A length of 12 millimeters is equivalent to about 0.47 inches, or slightly less than half an inch. The diameter of a dime is about 18 millimeters, so 12 millimeters is about two-thirds of the distance across the face of a dime.
In normal room light, the diameter of a human pupil is 3 to 4 millimeters, or one-fourth to one-third of 12 millimeters. A length of 12 millimeters constitutes about 4 percent of a foot. In metric terms, a meter consists of 1,000 millimeters, so 12 millimeters make up 1.2 percent of a meter. A centimeter consists of 10 millimeters.