A millimeter is equivalent to about 0.04 inch, or one twenty-fifth of an inch. A credit card is about 1 millimeter thick, as are an ordinary paper clip and a regular guitar string.
A millimeter constitutes about three one-thousandths, or less than 1 percent, of a foot. In metric terms, a meter consists of 1,000 millimeters, so 1 millimeter makes up one-thousandth of a meter. The relationship between millimeter and meter may be expressed as 1 millimeter = 0.001 meters. A centimeter consists of 10 millimeters, so 1 millimeter equals one-tenth of a centimeter. The abbreviation for millimeter is mm.