The 12 x 18 format measures 305 x 457 mm or 12 x 18 inches. An 12 x 18 paper fromat has an aspect ratio of 1:1.5. The Area of the 12 x 18 paper size is 139,385 mm² or 216 in² with a perimeter of 1,524 mm or 60 in. An 12 x 18 format has a diagonal length of 549.43 mm or 21.63 in.