A person measures in millimeters by comparing the length of the object in question with the tick marks on a metric ruler or the metric side of a standard ruler. The metric side is labeled in millimeters or centimeters with small tick marks that each represent one millimeter.
The standard ruler maintained by the U.S. National Bureau of Standards measures 6 inches on one side and 15 centimeters on the other. These measured lengths are close to each other as 1 inch equals 2.54 centimeters, or 254 millimeters. Each centimeter equals 10 millimeters. A common ruler used by schoolchildren is twice as long, at 12 inches, or 30 centimeters.