How to choose a trim size (without overthinking it)
Start with genre norms and reader expectations. Most fiction trends toward compact sizes, while workbooks and textbooks often benefit from larger pages. If you already have an interior PDF, preview how comfortable your margins and font size feel at each candidate trim size.
Trim size changes cover composition
A taller or wider trim changes how your typography sits on the front cover—especially at thumbnail size. If you’re designing for readability first, review KDP Cover Design Principles: Hierarchy, Contrast, and Readability before locking in your layout.
Trim size affects spine width (and your spine layout)
Spine width depends on page count and paper type, but trim size influences your page count—bigger pages often reduce pages, which can shrink the spine. Before you finalize spine text, read Spine Width for KDP: The Practical Guide (+ Calculator Tips) and confirm dimensions with the calculator.
Production setup: templates vs calculator
If you want a faster workflow, start from KDP Cover Templates (great for common trim sizes). If you want exact precision for your configuration, use the KDP Cover Size Calculator and then validate everything against KDP Cover Requirements (Bleed, DPI, Safe Zones, Barcode).