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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 for measurement.^[2] Worldwide, there has been a long process of independent conversions of countries from various local and traditional systems of measurement, beginning in France during the 1790s and spreading widely over the following two centuries, however metrication has not occurred in all countries and sectors.

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

Overview

In the world nearly all countries have metricated either through legislation or convention.^[3]

A country considered a "metric country" if its society, by legislation or otherwise, has substantially metricated its commercial and standard activities.^[4][5][6] (For common non-metric system of measurement in metric countries see exceptions).

The United Kingdom is metricated, having replaced nearly all of its commercial and standard activities with the metric system, however the government has the following legislated exceptions: road signage and the following products can be sold only in imperial measures: draught beer or cider by pint, milk in returnable containers by pint, and precious metals by troy ounce. Further, products can display an imperial measurement alongside the metric measurement but it can’t stand out more than the metric measurement.^{[7][8][9][10][11]} (See Metrication in the United Kingdom).

Some countries have legislated to metricate but have stalled in implementing a full change in society.^[12] For example:

• 
  Guyana have officially adopted the metric system, but have had some trouble over time implementing it.^[13][14]
  


• 
  Caribbean countries are "officially" metric and are moving toward total metrication implementation, but are slower than expected. For example Antigua and Barbuda, and Saint Lucia, are officially metric but are still in the process toward full conversion.^[15][16][17]
  

A country considered non-metric if its society, by legislation or otherwise, has not substantially metricated its commercial and standards activities.^[18][19][20] As of 2017^[update], seven countries are considered non-metric:^[21][22]

• The United States (and its associated states the Federated States of Micronesia, Marshall Islands and Palau), currently using U.S. customary units.


• 
  Liberia, currently using U.S. customary units.^[23]
  


• 
  Myanmar, currently using Burmese units.


• 
  Belize, currently using the Imperial units.^[24]
  

(Samoa is sometimes included in this list, however they have legislated and mandated from 1 January 2017 that all commercial engagements are to use the metric units.^[25])

Myanmar and Liberia have partially metricated, even without official legislation, due to international trade and standardisation.^[26] The government of Myanmar has stated that the country would metricate with a goal of completion by 2019.^[27][28][29] In 2018 the Liberian Commerce and Industry Minister announced that the Liberian government also has committed to metrication.^[30]

The United States has official legislation for metrication, however conversion was not mandatory and many industries chose not to convert, and unlike other countries, there is no government or major social desire to implement further metrication. (see Metrication in the United States.) The microstates of Federated States of Micronesia, Marshall Islands and Palau, have not metricated due to their close relationship as associated states to the United States.^[31]

The microstate of Belize, being a member of CARICOM is required to metricate to promote trade, but to date the imperial units are the standard in the country.^[32]

Other countries using the imperial system of measurement progressed with an official metrication programme during the second half of the 20th century or the first decade of the 21st century. Ireland, which began metric conversion in the 1970s, completed metrication of its road speed limits in early 2005.^[33]

Pre-metric measurement and the justification for metrication

Pre-metric measurement

Various societies developed their own systems of measurement. (See Previous System of Measurement in the table Conversion Table).

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 measurements 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".^[34] 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.^[35] However, revolutionary France was to produce the definitive International System of Units which has come to be used by most of the world today.

Justification for metrication

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 metric system was determined to be the preferred system and countries started to make the conversion, as the medieval ell and other systems of measurement were abandoned mostly because they were different from country to country.

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 (e.g. 10, 100, 1000). This contrasts with other systems of measurement, 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).

English units of length

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. (See Metric prefix). 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).

Standardisation of measures has contributed significantly to a countries GDP.

Country metrication

During the nineteenth century the metric system of measurement proved a convenient political compromise during the unification processes in the Netherlands, Germany and Italy. In 1814, Portugal became the first country not part of the French Empire to officially adopt a metric system. Spain found it expedient in 1858 to follow the French example and within a decade Latin America had also metricated. There was considerable resistance to metrication in the United Kingdom and in the United States, though once the United Kingdom announced its metrication program in 1965, the Commonwealth followed suit.

France

Napoleon Bonaparte introduced the Mesures usuelles.

The introduction of the metric system into France in 1795 was done on a district by district basis with Paris being the first district, but by modern standards the transition was poorly managed. Although thousands of pamphlets were distributed, the Agency of Weights and Measures who oversaw the introduction underestimated the work involved. Paris alone needed 500,000 metre sticks, yet one month after the metre became the sole legal unit of measure, they only had 25,000 in store.^{[36]: 269} This, combined with other excesses of the Revolution and the high level of illiteracy made the metric system unpopular.

Napoleon himself ridiculed the metric system, but as an able administrator, recognised the value of a sound basis for a system of measurement and under the décret impérial du 12 février 1812 (imperial decree of 12 February 1812), a new system of measure – the mesures usuelles or "customary measures" was introduced for use in small retail businesses – all government, legal and similar works still had to use the metric system and the metric system continued to be taught at all levels of education.^[37] The names of many units used during the ancient regime were reintroduced, but were redefined in terms of metric units. Thus the toise was defined as being two metres with six pied making up one toise, twelve pouce making up one pied and twelve lignes making up one pouce. Likewise the livre was defined as being 500 g, each livre comprising sixteen once and each once eight gros and the aune as 120 centimetres.^[38]

Louis Philippe I by means of the La loi du 4 juillet 1837 (the law of 4 July 1837) effectively revoked the use of mesures uselles by reaffirming the laws of measurement of 1795 and 1799 to be used from 1 May 1840.^[39][40] However, many units of measure, such as the livre (for half a kilogram), remained in everyday use for many years,^[40][41] and to a residual extent up to this day.

The Portuguese metric system

In August 1814, Portugal officially 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 liter and the libra (pound) = 1 kilogram.^[42]

The Dutch metric system

The Netherlands first used the metric system and then, in 1812, the mesures usuelles when it was part of the First French Empire. Under the Royal decree of 27 March 1817 (Koningklijk besluit van den 27 Maart 1817), the newly formed Kingdom of the Netherlands abandoned the mesures usuelles in favour of the "Dutch" metric system (Nederlands metrisch stelsel) in which metric units were given the names of units of measure that were then in use. Examples include the ons (ounce) which was defined as being 100 g.^[43]

The German Zollverein

Stone marking the Austro-Hungarian/Italian border at Pontebba displaying myriametres (10 km), a unit used in Central Europe in the 19th century.^[44]

At the outbreak of the French Revolution, much of modern-day Germany and Austria were part of the Holy Roman Empire which had become a loose federation of kingdoms, principalities, free cities, bishoprics and other fiefdoms, each with its own system of measurement, though in most cases such system were loosely derived from the Carolingian system instituted by Charlemagne a thousand years earlier.

During the Napoleonic era, there was a move among some of the German states to reform their systems of measurement using the prototype metre and kilogram as the basis of the new units. Baden, in 1810, for example, redefined the Ruthe (rods) as being 3.0 m exactly and defined the subunits of the Ruthe as 1 Ruthe = 10 Fuß (feet) = 100 Zoll (inches) = 1,000 Linie (lines) = 10,000 Punkt (points) while the Pfund was defined as being 500 g, divided into 30 Loth, each of 16.67 g.^[44][45]Bavaria, in its reform of 1811, trimmed the Bavarian Pfund from 561.288 g to 560 g exactly, consisting of 32 Loth, each of 17.5 g^[46] while the Prussian Pfund remained at 467.711 g.^[47]

After the Congress of Vienna there was a degree of commercial cooperation between the various German states resulting in the setting of the German Customs Union (Zollverein). There were however still many barriers to trade until Bavaria took the lead in establishing the General German Commercial Code in 1856. As part of the code the Zollverein introduce the Zollpfund (Customs Pound) which was defined to be exactly 500 g and which could be split into 30 'lot'.^[48] This unit was used for inter-state movement of goods, but was not applied in all states for internal use.

Although the Zollverein collapsed after the Austro-Prussian War of 1866, the metric system became the official system of measurement in the newly formed German Empire in 1872^[36]:350 and of Austria in 1875.^[49] The Zollpfund ceased to be legal in Germany after 1877.^[50]

Italy

Tablet showing conversions of legacy units of weights and measures to metric units, Vicopisano, Tuscany

The Cisalpine Republic, a North Italian republic set up by Napoleon in 1797 with its capital at Milan first adopted a modified form of the metric system based in the braccio cisalpino (Cisalpine cubit) which was defined to be half a metre.^[51] In 1802 the Cisalpine Republic was renamed the Italian Republic, with Napoleon as its head of state. The following year the Cisalpine system of measurement was replaced by the metric system.^[51]

In 1806, the Italian Republic was replaced by the Kingdom of Italy with Napoleon as its emperor. By 1812, all of Italy from Rome northwards was under the control of Napoleon, either as French Departments or as part of the Kingdom of Italy ensuring the metric system was in use throughout this region.

After the Congress of Vienna, the various Italian states reverted to their original system of measurements, but in 1845 the Kingdom of Piedmont and Sardinia passed legislation to introduce the metric system within five years. By 1860, most of Italy had been unified under the King of Sardinia Victor Emmanuel II and under Law 132 of 28 July 28, 1861 the metric system became the official system of measurement throughout the kingdom. Numerous Tavole di ragguaglio (Conversion Tables) were displayed in shops until 31 December 1870.^[51]

Spain

Until the ascent of the Bourbon monarchy in Spain in 1700, each of the regions of Spain retained its own system of measurement. The new Bourbon monarchy tried to centralise control and with it the system of measurement. There were debates regarding the desirability of retaining the Castilian units of measure or, in the interests of harmonisation, adopting the French system.^[52] Although Spain assisted Méchain in his meridian survey, the Government feared the French revolutionary movement and reinforced the Castilian units of measure to counter such movements. By 1849 however, it proved difficult to maintain the old system and in that year the metric system became the legal system of measure in Spain.^[52]

Don Carlos Ibáñez e Ibáñez de Ibero, Marquis of Mulhacén

The Government was urged by the Spanish Royal Academy of Science to approve the creation of a large-scale map of Spain in 1852. The following year Carlos Ibáñez e Ibáñez de Ibero was appointed to undertake this task. All the scientific and technical material had to be created. Carlos Ibáñez e Ibáñez de Ibero and Saavedra went to Paris to supervise the production by Brunner of a measuring instrument which they had devised and which they later compared with Borda's double-toise N°1 which was the main reference for measuring all geodetic bases in France and whose length was 3.8980732 metres.

In 1865 the triangulation of Spain was connected with that of Portugal and France. In 1866 at the conference of the Association of Geodesy in Neuchâtel, Ibáñez announced that Spain would collaborate in remeasuring the French meridian arc. In 1879 Ibáñez and François Perrier (representing France) completed the junction between the geodetic network of Spain and Algeria and thus completed the measurement of the French meridian arc which extended from Shetland to the Sahara.

In 1867 Russia, Spain and Portugal joined the "Europäische Gradmessung" (European Arc Measurement which would become the International Association of Geodesy). This same year at the second general conference of the European Arc Measurement held in Berlin, the question of an international standard unit of length was discussed in order to combine the measurements made in different countries to determine the size and shape of the Earth. The conference recommended the adoption of the metre and the creation of an international metre commission, according to the proposal of Johann Jacob Baeyer, Adolphe Hirsch and Carlos Ibáñez e Ibáñez de Ibero.

In November 1869 the French government issued invitations to join this commission. Spain accepted and Carlos Ibáñez e Ibáñez de Ibero took part in the Committee of preparatory research from the first meeting of the International Metre Commission in 1870. He became president of the permanent Committee of the International Metre Commission in 1872. In 1874 he was elected as president of the Permanent Commission of the European Arc Measurement. He also presided the General Conference of the European Arc Measurement held in Paris in 1875, when the association decided the creation of an international geodetic standard for the bases' measurement.^[53] He represented Spain at the 1875 conference of the Metre Convention, which was ratified the same year in Paris. The Spanish geodesist was elected as the first president of the International Committee for Weights and Measures. His activities resulted in the distribution of a platinum and iridium prototype of the metre to all States parties to the Metre Convention during the first meeting of the General Conference on Weights and Measures in 1889. Theses prototypes defined the metre right up until 1960.

United Kingdom and the Commonwealth

In 1824 the Weights and Measures Act imposed one standard 'imperial' system of weights and measures on the British Empire.^[54] The effect of this act was to standardise existing British units of measure rather than to align them with the metric system.

During the next eighty years a number of Parliamentary select committees recommended the adoption of the metric system, each with a greater degree of urgency, but Parliament prevaricated. A Select Committee report of 1862 recommended compulsory metrication, but with an "Intermediate permissive phase"; Parliament responded in 1864 by legalising metric units only for 'contracts and dealings'.^[55] The United Kingdom initially declined to sign the Treaty of the Metre, but did so in 1883. Meanwhile, British scientists and technologists were at the forefront of the metrication movement – it was the British Association for the Advancement of Science that promoted the CGS system of units as a coherent system^{[56]: 109} and it was the British firm Johnson Matthey that was accepted by the CGPM in 1889 to cast the international prototype metre and kilogram.^[57]

In 1895 another Parliamentary select committee recommended the compulsory adoption of the metric system after a two-year permissive period. The 1897 Weights and Measures Act legalised the metric units for trade, but did not make them mandatory.^[55] A bill to make the metric system compulsory in order to enable the British industrial base to fight off the challenge of the nascent German base passed through the House of Lords in 1904, but did not pass in the House of Commons before the next general election was called. Following opposition by the Lancashire cotton industry, a similar bill was defeated in 1907 in the House of Commons by 150 votes to 118.^[55]

In 1965 Britain commenced an official programme of metrication that, as of 2012^[update], had not been completed. The British metrication programme signalled the start of metrication programmes elsewhere in the Commonwealth, though India had started its programme in 1959, six years before the United Kingdom. South Africa (then not a member of the Commonwealth) set up a Metrication Advisory Board in 1967, New Zealand set up its Metric Advisory Board in 1969, Australia passed the Metric Conversion Act in 1970 and Canada appointed a Metrication Commission in 1971. Metrication in Australia, New Zealand and South Africa was essentially complete within a decade, while metrication in India and Canada is not complete. In addition the lakh and crore are still in widespread use in India, while in Canada the square foot is still widespread for commercial and residential advertisements and partially in construction because of the close trade relations with the United States, and the railways of Canada continue to measure their trackage in miles and speed limits in miles per hour because they also operate in the United States. Most other Commonwealth countries adopted the metric system during the 1970s.^[58]

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.^[59][60] The metric system is used by most businesses,^[61] 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.^[62]

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".^[63] These units consist of the recommendations of the General Conference on Weights and Measures,^[56] supplemented by some additional units of measure that may be used for specified purposes.^[64] 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^[65] and the Weights and Measures Act of 1864 and the Weights and Measures (Metric System) Act of 1896 legalised the metric system.^[66] 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.^[67]

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.^[68]

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.^[69]

In popular conversation, the stone unit is widely used for measuring a person's weight and feet and inches for height.

United States

In 1805 a Swiss geodesist Ferdinand Rudolph Hassler brought copies of the French metre and kilogram to the United States.^[70] In 1830 the Congress decided to create uniform standards for length and weight in the United States.^[71]Hassler was mandated to work out the new standards and proposed to adopt the metric system.^[71] The Congress opted for the British Parlementiary Standard from 1758 and the Troy Pound of Great Britain from 1824 as length and weight standards.^[71] Nevertheless the primary baseline of the US Coast Survey was measured in 1834 at Fire Island using four two-metre iron bars constructed after Hassler's specification in the United Kingdom and brought back in the United States in 1815.^[72][73] All distances measured in the US National Geodetic Survey were referred to the metre.^[74] In 1866 the United States Congress passed a bill making it lawful to use the metric system in the United States. The bill, which was permissive rather than mandatory in nature, defined the metric system in terms of customary units rather than with reference to the international prototype metre and kilogram.^{[75][76]:10–13} By 1893, the reference standards for customary units had become unreliable. Moreover, the United States, being a signatory of the Metre Convention was in possession of national prototype metres and kilograms that were calibrated against those in use elsewhere in the world. This led to the Mendenhall Order which redefined the customary units by referring to the national metric prototypes, but used the conversion factors of the 1866 act.^[76]:16–20 In 1896 a bill that would make the metric system mandatory in the United States was presented to Congress. Of the 29 people who gave evidence before the congressional committee who were considering the bill, 23 were in favor of the bill, but six were against. Four of the six dissenters represented manufacturing interests and the other two the United States Revenue service. The grounds cited were the cost and inconvenience of the change-over. The bill was not enacted. Subsequent bills suffered a similar fate.^[49]

In 1971 the US National Bureau of Standards completed a three-year study of the impact of increasing worldwide metric use on the US. 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 US, principally in the manufacturing and educational sectors. Public Law 93-380, enacted 21 August 1974, states that it is the policy of the US 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 US It established a US 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.^[77] In January 2007 NASA decided to use metric units for all future moon missions, in line with the practice of other space agencies.^[78]

Over time, the metric system has influenced the United States through international trade and standardisation. The use of the metric system was made legal as a system of measurement in 1866^[79] and the United States was a founding member of the International Bureau of Weights and Measures in 1875.^[80] 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.^[81] It has remained voluntary for federal and state road signage to use metric units, despite attempts in the 1990s to make it a requirement.^[82]

A 1992 amendment to the Fair Packaging and Labeling Act (FPLA), which took effect in 1994, required labels on federally regulated "consumer commodities"^[83] 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.^[84] 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.^[85]

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.^[86] At least two states, Kentucky and California, have even moved towards demetrication of highway construction projects.^[87][88][89]

Country metrication status

Countries by current metrication status:

Notes

Complexity of assessing metrication status

It is difficult to judge the degree to which ordinary people change to using metric in their daily lives.

In countries that have recently metricated, older segments of the population tend to still use the older system of measurement. Due to this 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.

Countries that are metric, are also required to adopt non-metric units due to trade with non-metric countries. This extends to manufacturing in Canada, where companies are able to use both imperial and metric units, since the major export market is the United States, but metric is required for both domestic use and for nearly all other exports.

Many Canadians still often use non-metric system of measurements in their daily lives for discussions of height and weight, though driver's licences and other official government documents record weight and height only in metric. In Canadian schools, however, the metric system 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 imperial units, and road signs use miles rather than kilometres.

The methods of metrication

There are four common methods that nations convert from traditional systems of measurement to the metric system. The first method is the direct changerover, or big bang adoption, which was used by India in the 1960s and several other nations including Australia and New Zealand. The second and third methods are phased adoption and parallel adoption of metric units over time and progressively outlaw traditional units or voluntarily fall out of favour, which was used by Unitied Kingdom, and was the original intention of metrication in the United States. These methods, however are slower and generally less complete and may result in duel systems of measurement. The fourth method is to redefine traditional units in metric terms. This has been used successfully where traditional units were ill-defined and had regional variations.

The direct changeover/big bang adoption method was 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 phased adoption method was to allow some industries, sectors, companies or goods and services to metricate, but letting others remain on the older system, and the parallel adoption method was to pass a law permitting the use of metric units in parallel with the traditional system of measurement, followed by education of metric units, then progressively ban the use of the non-metric system of measurement or have the older system fall out of favour. This has generally been a slow route to a country becoming metricated. The British Empire permitted the phased and paralleled adoption of metric measures in 1873, but the changeover was not completed in most Commonwealth countries until the 1970s and 1980s when governments adopted a direct change over method for metrication. Japan also adopted the phased/parallel methods and did complete its metrication process for 70 years. In the United Kingdom, metrication is still incomplete: road signage and the following products can be sold only in imperial measures: draught beer or cider by pint, milk in returnable containers by pint, and precious metals by troy ounce are areas yet to be phased in, and, products can display an imperial measurement parallel to the metric measurement.

A measuring cup, manufactured and sold in the United States (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.

The forth 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.^[42] 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").^[117] 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.

Further examples are, in Ireland and Australia, the imperial pint was redefined as 570ml in bars, pubs, and clubs (see Australian beer glasses).In English-speaking countries such as Australia, Canada, Ireland and New Zealand that have metricated, the product Quarter Pounder retains the name. In Latin America, the Spanish word galón may occasionally refer to a portable fuel container, often 5-20 L.

An interesting metrication method is known as implementing "soft" metric conversion, where rounded metric quantities are not adopted, rather a straight conversion expressing the traditional quantity in a metric unit. An example of this is butter in Canada, which is sold in a 454 gram package (and labelled as such), as it represents one pound. This is known as "soft metric" (as opposed to a "hard metric" system, where packages and measures are generally sold in "round numbers"; in a hard metric system, butter might come in a 450g or 500 g package).

Reasons for why metrication stalls

Metrication stalls according to Dr Vera for several reasons, notably:^[118]

1) Resistance to change, from consumers who suspect metrication will be accompanied by hidden price rises and by business people who see it as an imposition and an extra cost to their business.

2) Colonial powers (British and American) imposing their units of measurements on their colonies and overseas possessions.

3) British and American companies insisting on using their non-metric units for trade in goods, rather than the units used by the country they are trading with.

4) Poorer countries lacking resources to complete the change to metric units, or to enforce laws mandating their use.

Accidents and incidents

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.^[119]

An example of an accident of voluntary phased adoption across different sectors, that is having a country use of two different systems of measurement, was a contributing factor in the loss of the Mars Climate Orbiter in 1999. The National Aeronautics and Space Administration (NASA) specified metric units in the contract. NASA and other organisations 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.^[120]

Exceptions

As of 2018, the metric system officially predominates in most countries of the world; some non-metric systems of measurement, however are still used in many parts of the world in specific industries. For example:

• 
  Consumer electronics:
  
  
  
  • Television and monitor screen display size are still commonly cited in inches in many countries; however, in countries such as, France and South Africa, centimetres are often used for television sets, whereas CRT computer monitors and all LCD monitors are measured in inches.
  
  
  • The computer and printing industry Dots per inch and pixels per inch continue to be used in describing graphical resolution, however pixels per centimetre (ppcm) is increasingly used. The CSS3 media queries draft also introduces the unit dots per centimetre (dpcm) for resolution.^[121]
    
  
  
  • 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.
  
  
  • Non-metric printer paper size, US Letter, is still used in the United States, Canada, Chile, Mexico, Panama, the Dominican Republic and the Philippines.
  
  
  • 
    Font sizes for paper and screen are still predominantly given in non-metric typographic units, usually by the unit point. The size of the point has varied throughout the history of printing from 0.18 to 0.4 millimeters, and although there are still various definitions used today, the Desktop Publishing Point (DTP point) has become the de facto standard for printing. The DTP point is defined as ​¹⁄₁₂ of a pica, or equivalently ​¹⁄₇₂ of an international inch (about 0.353 mm). A proposed metric typographic unit is the quart (quarter millimetre, abbreviated ‘q’) defined as 250 μm (​¹⁄₄ mm). The quart is currently only specified by Japanese^[122] and German^[123] standardization bodies. Due to demand by Japanese typesetters, CSS adopted Q in 2015.^[124][125]
    
  
  
  • 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.^[126]
    
  
  
  
  


• Measurement of land and real estate area:
  
  
  
  • In Hong Kong, traditional Chinese and British imperial units are normally used instead of metric units in particular types of trade.
  
  
  • 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.
  
  
  • The imperial acre is still used in about a dozen countries for land measurement.
  
  
  • 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.
  
  
  
  


• Retention due to historic construction:
  
  
  
  • In the electronic industry, the dominant spacing for components is based on intervals of ¹⁄₁₀ in (2.54 mm), and a change would lead to compatibility issues, e.g., for connectors.
  
  
  • 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.
  
  
  • 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.
  
  
  
  


• Measurement of temperature:
  
  
  
  • 
    Fahrenheit is used as the official temperature scale only in the United States(including its unincorporated territories), its freely associated states in the Western Pacific (Palau, the Federated States of Micronesia and the Marshall Islands), the Bahamas and the British overseas territory the Cayman Islands.
  
  
  • In the United Kingdom and some Commonwealth countries, temperatures of domestic gas ovens are often specified using gas marks.^[127][128] Similarly, older French ovens and recipes book often use a scale based on the Fahrenheit scale: "Thermostat" (abbreviated "Th"), where Thermostat 1 equals 100 °F for conventional ovens, increasing by 50 °F for each whole number along the scale.^[129]
    
  
  
  
  


• 
  Thread count in the textile industry is frequently measured in "threads per inch" or "ends per inch".


• 
  Clothing size and Shoe size has not been standardised in many areas for the metric system, and use units such as "M" and "XL" to mean medium and extra large, or barleycorn for shoe sizes.


• The unit calorie is still used for food energy in some metric countries.


• In the sport of surfing, surfboards are usually designed, constructed, and sold in feet and inches.


• Measurement of fluid capacity:
  
  
  
  • 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.
  
  
  • 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.
  
  
  • World oil price is quoted in US dollars per Barrels.
  
  
  
  


• Measurement in vehicles and transportation:
  
  
  
  • The knot is still commonly used worldwide in meteorology, and in maritime and air navigation.
  
  
  • The length of small sail boats is often given in feet in popular conversation.
  
  
  • 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.
  
  
  • Automotive and bicycle wheel diameters are still usually but not always set as whole inch measurements (although tyre widths are measured in millimetres).
  
  
  • 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.^[130]
    
  
  
  • 
    Road distances and speed limits are still displayed in miles and miles per hour in the USA, UK, The Bahamas, Belize, Samoa, Saint. Lucia, St. Vincent & the Grenadines and St. Kitts & Nevis.
  
  
  
  

Air and sea transportation

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.^[131] Metrication is also gradually taking place in cargo weights and dimensions and in fuel volumes and weights.

In some countries like China, ex Soviet countries and Russia, the metric system is used in aviation (whereby in Russia altitudes above the transition level are given in feet)^[132][133]. Sailplanes use the metric system in many European countries.

See also

References

External links

• Online Metric Conversion Application