Metrication

History

Metrication began in France and spread outwards over the following century. The pattern set in France — abrupt introduction, partial retreat in the face of public resistance, then completion a generation later — recurs in most of the national programmes that followed. The metre was made the exclusive measure in 1801 under the French Consulate, but in 1812 Napoleon reintroduced familiar unit names, redefined in metric terms, as the mesures usuelles. These remained in use until Louis Philippe restored the metric system in full from 1840.

Two forces carried the system beyond France. The first was political consolidation. A single set of units was a convenient compromise wherever separate states were being merged, as in the Netherlands, the German states and Italy. Portugal adopted the system in 1814, the first country outside the French Empire to do so; Spain followed in 1849, and most of Latin America within the following decade, Chile having already converted by 1848. Where adoption was not driven by unification it was often driven by trade, and resistance was strongest in the United Kingdom and the United States.

The second force was science, and cartography in particular: national surveys could only be joined and compared if they shared a length standard. Britain and the United States, for all their resistance in trade, were among the first to adopt a metric standard for cartography. The sections below trace how individual countries made the change, and how the scientific need for common standards led to the Metre Convention of 1875.

France (1795–1840)

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. 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. 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. 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. However, many units of measure, such as the livre (for half a kilogram), remained in everyday use for many years, and to a residual extent up to this day.

Continental Europe in the 19th century

The countries below are listed in the order in which they first adopted metric units.

Italy (1797–1870)

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. 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.

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.

Netherlands (1799–1820)

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), which came into force in 1820 and 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.

Switzerland (1801–1877)

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.

Cartography pushed in the opposite direction to the cantonal foot. Guillaume-Henri Dufour founded a topographic office in Geneva in 1838 — the future Federal Office of Topography — which published the first official map of Switzerland between 1845 and 1864. The map commission chose decimal measures and adopted the metre as the linear unit, at a scale of 1:100,000; each sheet carried both a metric scale and one in Swiss leagues of 4,800 metres, and the sheet frames were graduated in centesimal minutes so that kilometres could be read directly along the meridians.

Under the 1848 Constitution the federal foot was to come into force throughout the country, but a committee chaired by Dufour argued for keeping the decimal metric system in the French-speaking cantons. In 1868 the metric system was legalised alongside the federal foot, a first step towards its definitive introduction; cantonal calibrators were supervised by a Federal Bureau of Verification created in 1862 and managed from 1864 by Heinrich von Wild. In 1875 responsibility for weights and measures passed from the cantons back to the Confederation, Switzerland (represented by Adolphe Hirsch) joined the Metre Convention, and a federal law imposed the metric system from 1 January 1877. Switzerland adopted the International System of Units in 1977.

Germany (1810–1877)

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. 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 while the Prussian Pfund remained at 467.711 g.

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'. 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 and of Austria in 1875. The Zollpfund ceased to be legal in Germany after 1877.

Portugal (1814)

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.

Spain (1849)

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. 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. Spain's national survey then made it a leading advocate of the metre internationally, as described below.

International standardisation (1832–1889)

Scientists pushed the metric system forward internationally well before most governments had adopted it at home. In 1832 Carl Friedrich Gauss proposed adding the second to the metre and the kilogram as base units, in the form of the CGS system, and in 1836 he founded the Magnetischer Verein with Alexander von Humboldt and Wilhelm Eduard Weber, the first international scientific association. It was followed by the Central European Arc Measurement (German: Mitteleuropäische Gradmessung), founded on the initiative of Johann Jacob Baeyer in 1863, and by the International Meteorological Organisation, whose second president, the Swiss Heinrich von Wild, represented Russia on the International Committee for Weights and Measures.

Geodesy made the need for common standards pressing. Surveyors needed standards that could be compared against one another to within fractions of a millimetre, which in practice meant comparison against Borda's toise, the reference for all French baselines, and careful control of temperature. Carlos Ibáñez e Ibáñez de Ibero, appointed in 1853 to produce a large-scale map of Spain, had a geodetic measuring apparatus built in Paris and compared it against Borda's double-toise N°1. Copies of the Spanish standard were made for Egypt, France and Germany.

Spain and Portugal joined the Central European Arc Measurement in 1866. At its 1867 conference in Berlin, a committee chaired by Otto Wilhelm von Struve recommended replacing the toise with the metre and creating a new international prototype metre against which national standards could be compared. In 1869 the Saint Petersburg Academy of Sciences invited the French Academy of Sciences to act jointly to secure universal scientific use of the metric system, and Napoleon III convened the International Metre Commission, which met in Paris in 1870. The Franco-Prussian War broke out and the Second French Empire collapsed, but the commission reconvened in 1872 with about thirty countries taking part, leading to the Metre Convention of 20 May 1875 and the creation of the International Bureau of Weights and Measures.

Ibáñez represented Spain at the 1875 conference and was elected the first president of the International Committee for Weights and Measures. His work resulted in the distribution of platinum–iridium prototypes of the metre to all states party to the convention at the first meeting of the General Conference on Weights and Measures in 1889; these prototypes defined the metre until 1960.

United Kingdom (1824–present)

The Weights and Measures Act 1824 (5 Geo. 4. c. 74) imposed one standard 'imperial' system of weights and measures on the British Empire. 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'. The United Kingdom initially declined to sign the Treaty of the Metre, but acceded in 1884. 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 and it was the British firm Johnson Matthey that was accepted by the CGPM in 1889 to cast the international prototype metre and kilogram.

In 1895, another parliamentary select committee recommended the compulsory adoption of the metric system after a two-year permissive period. The Weights and Measures (Metric System) Act 1897 (60 & 61 Vict. c. 46) legalised the metric units for trade, but did not make them mandatory. 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.

In 1965, with lobbying from British industry and the prospect of joining the Common Market, Britain began an official programme of metrication with 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.

As of 2026, goods sold by weight or measure must be sold in metric units, with three exceptions. Draught beer and cider must be served in set fractions or multiples of the pint. Milk in returnable containers may be sold by the pint, and precious metals by the troy ounce. Imperial units may also be shown alongside metric units, but not more prominently.

The government's 2022 consultation, Choice on units of measurement: markings and sales, drew around 100,000 responses, of which only about 1% favoured greater use of imperial units, and the existing rules on the sale of goods were retained. In December 2023 the government confirmed that it would not legislate for wider use of imperial measures, announcing instead a narrower change to the permitted pack sizes for wine. The Weights and Measures (Intoxicating Liquor) (Amendment) Regulations 2024 added 200 ml and 568 ml (one imperial pint) to the specified quantities for pre-packed still wine, and 500 ml and 568 ml for sparkling wine. The Product Regulation and Metrology Act 2025, which received royal assent on 21 July 2025, amended the Weights and Measures Act 1985 and gave the Secretary of State power to make regulations about units of measurement and the quantities in which goods are marketed.

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.

Surveys show that imperial units are still widely used for height, body weight, speed and long distances. Younger people are more likely to use metric units for short distances and for weighing goods.

United States (1830–present)

In 1805 a Swiss geodesist Ferdinand Rudolph Hassler brought copies of the French metre and kilogram to the United States. In 1830 the Congress decided to create uniform standards for length and weight in the United States. Hassler was mandated to work out the new standards and proposed to adopt the metric system. The Congress opted for the British Parliamentary Standard from 1758 and the Troy Pound of Great Britain from 1824 as length and weight standards. 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. All distances measured by the Survey of the Coast, Coast Survey, and Coast and Geodetic Survey were referred to the metre.

In 1866 the United States Congress passed a bill making it lawful to use the metric system for commercial and legal purposes in the United States, without displacing customary units. 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. The non-mandatory nature of the adoption has resulted in a much slower pace of adoption in the U.S. than in other countries. 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, 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”.

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. 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.

In 1971, the U.S. 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 U.S. to encourage educational agencies and institutions to prepare students to use the metric system of measurement with ease and facility as a part of the regular education program. On 23 December 1975, President Gerald Ford signed Public Law 94–168, the Metric Conversion Act of 1975. This act declares a national policy of coordinating the increasing use of the metric system in the US. It established a U.S. Metric Board whose functions as of 1 October 1982 were transferred to the Dept of Commerce, Office of Metric Programs, to coordinate the voluntary conversion to the metric system.

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

Other English-speaking countries

The British metrication programme signalled the start of metrication programmes elsewhere in the Commonwealth, though India had started its programme in 1956, nine 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 the programme stalled and was effectively abandoned in 1985, when the Metric Commission was wound up. 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. 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). Canadian railcars show weight figures in both imperial and metric. Most other Commonwealth countries adopted the metric system during the 1970s.

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. Hong Kong uses three systems (Chinese, imperial, and metric) and all three are permitted for use in trade.

Global status of metrication

Status by country/region

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

Notes

Methods of conversion

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 ran from 1 October 1958, when the Standards of Weights and Measures Act 1956 came into force, through October 1960, when metric weights became mandatory, to April 1962, when all remaining 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 United Kingdom legalised metric units for contracts and dealings in 1864 and for trade in 1897, 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. Imperial units shown alongside metric were due to become unlawful after 2009, but the time limit was removed and they may now 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. Portugal did the same in 1814, substituting traditional Portuguese names for the metric units, as described above. 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").

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.

Sectors and countries resistant to metrication

As of 2026, the metric system predominates in most of the world, however specific sectors are more resistant to metrication. For example:

Computers, cameras and televisions

• 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.
• Digital display sizes are usually advertised diagonally, using inches, along with centimetres supplementarily.
• 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.
• Photographic paper is still sold with imperial print sizes across the world in metric countries.
• The dominant pin spacing for electronic components is based on 1⁄10 in (2.54 mm).

Consumer goods

• Surfboards are usually designed, constructed, and sold in feet and inches.
• Shoes are still sold by the barleycorn in most English speaking countries, or the Paris Point in Europe.
• Abrasives, such as sandpaper and sharpening stones, are commonly graded by grit number, corresponding to the number of mesh openings per linear inch in the screen used to grade the particles.
• The fill power of down feather insulation is commonly expressed in "cubic inches per ounce".
• Fabric thread count is frequently sold in threads per square inch.
• Precious metals are often sold in troy ounces, even in mostly metric states.

Land, area and distances on the ground

• Golf courses are measured in yards in the , Canada, China, India, Ireland, Japan, the Mexico, Philippines, South Korea, Thailand, United Kingdom, UAE, United States, Venezuela, and Vietnam.
• 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 and rail transportation

• 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 nations.
• 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.
• 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 in six countries: Antigua and Barbuda, Dominica, Grenada, Saint Christopher and Nevis, Saint Lucia, and Saint Vincent and the Grenadines.
• United States gallons are still used for the sale of fuel in the United States and its territories (except for Puerto Rico, which has used litres since 1980), the three associated countries of the United States (the Federated States of Micronesia, the Marshall Islands and Palau), and in four other countries: Belize, Colombia, Haiti and Liberia.
• Both imperial gallons and United States gallons are used for the sale of fuel in the British Overseas Territory of the Turks and Caicos Islands and in the Bahamas.

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 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. 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). 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.

Legacy of previous units

• In Australia, pints are still used for beer, though a pint has been redefined to be 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.

United Kingdom

In the United Kingdom, goods sold by weight or measure must be sold in metric units, and imperial units may only be shown alongside them. Imperial units nonetheless remain in official use in two prominent areas: road signs give distances in miles and yards and speed limits in miles per hour, and draught beer and cider are sold by the pint. New cars must also carry fuel consumption figures in both miles per gallon and litres per 100 kilometres.

In everyday life, imperial units are still widely used for a person's height and weight, for speed and for longer distances, while younger people are more likely to use metric units for short distances and for weighing goods. There is little support for changing the current balance in either direction. In the government's 2022 consultation, 81.1% of responses favoured keeping imperial units as an option alongside metric units, while 17.6% wanted metric units only. In a 2023 YouGov poll, 62% opposed switching road signs and speed limits from miles to kilometres, while 22% supported it.

United States

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

A 1992 amendment to the Fair Packaging and Labeling Act (FPLA), which took effect in 1994, required labels on federally regulated "consumer commodities" to include both metric and US customary units. As of 2013, all but one U.S. state (New York) have passed laws permitting metric-only labels for the products they regulate.

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

Canada

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 U.S. or a Canadian (imperial) unit.

Belize

Belize, which is a former British colony, uses both the metric and British imperial systems. Miles are the most commonly used unit for measuring distance, and petrol is sold in US gallons (similar to some countries in Latin America).

Metrication mishaps

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 22300 pounds (10100 kg) of fuel instead of the required 22300 kilograms (49200 lb).

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.