A length of 8 millimeters is equivalent to about 0.31 inch, or slightly less than 1/3 inch. A human pupil measures about 3 to 4 millimeters in normal room light, so 8 millimeters approximates the combined diameters of two pupils.
A length of 8 millimeters constitutes about 3 percent of a foot. In metric terms, a meter consists of 1,000 millimeters, so 8 millimeters make up 0.8 percent of a meter. A centimeter consists of 10 millimeters, so 8 millimeters equal 0.8 centimeter. The diameter of a quarter is about 24 millimeters, which means that 8 millimeters is one-third of the diameter of a quarter.