Standard Book Sizes UK: Paperback, Hardback and A5 in mm, cm and Inches

Short answer: the standard UK book sizes are A-format 111 × 178 mm, B-format 129 × 198 mm and Demy 138 × 216 mm for paperbacks, and Demy, Royal 156 × 234 mm or Crown Quarto 189 × 246 mm for hardbacks and larger non-fiction. A5 is 148 × 210 mm.