Metrication 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 (imperial units) is still used in society.

, imperial units remain authorised as primary units only for road traffic signs and speed measurement (the mile, yard, foot and inch); for draught beer and cider and for milk sold in returnable containers (the pint); and for transactions in precious metals (the troy ounce). Their use is compulsory for road distance and speed signs and for draught beer and cider, but optional for milk and precious metals, which may equally be sold in metric units. Imperial units may also be used alongside metric units as "supplementary indications" for trade.

Due to metrication many imperial units have been phased out. In England, the national curriculum requires pupils to use metric units and to know approximate equivalences with the imperial units still in common use. The public is familiar with both systems, and which units people use depends more on context and age than on a general preference for one over the other: imperial units persist for personal height and weight, while younger people are more likely than older people to use metric units for cooking and short distances.

A royal commission considered a decimal system of weights and measures as early as 1819 and rejected it, and Parliament declined to make metric units compulsory on several occasions thereafter. Some industries and government agencies had nonetheless 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 (EEC), 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. From 1976 the government began issuing statutory instruments making metric quantities mandatory in certain sectors, an approach reinforced after a voluntary agreement among carpet retailers collapsed in 1977 when a large chain reverted to pricing by the square yard rather than the square metre to make its prices appear cheaper.

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 (generally the traditional imperial units formerly used) 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 and engineers have been at the forefront of the development of metrication. In 1861 a committee of the British Association for the Advancement of Science (BAAS), whose members included James Prescott Joule, William Thomson (later Lord Kelvin) and James Clerk Maxwell, began the work that led to the definition of several electrical units. In 1882 the British company Johnson, Matthey & Co manufactured standard metres and kilograms for the signatories of the Metre Convention.

Foundations for metrication (pre-1965) Nineteenth-century proposals A royal commission on weights and measures reported in 1819 against a decimal system, concluding that the existing subdivisions were more convenient in practice than a decimal scale and that there was no sufficient reason to alter the established standards of length. The commission acknowledged the French metre, giving its length in inches, but saw the advantage of a standard derived from nature as slight. Its recommendations formed the basis of the Weights and Measures Act 1824, which established a single set of imperial units for the whole United Kingdom, replacing the various English, Scottish and local measures then in use.

The difficulties of handling multiple currencies and systems of units at the Great Exhibition of 1851 triggered calls for the standardisation of units across Europe, with the metric system suggested as the natural choice. In 1854 the mathematician Augustus De Morgan was influential in setting up the Decimal Association to lobby for the decimalisation of both measurement and coinage. In 1862 the Select Committee on Weights and Measures favoured introducing decimalisation alongside metric weights and measures, and a Royal Commission on the introduction of the metric system reported in 1869.

A bill that would have mandated the use of the metric system throughout the British Empire passed its first and second readings in the House of Commons, but was rejected at committee stage as impractical. After pressure from the astronomers George Airy and Sir John Herschel, it was watered down to legalise the use of the metric system in contracts only, and passed in that form as a Private Member's Bill in 1864. Ambiguous wording in the 1864 act meant that traders who possessed metric weights and measures remained liable to arrest under the Weights and Measures Act 1835 (5 & 6 Will. 4 c. 63).

Failed attempts at compulsion Parliament passed the Weights and Measures (Metric System) Act 1897 (60 & 61 Vict. c. 46), which legalised metric units for all purposes without making them compulsory. A select committee reporting the same year went further, recommending that metrication become compulsory by 1899.

In 1902 an Empire conference decided that metrication should be compulsory across the British Empire. A Weights and Measures (Metric System) Bill originated by Lord Kelvin was introduced in the House of Lords by Lord Belhaven in 1904, supported by petitions in favour of the metric system. The government offered no opposition to the second reading on the understanding that the bill would be referred to a select committee. A select committee reported in 1907, and a bill was drafted proposing compulsory metrication by 1910 together with the decimalisation of coinage.

The matter was dropped in the face of war and depression, and was not raised again until the White Paper of 1951, which followed the Hodgson Committee report of 1949. That report unanimously recommended compulsory metrication and currency decimalisation within ten years, framing the question as whether to maintain two legal systems of measurement or to abolish the imperial one. It also recommended that any change be made in concert with the Commonwealth and the United States, that the United Kingdom adopt a decimal currency, and that the United Kingdom and United States harmonise their definitions of the yard by reference to the metre. British industry rejected the Hodgson report, but in 1959 the two countries redefined their yards as 0.9144 m exactly.

Renewed interest The British Standards Institution (BSI) began formally stimulating discussion about metrication in May 1962 by issuing a short statement on the subject. The introduction of the metric system was also discussed at the Fifth Commonwealth Standards Conference in Sydney in October 1962. In October 1963, following inquiries by its committees, the BSI stated that changes in the field of measurement were inevitable and that they should be directed towards making the metric system the primary system of weights and measures in the United Kingdom as soon as possible.

Principal phase of metrication (1965–1980)

In 1965, the then Federation of British Industries informed the British government that its members favoured the adoption of the metric system, though some sectors emphasised the need for a voluntary system of adoption. The Board of Trade, on behalf of the government, agreed to support a ten-year metrication programme. The programme was to be voluntary and the costs of change were to be borne where they fell, though the need for legislation was accepted without a date being set for it.

Work on adapting specifications started almost as soon as the government first gave its approval in 1965. The BSI took the lead in coordinating the efforts of industry, and where appropriate working with the International Organization for Standardization (ISO), CEE, CEN and CENELEC while the Royal Society liaised with professional societies, schools and the like. Initially the BSI targeted 1,200 basic standards which were converted to metric units by 1970. Most of the remaining 4,000 standards were converted in the ensuing five years.

There were three principal ways in which metrication was implemented: Hard metrication, which resulted in new products based on round metric quantities: for example A4 paper replaced both foolscap and quarto paper; and in rugby union 5-, 10-, and 22-metre lines replaced the 5-, 10-, and 25-yard lines respectively. Soft metrication, where existing standards were rewritten to convert imperial measurements to metric without changing the item itself. Revision of measurement techniques, in cases where the concepts behind the existing standard or practice were found to be archaic. One such revision was to define the strength of alcoholic drink as a percentage alcohol by volume rather than, in the case of whisky, in "degrees proof".

The Metrication Board

In July 1968, following the publication of a report from the Standing Joint Committee on Metrication, the government announced that an advisory metrication board would be set up as soon as possible, to oversee the metrication process, with a target completion date of the end of 1975. The report favoured the board being made up of part-time members drawn from commerce and industry, with government, education and consumer interests also being represented. In December 1968, the government announced the set-up of the Metrication Board to coordinate the metrication programme, with Lord Ritchie-Calder being appointed as chairman. By this time much of the groundwork, especially rewriting of many British Standards using metric units, had been done and many of the industries that stood to benefit from metrication had already metricated, or had a metrication programme in progress.

Policy review The general election of 18 June 1970 resulted in a change of government and four months later, on 27 October 1970, following an anti-metrication motion being tabled calling on the newly elected Conservative Party government not to continue with the previous government's metrication commitments, the government announced that a White Paper would be produced to examine the cost, savings, advantages and disadvantages of a change to the metric system. During the debate when the announcement was made, Conservative MPs complained that metrication was being introduced by stealth.