40 centimeters. Centimeter and inch are commonly used units of length, and the conversion formula between them is 1 inch = 3/10cm, so 12 inch is 40cm. The units that describe the length are kilometers, decimeters, centimeters, millimeters, microns, nanometers, picometers and femtometer. The conversion rate is 1 cm = 1 0mm, 1 decimeter = 1 0cm,1m = 1 decimeter =100cm,/kloc-0.