Metrication in the United Kingdom
is the act or process of converting to the metric system of measurement. The United Kingdom, through voluntary and mandated laws, has metricated most of government, industry, commerce, and scientific research to the metric system; however, the previous measurement system is still used in society. Imperial units as of 2024 remain mandated by law to still be used without metric units for speed and distance road signs, and the sizes of cider and beer sold by the glass, returnable milk containers and precious metals, and in some areas both measurement systems are mandated by law.
Due to metrication many Imperial units have been phased out. However, the national curriculum requires metric units and imperial units that still remain in common usage to be taught in state schools. As such, the public is familiar with both metric and Imperial units, and may interchange measurements in conversation, for example: distance and body measurements.
Adopting the metric system was discussed in Parliament as early as 1818 and some industries and government agencies had metricated, or were in the process of metricating by the mid-1960s. A formal government policy to support metrication was agreed by 1965. This policy, initiated in response to requests from industry, was to support voluntary metrication, with costs picked up where they fell. In 1969, the government created the Metrication Board as a quango to promote and coordinate metrication. The treaty of accession to the European Economic Community, which the United Kingdom joined in 1973, obliged the United Kingdom to incorporate into domestic law all EEC directives, including the use of a prescribed SI-based set of units for many purposes within five years. In 1978, after some carpet retailers reverted to pricing by the square yard rather than the square metre to try to make the prices appear cheaper, government policy shifted, and they started issuing directives making metrication mandatory in certain sectors.
In 1980, government policy shifted again to prefer voluntary metrication, and the Metrication Board was abolished. By the time the Metrication Board was wound up, all the economic sectors that fell within its remit except road signage and parts of the retail trade sector had metricated, and most pre-packaged goods were sold using the prescribed units. Mandatory use of prescribed units for retail sales took effect in 1995 for packaged goods and in 2000 for goods sold loose by weight. The use of "supplementary indications" or alternative units was originally to have been permitted for only a limited period, that period being extended a number of times due to public resistance, until in 2009 the requirement to ultimately cease use of traditional units alongside metric units was finally removed.
British scientists, philosophers and engineers have been at the forefront of the development of metrication. In 1861 a committee from the British Association for Advancement of Science, which members included James Prescott Joule, Lord Kelvin, and James Clerk Maxwell, defined several electrical metric units. In the 1870 the international prototype kilogram was manufactured by the British company Johnson, Matthey & Co.
Foundations for metrication (pre-1962)
Pre-1799
When James VI of Scotland inherited the English throne in 1603, England and Scotland had different systems of measure. Superficially the English and the Scots units of measure were similar – many had the same names – but there were differences in their sizes: in particular the Scots pint and gallon were more than twice the size of their English counterparts. In 1707, under the Act of Union, the Parliaments of England and Scotland were merged and the English units of measurement became the standard for the whole new Kingdom of Great Britain. The practical effect of this was that both systems were used in Scotland, and the Scottish measures remained in common use until the Weights and Measures Act 1824 outlawed them.File:Gunter's chain at Campus Martius Museum.JPG|thumb|left|upright|Gunter's chain – one of Britain's earliest decimalbased measuring devices greatly simplified the measurement of land area.
This period marked the Age of Enlightenment, when people started using the power of reason to reform society and advance knowledge. Britons played their role in the realm of measurement, laying down practical and philosophical foundations for a decimal system of measurement which were ultimately to provide the building blocks of the metric system.
One of the earliest decimal measuring devices, developed in 1620 by the English clergyman and mathematician Edmund Gunter, introduced two new units of measure – the chain and the link – and a new measuring device: Gunter's chain. Gunter's chain was 66 feet, or, in length and consisted of 100 links, making each link. The decimal nature of these units and of the device made it easy to calculate the area of a rectangle of land in acres and decimal fractions of an acre.
Having difficulties in communicating with German scientists, the Scottish inventor James Watt, in 1783, called for the creation of a global decimal measurement system. A letter of invitation, in 1790, from the French National Assembly to the British Parliament, to help create such a system using the length of a pendulum as the base unit of length received the support of the British Parliament, championed by John Riggs Miller, but when the French overthrew their monarchy and decided to use the meridional definition of the metre as their base unit, Britain withdrew support. The French continued alone and created the foundations of what is now called the Système International d'Unités and is the measurement system for most of the world.
1799–1962
The inherent problems associated with handling multiple currencies and systems of units encountered in the Great Exhibition of 1851 triggered calls for a standardisation of units across Europe with the metric system being suggested as the natural choice. In 1854, de Morgan was influential in setting up the "Decimal Association" to lobby for decimalisation of both measurement and coinage. In 1862, the Select Committee on Weights and Measures favoured the introduction of decimalisation to accompany the introduction of metric weights and measures. A further Royal Commission "on the question of the introduction of metric system of weights and measures" also reported in 1869.In 1863, a bill which would have mandated the use of the metric system throughout the British Empire, and which had passed its first and second readings in the House of Commons, was rejected at its Commons Committee stage as impractical, and so did not pass into law. The following year, after pressure from the astronomers George Airy and Sir John Herschel, the bill was watered down to merely legalise the use of the metric system in contracts. It was presented and passed as a Private Member's Bill. Ambiguous wording in the 1864 Act meant that traders who possessed metric weights and measures were still liable to arrest under the Weights and Measures Act 1835.
While the politicians were discussing whether or not to adopt the metric system, British scientists were in the forefront in developing the system. In 1845, a paper by James Prescott Joule proved the equivalence of mechanical and thermal energy, a concept that is vital to the metric system – in SI, power is measured in watts and energy in joules regardless of whether it is mechanical, electrical or thermal. By contrast, units such as the horsepower, British Thermal Unit, gasoline gallon equivalent, and foot-pound have no logical relationship to one another, as these units were independently defined before dimensional analysis was understood.
In 1861, a committee of the British Association for Advancement of Science including William Thomson, James Clerk Maxwell and Joule among its members was tasked with investigating the "Standards of Electrical Resistance". In their first report, they laid the ground rules for their work – the metric system was to be used and measures of electrical energy must have the same units as measures of mechanical energy.
In the second report, they introduced the concept of a coherent system of units whereby units of length, mass and time were identified as "fundamental units".
All other units of measure could be derived from these base units.
In 1873, another committee of the BAAS that also counted Maxwell and Thomson among its members and was tasked with "the Selection and Nomenclature of Dynamical and Electrical Units". They recommended the CGS system of units. The committee also recommended the names "dyne" and "erg" for the CGS units of force and energy. The CGS system became the basis for scientific work for the next seventy years.
In 1875, a British delegation was one of twenty national delegations to a convention in Paris that resulted in seventeen of the nations signing the Metre Convention on 20 May 1875, and the establishment of three bodies, the CGPM, CIPM and BIPM, that were charged with overseeing weights and measures on behalf of the international community. The United Kingdom was one of the countries that declined to sign the convention. In 1882 the British firm Johnson, Matthey & Co secured an agreement with the French government to supply 30 standard metres and 40 standard kilograms. Two years later the United Kingdom signed the treaty and the following year it was found that the standard yard which had been in use since 1855 had been shrinking at the rate of one part per million every twenty years. In 1889, one of the standard metres and one of the standard kilograms that had been cast by Johnson, Matthey & Co were selected at random as the reference standard and the other standards, having been cross-correlated with each other, were distributed to the signatory nations of the treaty.
Parliament passed the Weights and Measures Act 1897, legalising metric units for all purposes but not making them compulsory.
The situation was clarified in 1897 following another select committee which also recommended that metrication become compulsory by 1899. In 1902, an Empire conference decided that metrication should be compulsory across the British Empire. In 1904, scientist Lord Kelvin led a campaign for metrication and collected 8 million signatures of British subjects. On the opposition side, 1904 saw the establishment of the British Weights and Measures Association for "the purpose of defending and, where practicable, improving the present system of weights and measures". At this time 45% of British exports were to metricated countries. Parliament voted to set up a select committee on the matter.
This select committee reported in 1907 and a bill was drafted proposing compulsory metrication by 1910, including decimalisation of coinage.
The matter was dropped in the face of wars and depression, and would not be again raised until the White Paper of 1951, the result of the Hodgson Committee Report of 1949 which unanimously recommended compulsory metrication and currency decimalisation within ten years. The report said "The real problem facing Great Britain is not whether to adhere either to the Imperial or to the metric system, but whether to maintain two legal systems or to abolish the Imperial." The report also recommended that any change should be implemented in concert with the Commonwealth and the US, that the United Kingdom adopt a decimal currency and that the United Kingdom and United States harmonise their respective definitions of the yard using the metre as a reference. The Hodgson Report was originally rejected by British industry, but in 1959 the United Kingdom and United States redefined their respective yards to be 0.9144 m exactly.