Millimetri

The millimetre, symbol mm, is a unit of length in the International System of Units (SI). It is defined as one thousandth of a metre, so 1 mm = 0.001 m. The prefix milli- denotes 10^-3, placing the millimetre between the centimetre (10 mm) and the micrometre (0.001 mm).

In common usage, centimetres and metres are used for larger objects, while millimetres are used for smaller dimensions. Relations to other SI units include: 1 metre = 1000 millimetres, 1 centimetre = 10 millimetres, and 1 kilometre = 1,000,000 millimetres. One millimetre equals 1000 micrometres (µm).

Applications of the millimetre span science, engineering, manufacturing, and everyday measuring. It is suitable for dimensions such as the thickness of paper, the diameter of small fasteners, and the tolerances of mechanical parts. For example, printer paper is about 0.1 mm thick, a credit card is around 0.76 mm, and human hair typically measures tens of micrometres in diameter. In technical drawings, parts are often specified in millimetres, with tolerances frequently stated in tenths or hundredths of a millimetre.

Measurement tools used with millimetres include rulers, calipers, and micrometers, which can provide readings to sub-millimetre precision. When higher precision is required, measurements may be expressed in micrometres (µm) or converted to other units.

Linguistic note: in Italian, the unit is millimetro with plural millimetri; in English, it is millimetre (plural millimetres).