Metrication or metrification is the act or process of converting to the metric system of measurement.^[1] All over the world, countries have transitioned from local and traditional units of measurement to the metric system. This process began in France during the 1790s, and has persistently advanced over two centuries, accumulating into 95% of the world officially only using the modern metric system.^[2] Nonetheless, this also highlights that certain countries and sectors are either still transitioning or have chosen not to fully adopt the metric system.

The process of metrication is typically initiated and overseen by a country's government, generally motivated by the necessity of establishing a uniform measurement system for effective international cooperation in fields like trade and science. Governments achieve metrication through either mandatory changes to existing units within a specified timeframe or through voluntary adoption.

While metric use is mandatory in some countries and voluntary in others, all countries have recognised and adopted the SI, albeit to different degrees, including the United States. As of 2011, ninety-five percent of the world's population live in countries where the metric system is the only legal system of measurement.^{[3]: p. 49, ch 2}

According to the National Institute of Standards and Technology (NIST), there are only three countries that do not have mandatory metric laws (Liberia, Myanmar, and the United States),^[4][5] however a research paper completed by Vera (2011) stated in practice there were four additional countries (the United States COFA countries: Federated States of Micronesia, Marshall Islands and Palau, and Samoa. Samoa has since mandated metric trade^[6]).^{[3]: 60, 494–496}

In 2018, the Liberia government had pledged to adopt the metric system, but still mainly uses the US customary units.^[7] In 2013, the Myanmar Ministry of Commerce announced that Myanmar was preparing to adopt the metric system as the country's official system of measurement, and metrication in Myanmar began with some progress was made (road signs and temperature are legislated to be in metric), however there had been very little progress in local trade.^[8][9] As of 2023, the United States has a national policy of adopting the metric system, and all United States Government agencies are required to adopt it.^[10]

The metrication process can take years to implement and complete. For instance Guyana, adopted the metric system in 2002 and was only able to make it mandatory in local trade 2017 after the metric system was fully adopted in schools.^[11] Antigua and Barbuda, also officially metric, is moving slowly in its metrication process, with a new push in 2011 for all government agencies to convert by 2013 and the entire country to use the metric system by the first quarter of 2015.^[12] Other metric Caribbean countries, such as Saint Lucia officially metric 2000, are still in the process toward full conversion.^[13] The United Kingdom has officially embraced a dual measurement system. The United Kingdom as of 2007 halted its metrication process, and retain imperial units of the mile and yard in road markings, pints for returnable milk containers, and (with Ireland) for the pint for draught beer and cider sold in pubs.^[14] Throughout the 1990s, the European Commission helped accelerate the metrication process for member states, for the implemented the Units of Measure Directive to promote trade. This acceleration caused public backlash in the United Kingdom, and in 2007 the United Kingdom announced that it had secured permanent exemptions listed above and, to appease British public opinion and to facilitate trade with the United States, the option to include imperials units alongside metric units could continue indefinitely.^[14][15]

The United Kingdom and the United States face ongoing resistance toward metrication, rooted in a belief that their cultural identity is intertwined with the traditional measurement systems they historically have used.^[16] This has resulted in a review of mandatory sales and trade of metric units by the UK government^[17] and active opposition to metrication by some commentators in the US.^[18]

The metre was adopted as exclusive measure in 1801 under the French Consulate, then the First French Empire until 1812, when Napoleon decreed the introduction of the mesures usuelles which remained in use in France up to 1840 in the reign of Louis Philippe.^[19] Meanwhile, the metre was adopted by the Republic of Geneva.^[20] After the joining of canton of Geneva to Switzerland in 1815, Guillaume Henri Dufour published the first Swiss official map for which the metre was adopted as unit of length.^[21][22] A Swiss-French binational officer, Louis Napoléon Bonaparte was present when a baseline was measured near Zürich for Dufour map which would win the gold medal for the national map at the Exposition Universelle of 1855.^[23][24][25] Among the scientific instruments calibrated on the metre, which were displayed at the Exposition Universelle, was Brunner apparatus, a geodetic instrument devised for measuring the central baseline of Spain whose designer, Carlos Ibáñez e Ibáñez de Ibero would represent Spain at the International Statistical Institute. In addition to the Exposition Universelle and the second Statistical Congress held in Paris, an International Association for obtaining a uniform decimal system of measures, weights, and coins was created there in 1855.^{[26][27][28][29]} Copies of the Spanish standard would be made for Egypt, France and Germany.^[30][31] These standards were compared to each other and with Borda apparatus which was the main reference for measuring all geodetic baselines in France.^[32][33][34] These comparisons were essential, because of the expansibility of solid materials with raise in temperature. Indeed, one fact had constantly dominated all the fluctuations of ideas on the measurement of geodesic bases: it was the constant concern to accurately assess the temperature of standards in the field; and the determination of this variable, on which depended the length of the instrument of measurement, had always been considered by geodesists as so difficult and so important that one could almost say that the history of measuring instruments is almost identical with that of the precautions taken to avoid temperature errors.^[35] In 1867, the second general Conference of the European Arc Measurement recommended the adoption of the metre in replacement of the toise. In 1869, the Saint Petersburg Academy of Sciences sent to the French Academy of Sciences a report drafted by Otto Wilhelm von Struve, Heinrich von Wild and Moritz von Jacobi inviting his French counterpart to undertake joint action with a view to ensuring the universal use of the metric system in all scientific work.^[36] The same year, Napoleon III convened the International Metre Commission which was to meet in Paris in 1870. The Franco-Prussian War broke out, the Second French Empire collapsed, but the metre survived.^[37][38]

During the nineteenth century the metric system of weights and measures proved a convenient political compromise during the unification processes in the Netherlands, Germany and Italy. In 1814, Portugal became the second country not part of the French Empire to officially adopt the metric system. Spain found it expedient in 1849 to follow the French example and within a decade Latin America had also adopted the metric system, or had already adopted the system, such as the case of Chile by 1848. There was considerable resistance to metrication in the United Kingdom and in the United States. Despite this, they were actually the first countries in the World to use a metric standard for cartography.^{[39][40][41][34][42]}

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.^[43]: 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.^[44] 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.^[45] This intermediate step eased the transition to a metric-based system.

By the Loi du 4 juillet 1837 (the law of 4 July 1837), Louis Philippe I effectively revoked the use of mesures usuelles by reaffirming the laws of measurement of 1795 and 1799 to be used from 1 May 1840.^[46][47] However, many units of measure, such as the livre (for half a kilogram), remained in everyday use for many years,^[47][48] and to a residual extent up to this day.

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) (for simplicity at the expense of grammar, these are the singular forms of each name) while the Pfund was defined as 500 g, divided into 30 Loth, each of 16.67 g.^[49][50] 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^[51] while the Prussian Pfund remained at 467.711 g.^[52]

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 introduced the Zollpfund (Customs Pound) which was defined as exactly 500 g and could be split into 30 'lot'.^[53] 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 (the study of the Earth by the means of physics) preceded physics^{[citation needed]} 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).^[54][55][56] 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^[43]: 350 and of Austria in 1875.^[57] The Zollpfund ceased to be legal in Germany after 1877.^[58]

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 on the braccio cisalpino (Cisalpine cubit) which was defined to be half a metre.^[59] 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.^[59]

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 that the metric system was in use throughout this region.

After the Congress of Vienna, the various Italian states reverted to their original systems 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 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.^[59]

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 Napoleon'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: for instance the ons (ounce) was defined as 100 g.^[60]

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

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.^[61]

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.^[62] 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.^[62]

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.^[39][63]

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 founder of the Neuchâtel Observatory and Carlos Ibáñez e Ibáñez de Ibero Spanish representative, founder and first director of the Instituto Geográfico Nacional.^{[64][65][66][67][68]}

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.^[69] 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. These prototypes defined the metre right up until 1960.

In 1801, the Helvetic Republic at the instigation of Johann Georg Tralles promulgated a law introducing the metric system. However this 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. The Canton of Geneva adopted the metric system in 1813, the canton of Vaud in 1822, the canton of Valais in 1824 and the canton of Neuchâtel in 1857. In 1835, twelve cantons of the Swiss Plateau and the north-east adopted a concordat based on the federal foot (exactly 0.3 m) which entered into force in 1836. The cantons of central and eastern Switzerland, as well as the Alpine cantons, continued to use the old measures.^[20][70]

Guillaume-Henri Dufour founded in 1838 in Geneva a topographic office (the future Federal Office of topography), which published under his direction, from 1845 to 1864, the first official map of Switzerland, on the basis of new cantonal measurements. This map at 1:100,000 engraved on copper, suggested the relief by hatching and shadows. The map projection adopted by the commission was the Bonne projection, centred on the Bern Observatory (5° 6' 10.8'' east of Paris meridian), although this point was much closer to the western end of Switzerland than to its eastern end. But its position was well known, and there was no more central observatory. The scale was set at 1:100 000 because it was considered more suitable for a country as rugged as Switzerland than the 1:80 000 adopted for the large map of France and the two maps were in any case inconsistent, as the meridians of the map of Switzerland tilted in the opposite direction to those of the map of France. The map commission wanted to adopt decimal measures; and Switzerland did not have an already existing map which, like the Cassini map, used a scale close to 1:86 400, i.e. 1 line (1⁄12 of a French inch) to 100 toises (i.e. 600 French feet). The metre was adopted as a linear measure, and the entire map was divided into twenty-five sheets: five east–west and five north–south. Each sheet must have^{[clarification needed]} shown two scales, one purely metric, the other in Swiss leagues 4800 metres in length. The frame would be divided into sexagesimal minutes and centesimal minutes; the latter, each subdivided into ten parts, had the advantage of showing Kilometres in the direction of the meridians; so that there are new scales on the sides of the sheet to appreciate^{[clarification needed]} the distances.^[21][22][71]

According to the 1848 Constitution the federal foot was to come into force throughout the country. In Geneva, a committee chaired by Guillaume Henri Dufour militated in favor of maintaining the decimal metric system in the French-speaking cantons and against the standardization of weights and measures in Switzerland on the basis of the metric foot. In 1868 the metric system was legalized alongside the federal foot, which was a first step towards its definitive introduction. 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 responsibility for weights and measures was transferred back from the cantons to the Confederation, and Switzerland (represented by Adolphe Hirsch) joined the Metre Convention. The same year a federal law imposed the metric system from 1 January 1877. In 1977, Switzerland joined the International System of Units.^{[20][55][72][73][74]}

In 1824, the Weights and Measures Act imposed one standard 'imperial' system of weights and measures on the British Empire.^[75] 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'.^[76] 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^[77]: 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.^[78]

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.^[76] A bill to make the metric system compulsory to help the British industrial base 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 the House of Commons in 1907 by 150 votes to 118.^[76]

In 1965, the UK began an official programme of metrication, and as of 2024, in the United Kingdom the metric is the official measurement system for all regulated trading by weight or measure purposes, however imperial pint remains the sole legal unit for milk in returnable bottles and for draught beer and cider in British pubs. Imperial units are also legally permitted to be used alongside metric units on food packaging and price indications for goods sold loose.^[79] The UK government undertook a "Choice on units of measurement: consultation response", and found just over 1% of respondents wish to revert to an increase the use of imperial units, and a such kept the current regulations on the sale of goods.^[80]

In addition imperial units 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 given in inches only, but a piece of material priced per inch would be unlawful unless the metric price was also shown.

The general public still use imperial units in common langange for their height and weight, and imperial units are the norm when discussing longer distances such as journeys by car, but otherwise metric measurements are often used.^{[citation needed]}

In 1805 a Swiss geodesist Ferdinand Rudolph Hassler brought copies of the French metre and kilogram to the United States.^[81][82] In 1830 the Congress decided to create uniform standards for length and weight in the United States.^[83] Hassler was mandated to work out the new standards and proposed to adopt the metric system.^[83] The Congress opted for the British Parliamentary Standard from 1758 and the Troy Pound of Great Britain from 1824 as length and weight standards.^[83] Nevertheless, the primary baseline of the Survey of the Coast (renamed the United States Coast Survey in 1836 and the United States Coast and Geodetic Survey in 1878) was measured in 1834 at Fire Island using four 2-metre (6 ft 7 in) iron bars constructed after Hassler's specification in the United Kingdom and brought back in the United States in 1815.^[84][85][82] All distances measured by the Survey of the Coast, Coast Survey, and Coast and Geodetic Survey were referred to the metre.^[39][40] 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.^{[86][87]: 10–13} Ferdinand Rudolph Hassler's use of the metre in coastal surveying, 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”.^{[40][39][88][89]}

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.^{[87]: 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.^[57]

The United States mandated the acceptance of the metric system in 1866 for commercial and legal proceedings, without displacing their customary units.^[90] 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.^[91]

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.^[92]

In January 2007 NASA decided to use metric units for all future Moon missions, in line with the practice of other space agencies.^[93]

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 in Canada metrication has been halted since the 1970s. In that country, 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. Metric measurements on food products such as canned food are often merely the equivalent of the still widely used imperial units of measurement such as the ounce and the pound. Butter in Canada is sold in 454 g packagings, which is the equivalent of one pound. 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).^[94] Canadian railcars show weight figures in both imperial and metric. Most other Commonwealth countries adopted the metric system during the 1970s.^[95]

Apart from the United Kingdom and Canada, which have effectively halted their metrication programs, the great majority of 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.^[96] Hong Kong uses three systems (Chinese, imperial, and metric) and all three are permitted for use in trade.^[97]

Links in the country/region point to articles about metrication in that country/region.^[98]

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. Its 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,^[15] 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.^[61] 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").^[138]

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.

Surveys are performed by various interest groups or the government to determine 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 still to use the older units.^[139]

As of 2024, the metric system predominates in most of the world, however specific industries are more resistant to metrication. For example:

• Photograph and video cameras are standardised to mount to tripods using 1⁄4-20 and 3⁄8-16 screws, which are dimensioned in inches, by ISO 1222:2010.^[140]
• Thread count is frequently measured in threads per inch.^[141]
• Abrasives, such as sandpaper and sharpening stones, are commonly graded by grit, corresponding to the level of abrasiviness (mesh size) per square inch.
• For food energy the calorie is still commonly used.

• Dots per inch and pixels per inch are still used to describe graphical resolution with computers and printing.
• Data density for magnetic tape data storage is measured in bits per inch for density and bits per square inch for area density.^[142]
• Digital display sizes are usually advertised diagonally, using inches, along with centimetres supplementarily.
• Photographic paper are still sold with imperial print sizes across the world in metric countries.^[143]
• The dominant pin spacing for electronic components is based on 1⁄10 in (2.54 mm).

• Surfboards are usually designed, constructed, and sold in feet and inches.^[144]
• Shoes are still sold by the barleycorn in most English speaking countries, or the Paris Point in Europe.
• The fill power of down feather insulation is commonly expressed in "cubic inches per ounce".
• Precious metals are often sold in troy ounces, even in mostly metric states.^[145]

• Golf courses are measured in yards in the United States, Canada, Japan, South Korea, China, Thailand, the Philippines, Venezuela, Colombia, the Dominican Republic, Indonesia, Vietnam, and Cambodia. In the United Kingdom and Ireland, some golf courses are measured in yards while others are measured in metres.
• Office space and housing space is often sold and rented in square feet in Hong Kong, Singapore, Malaysia, Canada and India, square yards in India, Pakistan and Bangladesh, and tsubo/pyeong/"ping" in Japan, Korea, and Taiwan.

• Road signs for distance and speed limit are still displayed in yards, miles and miles per hour in the United States, United Kingdom, and multiple Caribbean states.
• Tyre pressure is commonly measured in pounds per square inch in multiple countries, including Argentina, Australia, Brazil, Canada, Chile, India, Ireland, Mexico, Peru, the United Kingdom, and the United States.
• Car and bicycle rim diameters are still usually set as whole inch measurements, and tyre widths are measured in millimetres.
• Engine power is usually measured in horsepower in most former USSR countries and German-speaking countries (this is metric horsepower rather than mechanical horsepower), although in the EU from 2010 the horsepower is permitted only as a supplementary unit.^[146]
• Imperial gallons are still used for the sale of fuel in four British Overseas Territories (Anguilla, the British Virgin Islands, the Cayman Islands, and Montserrat) and six states: Antigua and Barbuda, Dominica, Grenada, Saint Christopher and Nevis, Saint Lucia, and Saint Vincent and the Grenadines.
• United States gallons are used in the United States and its territories (except for Puerto Rico, which has used litres since 1980), the Bahamas, Belize, Colombia, the Dominican Republic, Ecuador, Guatemala, Haiti, Honduras, Liberia, Peru, and the Turks and Caicos Islands.^{[citation needed]}

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 1 852 metres (about 1.151 miles). It is not an SI unit. 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 since indicates temperature in Celsius.^[147] Metrication is also gradually taking place in cargo mass and dimensions and in fuel volume and mass.

In former Soviet countries and China, the metric system is used in aviation (whereby in Russia altitudes above the transition level are given in feet).^[148][149] 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.^[150]

• In Australia, pints are still used for beer (redefined to 570 ml (see Australian beer glasses)).
• McDonald's sells Quarter Pounders under their original name in all English-speaking countries despite metrication. In the Netherlands and Finland, it is known as a Quarter Pounder Cheese while Sweden also refers to it as QP Cheese. In Quebec, it is called a Quart de livre. In Latin America and Spain, it is called Cuarto de Libra con Queso. In Taiwan, it is known as the four-ounce beef burger.
• Inch-based spare parts are occasionally kept to service American and pre-1950s machines, but screws are changed to metric thread at maintenance.
• 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, some of the population continues to resist metrication to varying degrees. The traditional imperial measures are preferred by a majority and continue to have widespread use in some applications.^[151][152] The metric system is used by most businesses,^[153] 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.^[154]

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".^[155] These units consist of the recommendations of the General Conference on Weights and Measures,^[77] supplemented by some additional units of measure that may be used for specified purposes.^[156] 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^[157] and the Weights and Measures Act of 1864 and the Weights and Measures (Metric System) Act of 1896 legalised the metric system.^[158]

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 occurred 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.^[159]

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.^[160]

Immediately following the United Kingdom's vote to withdraw from the European Union, it was reported that some retailers requested to revert to 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.^[161]

The UK government started a consultation on 3 June 2022 on the choice of units of measurement markings.^[162]

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

Fuel economy is often advertised in miles per imperial gallon, which may lead to some confusion for users of US gallons for American manufactured cars.^[163]

Heating, air conditioning, and gas cooking appliances occasionally display power in British thermal units per hour (BTU/h).^[164]

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^[165] and the United States was a founding member of the International Bureau of Weights and Measures in 1875.^[166] 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.^[167] Attempts in the 1990s to make it mandatory for federal and state road signage to use metric units failed and it remains voluntary.^[168]

A 1992 amendment to the Fair Packaging and Labeling Act (FPLA), which took effect in 1994, required labels on federally regulated "consumer commodities"^[169] 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.^[170]

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.^[171] At least two states, Kentucky and California, have even moved towards demetrication of highway construction projects.^{[172][173][174]}

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.^[175]

Belize, which is a former British colony, uses both the metric and British imperial systems.^[176][177] Miles are the most commonly used unit for measuring distance,^[178] and gasoline is sold in US gallons (similar to neighboring countries in Central America).

Confusion over units during the process of metrication can sometimes lead to accidents. 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 (10,100 kg) of fuel instead of the required 22,300 kilograms (49,200 lb).^[179]

While not strictly an example of national metrication, the use of two different measurement systems 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.^[180]

• History of the metric system
• Metre Convention
• Metrication opposition
• Conversion of units
• Metric engine (American expression)
• Language reform
• Preferred numbers
• Spread of the Latin script
• Decimalisation