How to Handle Custom Trim Sizes on KDP

Most KDP authors never think twice about trim size — they pick 6″ × 9″ or 5.5″ × 8.5″ from the dropdown and move on. But if you're formatting a workbook, a square photo book, a reprint of a public-domain title, or a series that has to match dimensions from another printer, the standard list might not fit. KDP does allow custom trim sizes for paperback — within specific bounds — but hardcover is locked to five fixed options no matter what you need. This guide walks through exactly where those boundaries sit, how to calculate a full cover layout once you've chosen a non-standard size, and how to decide whether custom is actually the right call for your project.