Metrication

National Convention decree of April 1795 establishing the metric system

The introduction of the metric system into France in 1795 was done on a district by district basis with Paris being the first district. 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.[21]^:269 This, combined with the excesses of the Revolution and the high level of illiteracy in 18th century France, 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. Under the décret impérial du 12 février 1812 (imperial decree of 12 February 1812), a new system of measure – the mesures usuelles ("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.[22] That system reintroduced the names of many units used during the ancient regime, but their values were redefined in terms of metric units. Thus the toise was defined as being two metres, with six pieds making up one toise, twelve pouces 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.[23] This intermediate step eased the transition to a metric-based system.

By the La loi du 4 juillet 1837 (the law of 4 July 1837), Louis Philippe I effectively revoked the use of mesures uselles by reaffirming the laws of measurement of 1795 and 1799 to be used from 1 May 1840.[24][25] However, many units of measure, such as the livre (for half a kilogram), remained in everyday use for many years,[25][26] and to a residual extent up to this day.

Conversion table in an 1848 German schoolbook showing the metric equivalent of the foot in use in over two dozen countries, including several German states

Stone marking the Austro-Hungarian/Italian border at Pontebba displaying myriametres (10 km), a unit used in Central Europe in the 19th century[27]

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 the systems were loosely derived from the Carolingian system instituted by Charlemagne a thousand years earlier.

During the Napoleonic era, some of the German states moved 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.[27][28] 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[29] while the Prussian Pfund remained at 467.711 g.[30]

After the Congress of Vienna there was a degree of commercial cooperation between the various German states resulting in 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'.[31] This unit was used for inter-state movement of goods, but was not applied in all states for internal use.

In 1832, Carl Friedrich Gauss studied the Earth's magnetic field and proposed adding the second to the basic units of the metre and the kilogram in the form of the CGS system (centimetre, gram, second). In 1836, he founded the Magnetischer Verein, the first international scientific association, in collaboration with Alexander von Humboldt and Wilhelm Edouard Weber. Geophysics or the study of the Earth by the means of physics preceded physics and contributed to the development of its methods. It was primarily a natural philosophy whose object was the study of natural phenomena such as the Earth's magnetic field, lightning and gravity. The coordination of the observation of geophysical phenomena in different points of the globe was of paramount importance and was at the origin of the creation of the first international scientific associations. The foundation of the Magnetischer Verein would be followed by that of the Central European Arc Measurement (German: Mitteleuropaïsche Gradmessung) on the initiative of Johann Jacob Baeyer in 1863, and by that of the International Meteorological Organisation whose second president, the Swiss meteorologist and physicist, Heinrich von Wild represented Russia at the International Committee for Weights and Measures (CIPM).[32][33][34] In 1867, the European Arc Measurement (German: Europäische Gradmessung) called for the creation of a new, international prototype metre (IPM) and the arrangement of a system where national standards could be compared with it. The French government gave practical support to the creation of an International Metre Commission, which met in Paris in 1870 and again in 1872 with the participation of about thirty countries. The Metre Convention was signed on 20 May 1875 in Paris and the International Bureau of Weights and Measures was created under the supervision of the CIPM.

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[21]^:350 and of Austria in 1875.[35] The Zollpfund ceased to be legal in Germany after 1877.[36]

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.[37] In 1802 the Cisalpine Republic was renamed the Italian Republic, with Napoleon as its head of state. The following year the Cisalpine system of measure was replaced by the metric system.[37]

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.[37]

The Netherlands (as the revolutionary Batavian Republic) began to use the metric system from 1799 but, as with its co-revolutionaries in France, encountered numerous practical difficulties. Subsequently, as part of the First French Empire since 1809, the Netherlands used Napoloeon's mesures usuelles from their introduction in 1812 until the fall of his Empire in 1815. Under the (Dutch) Weights and Measures Act of 21 August 1816 and 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 simply given the names of units of measure that were then in use. Examples include the ons (ounce) which was defined as being 100 g.[38]

In 1875, Norway was the first country to ratify the metre convention, and it was seen as an important step for Norway to gain independence. The decision to adopt the metric system is said to have been the Norwegian Parliament's fastest decision in peace time.

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 litre and the libra (pound) = 1 kilogram.[39]

Until the ascent of the Bourbon monarchy in Spain in 1700, each region of Spain had 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.[40] 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.[40]

Don Carlos Ibáñez e Ibáñez de Ibero, Marquis of Mulhacén

The Spanish Royal Academy of Science urged the Government 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. 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 by definition 3.8980732 metres at a specified temperature.[41][42]

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 1866, Spain and Portugal joined the Central European Arc Measurement which would become the European Arc Measurement the next year. In 1867 at the second general conference of the geodetic association 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 proposed according to recommendations drawn up by a committee chaired by Otto Wilhelm von Struve director of the Pulkovo Observatory in St. Petersburg the adoption of the metre and the creation of an international metre commission, after a preliminary discussion held in Neuchâtel between Johann Jacob Baeyer director of the Royal Prussian Geodetic Institute, Adolphe Hirsch member of the Swiss Geodetic Commission and Carlos Ibáñez e Ibáñez de Ibero Spanish representative, founder and first director of the Instituto Geográfico Nacional.[43][44][45][46][47]

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.[48] 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.

In 1801, the Helvetic Repulic at the instigation of Johann Georg Tralles promulgated a law introducing the metric system which was never applied, because in 1803 the competence for weights and measures returned to the cantons. On the territory of the current canton of Jura, then annexed to France (Mont-Terrible), the metre was adopted in 1800. Canton of Geneva adopted the metric system in 1813, canton of Vaud in 1822, canton of Valais in 1824 and canton of Neuchâtel in 1857. Twelve cantons of the Swiss Plateau and the north-east adopted in 1835 a concordat based on the mesure usuelles which entered in force in 1836. The cantons of central and eastern Switzerland, as well as the Alpine cantons continued to use the old measures.[49]

According to the 1848 Constitution the mesures usuelles were to come into force thoughout the country. In 1868 the metric system was legalized alongside the mesures usuelles. Cantonal calibrators were supervised by a Federal Bureau of Verification created in 1862, whose management was entrusted to Heinrich von Wild from 1864. In 1875, the competence for weights and measures was assigned to the Confederation and Switzerland represented by Adolphe Hirsch joined the Metre Convention. The same year a federal law imposed the metric system from January 1, 1877. In 1977, Switzerland joined the International System of Units.[49][33][50]

In 1824 the Weights and Measures Act imposed one standard 'imperial' system of weights and measures on the British Empire.[51] 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'.[52] 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[53]^: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.[54]

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.[52] 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.[52]

In 1965 the UK began an official programme of metrication that, as of 2021, has not been completed and has effectively been halted.

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.[21][55] The Imperial pint also remains a permitted unit for milk in returnable bottles and for draught beer and cider in British pubs.[55] 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.

Most of the general public still measure their height and weight in imperial units, and imperial units are the norm for daily conversation, particularly when describing longer distances. While metric units are the legal trading units for most purposes, many Brits have learned to mentally convert from metric to the more familiar imperial units. One example of this mental conversion is 100 grams approximately equals 3.5 ounces. Many products, mostly those that are imported, come in multiples of 100 grams, so many Brits multiply the amount of grams, rounded to the nearest hundred, by 3.5 and get the approximate conversion in ounces. Similar methods apply with converting between other metric and imperial units, but generally everybody has their own methods.

Ferdinand Rudolph Hassler, first superintendent of the United States Coast Survey

In 1805 a Swiss geodesist Ferdinand Rudolph Hassler brought copies of the French metre and kilogram to the United States.[56][57] In 1830 the Congress decided to create uniform standards for length and weight in the United States.[58] Hassler was mandated to work out the new standards and proposed to adopt the metric system.[58] The Congress opted for the British Parliamentary Standard from 1758 and the Troy Pound of Great Britain from 1824 as length and weight standards.[58] 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.[59][60][57] All distances measured in the US National Geodetic Survey were referred to the metre.[41][61] 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.[62][63]^:10–13 Ferdinand Rudolph Hassler's use of the metre in coastal survey, which had been an argument for the introduction of the Metric Act of 1866 allowing the use of the metre in the United States, probably also played a role in the choice of the metre as international scientific unit of length and the proposal by the European Arc Measurement (German: Europäische Gradmessung) to “establish a European international bureau for weights and measures”.[61][41][64][65]

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.[63]^:16–20 In 1896 a bill that would make the metric system mandatory in the United States was presented to Congress. Twenty-three of the 29 people who gave evidence before the congressional committee who were considering the bill were in favor of it, but six were against. Four of these six dissenters represented manufacturing interests and the other two were from 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.[35]

The United States mandated the acceptance of the metric system in 1866 for commercial and legal proceedings, without displacing their customary units.[66] The non-mandatory nature of the adoption of the SI has resulted in a much slower pace of adoption in the US than in other countries.[67]

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.[68]

In January 2007 NASA decided to use metric units for all future moon missions, in line with the practice of other space agencies.[69]

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 Canada is not complete. 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. The railways of Canada such as the Canadian National and Canadian Pacific as well as commuter rail services continue to measure their trackage in miles and speed limits in miles per hour because they also operate in the United States (although urban railways including subways and light rail have adopted kilometres and kilometres per hour).[70] Canadian railcars show weight figures in both imperial and metric. Most other Commonwealth countries adopted the metric system during the 1970s.[71]

Apart from the United Kingdom which has effectively halted its metrication program, all other countries using the imperial system have 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.[72] Hong Kong uses three systems (traditional, Imperial and metric) and all three are permitted for use in trade.[73]

Countries using the metric, imperial and US customary systems as of 2019 (based on popular and official usage).

Countries by current metrication status (based on official usage only) (2016):

  Complete

  Almost complete

  Some adoption to partially complete

  Little adoption

Links in the country/region point to articles about metrication in that country/region.[76]

Notes

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. 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 was the first Commonwealth country to use this method of conversion. It'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. Two industrialized Commonwealth countries, Australia and New Zealand also did a quick conversion to metric.

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 other than India and Australia until the 1970s and 1980s when governments took an active role in metric conversion. In the United Kingdom and Canada, the process is still incomplete. Japan also followed this route and did not complete the changeover for 70 years. 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,[13] 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.[39] 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").[111] 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 US, but metric is required for both domestic use and for nearly all other exports. This may be due to the overwhelming influence of the neighbouring United States; similarly, many Canadians still often use non-metric measurements in day-to-day discussions of height and weight, 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). 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. Similarly, one of the standard sizes of some liquor bottles is 1.14 litres, which translates to 40 ounces (one Imperial/Canadian quart). This is considered a "soft" metric conversion.

In Ireland, metric is the official unit of measurement though most people would not understand metric units for measuring the body, distances or area. Conversely British imperial units would generally not be understood for temperature. For non-body weights, metric is gradually replacing imperial in everyday use, possibly due to weights on packaging being in metric.

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.[121][122] The metric system is used by most businesses,[123] 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.[124]

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".[125] These units consist of the recommendations of the General Conference on Weights and Measures,[53] supplemented by some additional units of measure that may be used for specified purposes.[126] 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[127] and the Weights and Measures Act of 1864 and the Weights and Measures (Metric System) Act of 1896 legalised the metric system.[128] 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.[129]

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.[130]

Following the United Kingdom's vote to withdraw from the European Union, retailers have begun to request to go back to using imperial units, with some reverting without permission. A poll following the 2016 vote also found that 45% of Britons sought to revert to selling produce in imperial units.[131]

Imperial units remain in common everyday use for human body measurements, in particular stones and pounds for weight, and feet and inches for height.

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.

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[132] and the United States was a founding member of the International Bureau of Weights and Measures in 1875.[133] 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.[134] Attempts in the 1990s to make it mandatory for federal and state road signage to use metric units failed and it remains voluntary.[135]

A 1992 amendment to the Fair Packaging and Labeling Act (FPLA), which took effect in 1994, required labels on federally regulated "consumer commodities"[136] 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.[137]

After many years of informal or optional metrication, the American public and much of the private business and industry still use US customary units today.[138] At least two states, Kentucky and California, have even moved towards demetrication of highway construction projects.[139][140][141]

Canada 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.[142]

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, and now indicates temperature in Celsius.[143] 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).[144][145] Sailplanes use the metric system in many European countries.

In 1975, the assembly of the International Maritime Organization (IMO) decided that future conventions of the International Convention for the Safety of Life at Sea (SOLAS) and other future IMO instruments should use SI units only.[146]