Square Millimeters
Abbreviation/Symbol:
mm²
sq mm
Unit of:
Worldwide use:
A square millimeter (mm²) is the area of a square 1mm on each side, one millionth of a square meter. Wires and cables are specified by cross-sectional area in it, and 1.5mm² and 2.5mm² are the standard sizes for household wiring in the UK and Europe.
To go from square millimeters to square centimeters divide by 100, not 10 – 250mm² is 2.5cm². For square meters divide by 1,000,000.
Definition:
The square millimeter is derived from the base unit of length, the millimeter, which is equal to one thousandth of a meter. The square millimeter is defined as the area of a square with sides measuring one millimeter in length.
Origin:
The square millimeter comes from the metric system developed in France in the 1790s, and is the area of a square one millimeter on each side. Electrical cable sizes are quoted in it, eg 2.5mm² for UK ring main cable.
The milli- prefix means a thousandth, but it applies to both sides of the square – so a square millimeter is one millionth of a square meter, not one thousandth. That makes 1,000,000mm² in 1m².
Usage context:
Cable cross-sections are given in mm²: 1.5 and 2.5 mm² are the usual sizes of household wiring in the UK and Europe. Chip dies are measured in square millimeters too.
Engineers quote stress in newtons per square millimeter – 1 N/mm² is exactly 1 megapascal – and a full-frame camera sensor (36 mm by 24 mm) is 864 mm².