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A speedometer used in American and British cars, showing the speed of the vehicle in miles per hour (outer, in white) and kilometres per hour (inner, in red) – for Canadian[1] vehicles the scales are reversed, or in km/h only.
A speedometer from an Australian car, showing the speed of the vehicle only in km/h, as in almost all countries.

Metrication or metrification is conversion to the metric system of units of measurement.[2] Worldwide, there has been a long process of independent conversions of countries from various local and traditional systems, beginning in France during the 1790s and spreading widely over the following two centuries, but the metric system has not been fully adopted in all countries and sectors.

An example of metrication in 1860 when Tuscany became part of modern Italy


Since 2006, three countries formally do not use the metric system as their main standard of measurement: the United States, Myanmar, and Liberia.[3] In the United Kingdom metric is the official system for most regulated trading by weight or measure purposes, but some imperial units remain the primary official unit of measurement. For example, miles, yards, and feet remain the official units for road signage – and use of imperial units is widespread.[4][5] The Imperial pint (expressed in metric units) also remains a permitted unit for milk in returnable bottles and for draught beer and cider in British pubs.[4] Imperial units are also legal for use alongside metric units on food packaging and price indications for goods sold loose, and may be used exclusively where a product is sold by description, rather than by weight/mass/volume. E.g. Television screen and clothing sizes tend to be denominated in inches only, but a piece of material priced per inch would be unlawful unless the metric price was also shown.

Some sources now identify Liberia as metric, and the government of Myanmar has stated that the country would metricate with a goal of completion by 2019.[6][7] Both Myanmar and Liberia are substantially metric countries, trading internationally in metric units. Visiting advocates of metrication have stated that they use metric units for many things internally with exceptions such as old petrol pumps in Myanmar, calibrated in British Imperial gallons. These and other countries have adopted metric measures to some degree through international trade and standardization[8] for example, Sierra Leone switched to selling fuel by the litre in May 2011.[9] The United States mandated the acceptance of the metric system in 1866 for commercial and legal proceedings, without displacing their customary units.[10] The United Kingdom followed suit in 1897 and did not make the use of metric units compulsory for most purposes until 1995.

In 1971 the National Bureau of Standards completed a three-year study of the impact of increasing worldwide metric use on the U.S. The study concluded with a report to Congress entitled A Metric America – A Decision Whose Time Has Come. Since then metric use has increased in the U.S., principally in the manufacturing and educational sectors. Public Law 93-380, enacted 21 August 1974, states that it is the policy of the U.S. to encourage educational agencies and institutions to prepare students to use the metric system of measurement with ease and facility as a part of the regular education program. On 23 December 1975, President Gerald Ford signed Public Law 94-168, the Metric Conversion Act of 1975. This act declares a national policy of coordinating the increasing use of the metric system in the U.S. It established a U.S. Metric Board whose functions as of 1 October 1982 were transferred to the Dept of Commerce, Office of Metric Programs, to coordinate the voluntary conversion to the metric system.[11]

Most countries have adopted the metric system officially over a transitional period where both units are used for a set period of time. Some countries such as Guyana, for example, have officially adopted the metric system, but have had some trouble over time implementing it.[12] Antigua and Barbuda, also "officially" metric, is moving toward total implementation of the metric system, but slower than expected. The government had announced that they have plans to convert their country to the metric system by the first quarter of 2015.[13] Other Caribbean countries such as Saint Lucia are officially metric but are still in the process toward full conversion.[14]

The European Union used the Units of Measure Directive to attempt to achieve a common system of weights and measures and to facilitate the European Single Market. Throughout the 1990s, the European Commission helped accelerate the process for member countries to complete their metric conversion processes. Among them is the United Kingdom where laws in some or all contexts mandate or permit many imperial measures, such as miles and yards for road-sign distances, road speed limits in miles per hour, pints of beer, and inches for clothes.[15] The United Kingdom secured permanent exemptions for the mile and yard in road markings, and (with Ireland) for the pint (Imperial) of draught beer sold in pubs[16] (see Metrication in the United Kingdom). In 2007, the European Commission also announced that (to appease British public opinion and to facilitate trade with the United States) it was to abandon the requirement for metric-only labelling on packaged goods, and to allow dual metric–imperial marking to continue indefinitely.[16]

Other countries using the imperial system completed official metrication during the second half of the 20th century or the first decade of the 21st century. The most recent to complete this process was the Republic of Ireland, which began metric conversion in the 1970s and completed it in early 2005.[17]

In January 2007 NASA decided to use metric units for all future moon missions, in line with the practice of other space agencies.[18]

The United States, the United Kingdom, and Canada have some active opposition to metrication, particularly where updated weights and measures laws would make obsolete historic systems of measurement. Other countries, like France and Japan, that once had significant popular opposition to metrication now have complete acceptance of metrication.[citation needed]

Before the metric system[edit]

For more details on this topic, see History of measurement.

The Roman empire used the pes (foot) measure. This was divided into 12 unciae ("inches"). The libra ("pound") was another measure that had wide effect on European weight and currency long after Roman times, e.g. lb, £. The measure came to vary greatly over time. Charlemagne was one of several rulers who launched reform programmes of various kinds to standardise units for measure and currency in his empire, but there was no real general breakthrough.

In medieval Europe, local laws on weights and measures were set by trade guilds on a city-by-city basis. For example, the ell or elle was a unit of length commonly used in Europe, but its length varied from 40.2 centimetres in one part of Germany to 70 centimetres in The Netherlands and 94.5 centimetres in Edinburgh. A survey of Switzerland in 1838 revealed that the foot had 37 different regional variations, the ell had 68, there were 83 different measures for dry grain, 70 measures for fluids and 63 different measures for "dead weights".[19] When Isaac Newton wrote Philosophiae Naturalis Principia Mathematica in 1687, he quoted his measurements in Parisian feet so readers could understand the size. Examples of efforts to have local intercity or national standards for measurements include the Scottish law of 1641, and the British standard imperial system of 1824, which is still commonly used in the United Kingdom. At one time Imperial China had successfully standardised units for volume throughout its territory, but by 1936 official investigations uncovered 53 values for the chi varying from 200 millimetres to 1250 millimetres; 32 values of the cheng, between 500 millilitres and 8 litres; and 36 different tsin ranging from 300 grams to 2500 grams.[20] However, revolutionary France was to produce the definitive International System of Units which has come to be used by most of the world today.

The desire for a single international system of measurement came from growing international trade and the need to apply common standards to goods. For a company to buy a product produced in another country, they need to ensure that the product would arrive as described. The medieval ell was abandoned in part because its value could not be standardised. One primary advantage of the International System of Units is simply that it is international, and the pressure on countries to conform to it grew as it became increasingly an international standard. However, it also simplified the teaching and learning of measurement as all SI units are based on a handful of base units (in particular, the metre, kilogram and second cover the majority of everyday measurements), using decimal prefixes to cover all magnitudes. This contrasts with pre-metric units, which largely have names that do not relate directly to one another (e.g. inch, foot, yard, mile) and are related to one another by inconsistent ratios which must simply be memorised, (e.g. 12, 3, 1760). As the values in an SI expression are always decimal (i.e. without vulgar fractions) and mixed units (such as "feet and inches") are not used with SI, measurements are easy to add or multiply. Moreover, scientific measurement and calculation are greatly simplified as the units for electricity, force etc. are part of the SI system and hence are all interrelated in a coherent manner (e.g. 1 J = 1 kg·m2·s−2 = 1 V·A·s). Standardization of measures has contributed significantly to the industrial revolution and technological development in general[according to whom?]. SI is not the only example of international standardization; several powerful international standardization organizations exist for various industries, such as the International Organization for Standardization (ISO), the International Electrotechnical Commission (IEC), and the International Telecommunication Union (ITU).

Forerunners of the metric system[edit]

Decimal numbers are an essential part of the metric system, with only one base unit and multiples created on the decimal base, the figures remain the same. This simplifies calculations. Although the Indians used decimal numbers for mathematical computations, it was Simon Stevin who in 1585 first advocated the use of decimal numbers for everyday purposes in his booklet De Thiende (old Dutch for 'the tenth'). He also declared that it would only be a matter of time before decimal numbers were used for currencies and measurements. His notation for decimal fractions was clumsy, but this was overcome with the introduction of the decimal point, generally attributed to Bartholomaeus Pitiscus who used this notation in his trigonometrical tables (1595).[21]

In his Essay towards a Real Character and a Philosophical Language, published in 1668, John Wilkins proposed a system of measurement that was very similar in concept to today's metric system. He proposed retaining the second as the basic unit of time and proposed that the length of a pendulum which crossed the zero position once a second (i.e. had a period of two seconds) should be the base unit of length. This length, for which he proposed the name "standard", would have been 994 mm. His base unit of mass, which he proposed calling a "hundred", would have been the mass of a cubic standard of distilled rainwater. The names that he proposed for decimal multiples and subunits of his base units of measure were the names of units of measure that were in use at the time.[22][23]

In 1670, Gabriel Mouton published a proposal that was in essence similar to Wilkins' proposal, except that his base unit of length would have been 1/1000 of a minute of arc (about 1.852 m) of geographical latitude. He proposed calling this unit the virga. Rather than using different names for each unit of length, he proposed a series of names that had prefixes, rather like the prefixes found in SI.[24]

In 1790, Thomas Jefferson submitted a report to the United States Congress in which he proposed the adoption of a decimal system of coinage and of weights and measures. He proposed calling his base unit of length a "foot" which he suggested should be either 310 or 13 of the length of a pendulum that had a period of one second – that is 310 or 13 of the "standard" proposed by Wilkins over a century previously. This would have equated to 11.755 English inches (29.8 cm) or 13.06 English inches (33.1 cm). Like Wilkins, the names that he proposed for multiples and subunits of his base units of measure were the names of units of measure that were in use at the time.[25] The great interest in geodesy during this era, and the measurement system ideas that developed, influenced how the continental U.S. was surveyed and parceled. The story of how Jefferson's full vision for the new measurement system came close to displacing the Gunter chain and the traditional acre, but ended up not doing so, is explored in Andro Linklater's Measuring America.[26]

Conversion process[edit]

The metric system was officially introduced in France in December 1799. In the 19th century, the metric system was adopted by almost all European countries: Portugal (1814);[27] Netherlands, Belgium and Luxembourg (1820); Switzerland (1835); Spain (1850s); Italy (1861); Germany (1870, legally from 1 January 1872);[28] and Austria-Hungary (1876, but the law was adopted in 1871).[29] Thailand did not formally adopt the metric system until 1923, but the Royal Thai Survey Department used it for cadastral survey as early as 1896.[30] Denmark and Iceland adopted the metric system in 1907.

Chronology and status of conversion by country[edit]

Countries by date of metrication. Colours red to green show the pattern of metrication from 1795 to 1998. Black identifies countries that have not adopted the metric system as the primary measurement system. White identifies countries that already used the metric system at the time they gained their independence.
Countries by current metrication status:
  Almost Complete
  Partially Complete
  Little Adoption

Links in the country point to articles about metrication in that country.[31]

Year metrication
process started[32][33][34]
Country Previous system
of measure
Status of
1795 France French 5Complete
1814 Portugal[Note 1] Portuguese 5Complete
1820 Belgium Various 5Complete
1820 Netherlands Dutch 5Complete
1848 Chile Spanish 3Almost entirely complete
1852[35] Mexico Variants of Spanish 4Complete (some national and regional units are still in use and some United States customary units also in use in some industries)
1852 Spain Spanish 5Complete, however many pre-metrics are still in use: for land - ferrado, fanega, atahúlla and quiñón; for weight - libra and arroba; for liquids - cuartilla; for screen sizes - pulgada (23 mm).
1861 Italy Various 5Complete
1862[36] Brazil Portuguese 5Complete
1862 Peru Spanish 4Almost entirely complete
1863 Uruguay Spanish 5Complete
1864 Romania Romanian 5Complete
1868 North German Confederation Various 5Complete
1869 South German states Various 5Complete
1871 Austria Various 5Complete
1872[Note 2] Germany Various 5Complete
1873[37] Serbia Various 5Complete
1875 Ottoman Empire/Turkey Ottoman 5Complete
1875[38] Norway Norwegian 5Complete
1876 Sweden Swedish 5Complete
1876 Switzerland Various 5Complete
1876 Hungary Various 5Complete
1885 Japan[39] Japanese 3Almost entirely complete
1886 Argentina Spanish 5Complete
1886[40] Finland Finnish 5Complete
1899 Paraguay Variants of Spanish 5Complete
1907 Denmark Danish 5Complete
1907 Iceland Icelandic / Danish 5Complete
1907 Philippines Spanish and own. 3Almost entirely complete
1908 Costa Rica Spanish 5Complete
1912 Dominican Republic Spanish 5Complete
1918[41][42] Russia Russian 5Complete
1923[43][44] Thailand Various 3Almost entirely complete
1925 China Chinese 3Almost entirely complete
1929 Estonia Estonian 5Complete
1946 Indonesia Various 3Almost entirely complete
1954 India Various 5Complete
1954 Sudan Various 5Complete
1959[45] Greece Old Greek 5Complete
1963 Laos Unknown 5Complete
1963 Vietnam Vietnamese 5Complete
1965[Note 3] United Kingdom Imperial 2Partially complete
1967 Ireland Imperial[Note 4] 5Complete
1967 Pakistan Imperial 5Complete
1969 New Zealand Imperial 5Complete
1970 Australia Imperial 5Complete
1971 South Africa[Note 5] Imperial 5Complete
1972 Malaysia Imperial and Malay 2Partially complete
1973 Canada Imperial 2Partially complete
1975[Note 6][46] United States of America United States customary units 1Some adoption[47]
1976[48] Sri Lanka Imperial 3Almost entirely complete[49]
1998 Jamaica Imperial 2Partially complete
2005 Saint Lucia Imperial 5Complete
Indeterminate Liberia[Note 7] Imperial[46] 1Some adoption[Note 8][50]
2011[Note 9][51]
Burma (Myanmar) Burmese, Imperial[46] 1Some adoption[53]
Announcement of full metrication, with technical assistance from the German National Metrology Institute[54]


  1. ^ Including Portuguese colonies, most of them now independent countries.
  2. ^ Many German states, particularly those under French tutelage during the Napoleonic Wars (Rheinbund) adopted the metre 1806–15.
  3. ^ Phased transition announced in 1965
  4. ^ Old Irish units of measurement prior to 1807. See also Irish measure.
  5. ^ Including South-West Africa, now Namibia
  6. ^ First adopted in 1866, not enacted until Signing of the Metric Conversion Act in 1975
  7. ^ See also Liberia measurement system.
  8. ^ The Liberian government has begun transitioning from use of imperial units to the metric system. However, this change has been gradual, with government reports concurrently using both systems.
  9. ^ In June 2011, the Burmese government's Ministry of Commerce began discussing proposals to reform the measurement system in Burma and adopt the metric system used by most of its trading partners.

There are three common ways that nations convert from traditional measurement systems to the metric system. The first is the quick, or "Big-Bang" route which was used by India in the 1960s and several other nations including Australia and New Zealand since then. The second way is to phase in units over time and progressively outlaw traditional units. This method, favoured by some industrial nations, is slower and generally less complete. The third way is to redefine traditional units in metric terms. This has been used successfully where traditional units were ill-defined and had regional variations.

The "Big-Bang" way is to simultaneously outlaw the use of pre-metric measurement, metricate, reissue all government publications and laws, and change education systems to metric. India's changeover lasted from 1 April 1960, when metric measurements became legal, to 1 April 1962, when all other systems were banned. The Indian model was extremely successful and was copied over much of the developing world.

The phase-in way is to pass a law permitting the use of metric units in parallel with traditional ones, followed by education of metric units, then progressively ban the use of the older measures. This has generally been a slow route to metric. The British Empire permitted the use of metric measures in 1873, but the changeover was not completed in most Commonwealth countries until the 1970s and 1980s when governments took an active role in metric conversion. Japan also followed this route and did not complete the changeover for 70 years. In the United Kingdom, the process is still incomplete. By law, loose goods sold with reference to units of quantity have to be weighed and sold using the metric system. In 2001, the EU directive 80/181/EEC stated that supplementary units (imperial units alongside metric including labelling on packages) would become illegal from the beginning of 2010. In September 2007,[16] a consultation process was started which resulted in the directive being modified to permit supplementary units to be used indefinitely.

The third method is to redefine traditional units in terms of metric values. These redefined "quasi-metric" units often stay in use long after metrication is said to have been completed. Resistance to metrication in post-revolutionary France convinced Napoleon to revert to mesures usuelles (usual measures), and, to some extent, the names remain throughout Europe. In 1814, Portugal adopted the metric system, but with the names of the units substituted by Portuguese traditional ones. In this system, the basic units were the mão-travessa (hand) = 1 decimetre (10 mão-travessas = 1 vara (yard) = 1 metre), the canada = 1 litre and the libra (pound) = 1 kilogram.[27] In the Netherlands, 500 g is informally referred to as a pond (pound) and 100 g as an ons (ounce), and in Germany and France, 500 g is informally referred to respectively as ein Pfund and une livre ("one pound").[55] In Denmark, the re-defined pund (500 g) is occasionally used, particularly among older people and (older) fruit growers, since these were originally paid according to the number of pounds of fruit produced. In Sweden and Norway, a mil (Scandinavian mile) is informally equal to 10 km, and this has continued to be the predominantly used unit in conversation when referring to geographical distances. In the 19th century, Switzerland had a non-metric system completely based on metric terms (e.g. 1 Fuss (foot) = 30 cm, 1 Zoll (inch) = 3 cm, 1 Linie (line) = 3 mm). In China, the jin now has a value of 500 g and the liang is 50 g.

It is difficult to judge the degree to which ordinary people change to using metric in their daily lives. In countries that have recently changed, older segments of the population tend to still use the older units. Also, local variations abound in which units are round metric quantities or not. In Canada, for example, ovens and cooking temperatures are usually measured in degrees Fahrenheit and Celsius. Except for in cases of import items, all recipes and packaging include both Celsius and Fahrenheit, so Canadians are typically comfortable with both systems of measurement. This extends to manufacturing, where companies are able to use both imperial and metric units, since the major export market is the U.S., but metric is required for both domestic use and for nearly all other export. This may be due to the overwhelming influence of the neighbouring United States; similarly, many English Canadians[citation needed] (unlike most French Canadians[citation needed]) still often use non-metric measurements in day-to-day discussions of height and weight, and for clothing sizes, which are invariably measured in inches, though most driver's licences and other official government documents record weight and height only in metric (Saskatchewan driver licences, prior to the introduction of the current one-piece licence, indicated height in feet and inches but have switched to centimetres following the new licence format[citation needed]). In Canadian schools, however, metric is the standard, except when it comes up in recipes, where both are included, or in practical lessons involving measuring wood or other materials for manufacturing. In the United Kingdom, degrees Fahrenheit are seldom encountered (except when some people talk about hot summer weather), while other metric units are often used in conjunction with older measurements, and road signs use miles rather than kilometres. Another example is "hard" and "soft" metric. Canada converted liquid dairy products to litre, 500 mL, and 250 mL sizes, which caused some complaining at the time of the conversion, as a litre of milk is slightly over 35 imperial fluid ounces, while the former imperial quart used in Canada was 40 ounces. This is an example of a "hard" metric conversion. Conversely, butter in Canada is sold primarily in a 454 g package, which converts to one Imperial pound. This is considered a "soft" metric conversion. Such countries could be said to be "semi-metric". However, unlike in the rest of Canada, metrication in the Francophone province of Quebec has been more implemented and metric measures are more consistently used in Quebec than elsewhere in Canada.


As of 2015, in most countries of the world, the metric system officially predominates; traditional units, however, are still used in many places and industries. For example:

  • Photo and video cameras are standardized to mount to tripods using 1/4-20 and 3/8-16 screws, which are dimensioned in inches, as per ISO 1222:2010,[56]
  • Automobile tyre pressure is commonly measured in units of psi in several countries including Brazil, Peru, Argentina, Australia, and Chile, the UK and the US.
  • Automotive engine power is usually measured in horsepower (rather than in kilowatts) in Russia, most other ex-USSR countries and German-speaking countries (note that this is typically "metric horsepower" rather than imperial horsepower), although in the EU from 2010 the horsepower is permitted only as a supplementary unit.[57]
  • In Hong Kong, traditional Chinese and British imperial units are normally used instead of metric units in particular types of trade.
  • Amongst construction workers in Northern Europe, planks and nails are often called by their old inch-based names.
  • The length of small sail boats is often given in feet in popular conversation.
  • Office space is often rented in traditional units, such as square feet in Hong Kong and India, tsubo/pyeong/"ping" in Japan, Korea and Taiwan.
  • In plumbing, some pipes and pipe threads are still designated in inch sizes due to historic international acceptance of particular sequences of pipe sizes and pipe threads, such as BSP / ISO 7 / EN 10226 threads.
  • In the United Kingdom and some Commonwealth countries, temperatures of domestic gas ovens are often specified using gas marks.[citation needed]
  • Automotive wheel diameters are still usually but not always set as whole inch measurements (although tyre widths are measured in millimetres).
  • Dots per inch and pixels per inch continue to be used in describing graphical resolution in the computer and printing industry.
  • Television and monitor screen diagonals are still commonly cited in inches in many countries; however, in countries such as Australia, France and South Africa, centimetres are often used for television sets, whereas CRT computer monitors and all LCD monitors are measured in inches.
  • Many large format computer printers, commonly known as plotters, have carriage widths measured in inches. Common widths are 24 in (610 mm), 36 in (910 mm), 44 in (1,100 mm) and 60 in (1,500 mm). While metric media sizes are often quoted (e.g. A0, A1), rolls of film, plain paper or photographic paper are normally sold in these widths, giving rise to wastage when they are trimmed. Rolls lengths are variously quoted in feet or metres.
  • In the electronic industry, the dominant spacing for components is based on intervals of 110 in (2.54 mm), and a change would lead to compatibility issues, e.g., for connectors.
  • Within the mechanical industry, inch-based spare parts can occasionally be kept, e.g., to service American or pre-World War II machines, but at maintenance, screws may be exchanged to metric thread.
  • In Ireland, the only legal exception to the metrication process was the pint in bars, pubs, and clubs (although alcohol sold in any other location is in metric units (usually 330 ml (bottled beer), 500 ml (canned beer), 750 ml (wine), or 1 L or 700 ml (spirits))).
  • In Australia, a pint of beer was redefined to 570 ml (see Australian beer glasses).
  • In both metric and non-metric countries, racing bicycle frames are generally measured in centimetres, while mountain bicycle and other frames are measured in either or both.
  • In the sport of surfing, surfboards are usually designed, constructed, and sold in feet and inches.
  • In Spain and former colonies i.e. Americas and Philippines certain pre-metric units are still used, e.g., the quiñón for land measurement in the Philippines, the fanega, ferrado and atahúlla to name three used in Spain and other former possessions.
    • The pulgada (inch) is 23 mm, 2 mm shorter than the English inch.
  • In many long-time metric countries, when non-metric units are used, it is often to give rough estimates in a short form, while accurate measures always are metric, e.g., "6 feet" may feel less exact and shorter to say than "1.8 metres" or "180 cm".[citation needed] Measurement tools for inches are generally rare to find there, only on the other side of some carpenter's rulers, and may present a variation between national legacy inches and British/US inches, easily causing significant measurement errors if used.
  • The Imperial gallon is used as a unit of measure for fuel in Anguilla, Antigua and Barbuda, Burma, the Cayman Islands, Dominica, Grenada, Montserrat, St Kitts and Nevis and St Vincent and the Grenadines. In the United Kingdom fuel efficiency is officially recorded in miles per imperial gallon although fuel is sold in litres. [58]
  • The US gallon is used in the Bahamas, Belize, British Virgin Islands, Colombia, Dominican Republic, Ecuador, El Salvador, Guatemala, Haiti, Honduras, Liberia, Peru, Turks and Caicos Islands, and the US, especially for pricing.[58]
  • In Latin America, as SI units are standard, litres are used as well, e.g., often regarding fuel economy (km per litre), and the Spanish word galón may occasionally refer to a portable fuel container, often 5-20 L.
  • Road distances and speed limits are still displayed in miles and miles per hour respectively in the USA, UK, Burma, and various Caribbean nations.[59]

In some countries (such as Antigua and Barbuda, see above), the transition is still in progress. The Caribbean island nation of Saint Lucia announced metrication programmes in 2005 to be compatible with CARICOM.[60]

United Kingdom[edit]

Examples of official metrication of British grocery products. Two of the four items are purely metric. Milk is often sold as "1.136 litres / 2 pints". The sausages are labelled "340 g / 12 oz"

In the United Kingdom, some of the population continue to resist metrication to varying degrees. The traditional imperial measures are preferred by a majority and continue to have widespread use in some applications.[61][62] The metric system is used by most businesses,[63] and is used for most trade transactions. Metric units must be used for certain trading activities (selling by weight or measure for example), although imperial units may continue to be displayed in parallel.[64]

British law has enacted the provisions of European Union directive 80/181/EEC, which catalogues the units of measure that may be used for "economic, public health, public safety and administrative purposes".[65] These units consist of the recommendations of the General Conference on Weights and Measures,[66] supplemented by some additional units of measure that may be used for specified purposes.[67] Metric units could be legally used for trading purposes for nearly a century before metrication efforts began in earnest. The government had been making preparations for the conversion of the Imperial unit since the 1862 Select Committee on Weights and Measures recommended the conversion[68] and the Weights and Measures Act of 1864 and the Weights and Measures (Metric System) Act of 1896 legalised the metric system.[69] In 1965, with lobbying from British industries and the prospects of joining the Common Market, the government set a 10-year target for full conversion, and created the Metrication Board in 1969. Metrication did occur in some areas during this time period, including the re-surveying of Ordnance Survey maps in 1970, decimalisation of the currency in 1971, and teaching the metric system in schools. No plans were made to make the use of the metric system compulsory, and the Metrication Board was abolished in 1980 following a change in government.[70]

The United Kingdom avoided having to comply with the 1989 European Units of Measurement Directive (89/617/EEC), which required all member states to make the metric system compulsory, by negotiating derogations (delayed switchovers), including for miles on road signs and for pints for draught beer, cider, and milk sales.[71]

Following the United Kingdom's referendum to exit the European Union, retailers have begun to request to go back to using Imperial Units, with some reverting without permission. A poll following the EU vote also found that 45% of Britons sought to revert to selling produce in Imperial Units.[72]

In popular conversation, the stone unit is still used for body weight by some people though people are starting to weigh themselves in kilos as pre-metric weighing machines become obsolete, and feet and inches are used to describe height by some as well.

United States and Canada[edit]

A measuring cup, manufactured and sold in the US (circa 1980) features graduations in both metric and US customary systems. Held in the right hand, a person would have the metric graduations in front, facing them. But in filling the measuring cup from another container, right-handed people would hold it in their left hand, and read from the customary graduations.

Over time, the metric system has influenced the United States through international trade and standardization. The use of the metric system was made legal as a system of measurement in 1866[73] and the United States was a founding member of the International Bureau of Weights and Measures in 1875.[74] The system was officially adopted by the federal government in 1975 for use in the military and government agencies, and as preferred system for trade and commerce.[75] It has remained voluntary for federal and state road signage to use metric units, despite attempts in the 1990s to make it a requirement.[76]

A 1992 amendment to the Fair Packaging and Labeling Act (FPLA), which took effect in 1994, required labels on federally regulated "consumer commodities"[77] to include both metric and US customary units. As of 2013, all but one US state (New York) have passed laws permitting metric-only labels for the products they regulate.[78] Likewise, Canada also legally allows for dual labelling of goods provided that the metric unit is listed first and that there is a distinction of whether a liquid measure is a US or a Canadian (Imperial) unit.[79]

Today, the American public and much of the private business and industry still use US customary units despite many years of informal or optional metrication.[80] At least two states, Kentucky and California, have even moved towards demetrication of highway construction projects.[81][82][83]

Air and sea transportation[edit]

Air and sea transportation commonly use the nautical mile. This is about one minute of arc of latitude along any meridian arc and it is precisely defined as 1852 metres (about 1.151 statute miles). It is not an SI unit (although it is accepted for use in the SI by the BIPM). The prime unit of speed or velocity for maritime and air navigation remains the knot (nautical mile per hour).

The prime unit of measure for aviation (altitude, or flight level) is usually estimated based on air pressure values, and in many countries, it is still described in nominal feet, although many others employ nominal metres. The policies of the International Civil Aviation Organization (ICAO) relating to measurement are:

  • there should be a single system of units throughout the world
  • the single system should be the SI
  • the use of the foot for altitude is a permitted variation.

Consistent with ICAO policy, aviation has undergone a significant amount of metrication over the years. For example, runway lengths are usually given in metres. The United States metricated the data interchange format (METAR) for temperature reports in 1996.[84] Metrication is also gradually taking place in cargo weights and dimensions and in fuel volumes and weights.

Accidents and incidents[edit]

Confusion over units during the process of metrication can sometimes lead to accidents. One of the most notable examples was during metrication in Canada. In 1983, an Air Canada Boeing 767, nicknamed the "Gimli Glider" following the incident, ran out of fuel in midflight. The incident was caused, in a large part, by the confusion over the conversion between litres, kilograms, and pounds, resulting in the aircraft receiving 22,300 pounds of fuel instead of the required 22,300 kg.[85]

While not strictly an example of national metrication, the use of two different systems was a contributing factor in the loss of the Mars Climate Orbiter in 1998. The National Aeronautics and Space Administration (NASA) specified metric units in the contract. NASA and other organizations worked in metric units, but one subcontractor, Lockheed Martin, provided thruster performance data to the team in pound force-seconds instead of newton-seconds. The spacecraft was intended to orbit Mars at about 150 kilometres (93 mi) in altitude, but the incorrect data meant that it descended to about 57 kilometres (35 mi). As a result, it burned up in the Martian atmosphere.[86]

On 25 September 2009, the British Department for Transport published a draft version of legislation to amend its road signs legislation[87] for comment. Among the proposed changes was an amendment to existing legislation to make dual-unit height and width warning and restriction signs mandatory. This was justified in Paragraph 53 of the Impact Analysis[88] by the text "... Based on records from Network Rail's incident logs since April 2008, approximately 10 to 12 percent of bridge strikes involved foreign lorries. This is disproportionately high in terms of the number of foreign lorries on the road network." This proposal was shelved with the change of government in 2010, though many bridges are now signed both ways. The latest signage guidance consultation has proposed this once again.

See also[edit]


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External links[edit]