Metrication or metrification is the act or process of converting to the metric system of measurement. 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 continued over more than two centuries. , about 95% of the world's population lived in countries where the modern metric system was the only legal system of measurement. Certain countries and sectors are nonetheless either still transitioning or have chosen not to fully adopt the metric system.
Overview 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.
According to the U.S. National Institute of Standards and Technology (NIST), only three countries do not have mandatory metric laws: Liberia, Myanmar and the United States. A research paper by Vera (2011) identified further countries where metric use was not mandatory in practice, including the three Compact of Free Association states: the Federated States of Micronesia, the Marshall Islands and Palau.
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 made (road signs and temperature are legislated to be in metric), although there has been very little progress in local trade. In 2018, the Liberian government had pledged to adopt the metric system. The Liberia Standards Authority, established by law in 2022 and operationalised in 2024, replaced the country's National Standards Laboratory and was given responsibility for national measurement in trade and for traceability to SI units.
, the United States has a national policy of adopting the metric system based on the Metric Conversion Act of 1975, amended by the Omnibus Trade and Competitiveness Act of 1988, and Presidential Executive Order 12770 of 1991, and all United States government agencies are required to adopt it.
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. 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. Other metric Caribbean countries, such as Saint Lucia officially metric 2000, are still in the process toward full conversion.
Other countries have begun the process but chosen not to finish it. The United Kingdom, for example, began metricating in 1965, but in 1979 the government decided against any further compulsory conversion. Metric units are now the primary legal units for trade, but imperial units are still used on road signs and for draught beer, and may be shown alongside metric units in shops. After a public consultation, the government confirmed in 2023 that it would keep these rules unchanged.
The United Kingdom and the United States face ongoing resistance toward metrication, which may be partially rooted in a belief that their cultural identity is intertwined with the traditional measurement systems they historically have used.
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 (imperial decree of 12 February 1812), a new system of measure – the ("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 was defined as being two metres, with six making up one toise, twelve making up one pied and twelve making up one pouce. Likewise the 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 (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,Crease (2011) refers to: 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 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 (), the newly formed Kingdom of the Netherlands abandoned the in favour of the "Dutch" metric system (), 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 (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)