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About the units

• Square millimetres (mm²) – A square millimetre is a square one millimetre on a side, one millionth of a square metre – not a thousandth, because the prefix is squared with the unit. It is a legal unit in the EU and the UK and the usual measure for the cross-section of electrical conductors. Area is not thickness: a solid 2.5 mm² conductor is only 1.78 millimetres across. A newton per square millimetre is exactly one megapascal. The SI Brochure forbids abbreviations such as “sq. mm”; the symbol is mm².
• Square centimetres (cm²) – A square centimetre is one ten-thousandth of a square metre and a legal unit in the EU and the UK. Food labelling law uses it: on a pack whose largest surface is under 80 cm² the mandatory text may have an x-height of 0.9 mm instead of 1.2 mm, and below 25 cm² no nutrition declaration is needed. A square inch is exactly 6.4516 cm². The trap is the old pressure unit “kg/cm²”, really a kilogram-force per square centimetre, exactly 0.980665 bar.
• Square metres (m²) – The square metre is the SI coherent unit of area, formed from the metre, and a legal unit throughout the EU. It is the measure for floor areas, land areas and pricing by area. A full-size football pitch is 100–110 by 64–75 metres, so 6,400–8,250 square metres. One square metre is 10.76 square feet. The common slip is to read a square metre as a metre squared: ten square metres is an area of ten square metres, not a square ten metres on a side.
• Hectares (ha) – A hectare is 10,000 square metres, a square 100 metres on a side. The unit was adopted by the International Committee for Weights and Measures as early as 1879 and is legal in the EU for farmland and building land. It is used in agriculture, forestry, planning and land registers across Europe. A full-size football pitch is 0.64–0.83 hectares. Write “ha”, not “Ha”. The hectare is not an SI unit but a historical name for the square hectometre, and it is not the are, which is 100 square metres.
• Square kilometres (km²) – A square kilometre is a square one kilometre on a side, 1,000,000 square metres or 100 hectares, and a legal unit in the EU and the UK. It is the measure for districts, counties and countries, and the Office for National Statistics gives population density as usual residents per square kilometre of land, leaving out inland water. One square kilometre is 247 acres or 0.386 square miles. The trap is scaling: ten kilometres by ten is 100 square kilometres, not ten.
• Square feet (ft²) – A square foot is the square of the international foot and therefore exactly 0.09290304 square metres. It is the measure for floor and office areas in the US and partly in the UK, while in the EU it may appear only as a supplementary indication. A thousand square feet is 92.9 square metres. The factor against the square metre is 10.76, the square of 3.28, and it is one of the conversions where guesses go furthest wrong. American areas from before 2023 may rest on the old survey foot.
• Acres (ac) – An acre is 4,840 square yards, which with the yard at exactly 0.9144 metres gives 4,046.8564224 square metres, or 43,560 square feet. It is used for farmland in the US, the UK and Ireland. In the EU the acre is not a legal trade unit; it was allowed to survive for a transitional period in the countries where it was permitted in 1973. An acre is just under 0.405 hectares, and a hectare is 2.47 acres. American areas from before 2023 may rest on the slightly larger survey acre.
• Square miles (mi²) – A square mile is 640 acres, and with the yard at exactly 0.9144 metres it is exactly 2,589,988.110336 square metres, or 2.59 square kilometres. It is used for land areas in the US and the UK, though British trade may use it only as a supplementary indication. The City of London is known as the Square Mile, and its corporation puts it at 1.12 square miles, some 2.90 square kilometres. The trap is scaling: two miles by two is four square miles, not two.