting that size. This matters because the page count will dictate spine width, unit cost, and potentially the binding method.
As you consider these possibilities, I’d recommend trying a small-scale test. Take a single chapter or a few pages from your manuscript and format them in different sizes. Keep the font choice consistent while adjusting margin size and line spacing until the text is legible in each version. Consider how many pages you have in total, the length of your lines, the spacing around headers and footers, and whether the page feels crowded or open. And if your project will contain images or activities, use those as examples in the test as well rather than just text.
Also think about the trim size options available to your chosen printer or publisher. Many commercial printers will only print certain trim sizes because it’s easier for them to do it this way, while a few might also charge a premium for special sizes. Print-on-demand platforms also often offer their own accepted trim sizes, along with paper options and binding types. You don’t have to learn all these things yourself, but it’s worth checking the options your printer will offer before you finalize your book’s layout. Changing trim sizes too late in the process will impact everything from the page count to cover size to image size to table of contents length.
One quick way of telling if you have arrived at the best trim size is if it does all the work it can for the text without making any of the other issues more difficult. Margins have space. The gutter is not obscuring text. Headers, footers, and page numbers look consistent and easy to read. A reader can easily identify it as belonging in the same category, but won’t be reminded of another specific book. When all of these things align, the trim size is not a guess but rather a smart publishing choice.