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About the units

• Millimetres (mm) – A millimetre is one thousandth of a metre, since the SI prefix milli means 10⁻³, and a legal unit throughout the EU and in UK trade. It is the measure for small lengths and for rainfall: a millimetre of rain is one litre of water on every square metre, so 10 mm puts 10 litres on each. The trap is the word mil. In American usage a mil is a thousandth of an inch, 0.0254 millimetres, while in Swedish a mil is ten kilometres.
• Centimetres (cm) – A centimetre is one hundredth of a metre, since the SI prefix centi means 10⁻². As a decimal submultiple of the metre it is a legal unit throughout the EU, and it is the everyday measure for height, clothing sizes and furniture in metric countries. It also appears on US packaging beside the inch figure. An inch is 2.54 centimetres, so a six-inch screen measures just over 15 centimetres diagonally. In the old CGS system the centimetre was a base unit. Write cm, never “cms”.
• Metres (m) – The metre is the SI base unit of length, fixed by the General Conference on Weights and Measures. Since 1983 it has been the distance light travels in a vacuum in 1/299,792,458 of a second, restated in 2019 as a defined value for the speed of light. It is a legal unit throughout the EU, and the US yard and pound are derived from it. A full-size football pitch is 100–110 metres long. Note the spelling: metre in Britain, meter in America, where the word also means a gauge.
• Kilometres (km) – A kilometre is 1,000 metres, since the prefix kilo means 10³. It is a legal unit in the EU and the measure used for road and map distances. Swedish speed limits are set in kilometres per hour, 50 in built-up areas and 70 outside them. One kilometre is 0.621 miles, a little over six tenths. The names are the real trap: a Swedish mile is 10 kilometres, a statute mile 1.609 kilometres and a nautical mile 1.852 kilometres.
• Swedish miles (mil) – A Swedish mile is 10 kilometres. No authority fixes it: it appears neither in the SI nor in the EU units directive, and Swedish law prescribes SI units. It survives in everyday speech in Sweden and Norway and in official statistics, where the Swedish transport agency reports car mileage in billions of mil. Road signs and trade use kilometres. The older Swedish mile was longer, 36,000 feet or 10,688.54 metres, and lasted until the metric system arrived in 1889.
• Inches (in) – An inch is one thirty-sixth of a yard, which has made it exactly 2.54 centimetres since the international yard and pound agreement of 1959. It is used worldwide for screen diagonals, pipe and screw sizes and tyre and wheel dimensions, and as a general length in the US. Within the EU it may appear only as a supplementary indication beside the metric figure. Inches are written as fractions in trade, 3/8 in, but as decimals in engineering. The symbol ″ also means an arcsecond.
• Feet (ft) – A foot is by definition one third of a yard, and the yard has been exactly 0.9144 metres since 1959. That makes the foot exactly 0.3048 metres, or twelve inches. It is the everyday unit for building and for height in the US, and altitudes are given in feet even in European airspace. The US survey foot was slightly longer, 1200/3937 of a metre, and was retired on 1 January 2023. The gap of two parts per million shows up only in old survey data.
• Yards (yd) – A yard has been exactly 0.9144 metres since the yard and pound agreement of 1959, and divides into three feet or 36 inches. In British trade it may appear only as a supplementary indication, but EU law exempted British and Irish road signs, and a British sign may still give a distance under half a mile in yards, to the nearest 10. The trap on those signs is the letter m, which means miles, not metres. And 100 yards is only 91.44 metres, 8.6% short.
• Miles (mi) – A mile is 1,760 yards, and since the yard is exactly 0.9144 metres the mile is exactly 1,609.344 metres, or 5,280 feet. It is the unit on road signs and speed limits in the US and the UK, and EU law makes an explicit exception for British and Irish road signs. For trade elsewhere in the EU it may appear only as a supplementary indication. Do not confuse it with the nautical mile of 1,852 metres or the Swedish mile of 10,000 metres.
• Nautical miles (M) – A nautical mile is exactly 1,852 metres, set by an international hydrographic conference in 1929 and roughly one minute of latitude. It is the distance unit of shipping and aviation, and the EU units directive leaves room for units laid down in international transport agreements. The Territorial Sea Act 1987 defines it as 1,852 metres and sets the UK’s territorial sea at 12 nautical miles, 22.2 kilometres. There is no agreed symbol. Until 1954 the United States used 6,080.20 feet, about 1,853 metres.