Millimeter is a unit of length that is one thousandth of a meter. It's often used in measurements to make them more understandable to the average person. Millimeters are also commonly used to describe the sizes and dimensions of things. Millimeter is the smallest unit of measurement in the International System of Units (SI). It is also called a subdivision of a centimeter. One millimeter is about the width of a human hair.

The inch is a commonly used length unit in the United States and many other countries. An inch is equal to 0.00039 meters. The inch was originally a Roman unit of length, equal to 1/12 of a foot. It became an SI unit in the early 19th century, and is now one of the six standard lengths interdependent with meters (along with meter, kilometer, millimeter, micro meter, and nano meter). In 1935, theUSEb added the “point” to the name of the inch; thus 0.0003936 meters becomes 3.62 points, or 1/32nd of an inch.