Millimeters

The millimeter (symbol: mm) is a unit of length in the metric system and in the International System of Units (SI). It is a derived unit equal to one thousandth of a meter: 1 mm = 0.001 m.

It is commonly used for measuring small distances and dimensions in engineering, machining, science, and everyday objects. The millimeter sits between the centimeter (10 mm) and the micrometer (0.001 mm) in the metric hierarchy. Everyday conversions include 10 millimeters in a centimeter and 1000 millimeters in a meter. In contexts where imperial units are used, lengths are often converted to inches; 1 inch equals exactly 25.4 millimeters.

In practice, millimeters are measured with tools such as calipers, micrometers, or rulers with metric graduations. Precision machining and tolerances are often specified in hundredths or thousandths of a millimeter (for example, 0.01 mm or 0.001 mm). The prefix milli- comes from French mille, meaning thousand, reflecting its role as one thousandth of a meter.

The millimeter emerged with the adoption of the metric system in the 18th and 19th centuries and is now the standard unit for small lengths in most scientific, technical, and consumer contexts.