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Metrication, is the act or process of converting to the metric system of measurement. The United Kingdom, through voluntary and mandated laws have metricated most of government, industry, commerce, and scientific research to the metric system, however the previous measurent system (Imperial units) is still used in society. Imperial units as of 2024 still 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.

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. The public is familar 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 (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. 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 (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, 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 (BAAS), 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.

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 one chain (20 metres) in length (i.e. one tenth of a furlong) and consisted of 100 links, making each link 1⁄1000 furlong (200 millimetres). 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 (5 & 6 Will 4 c. 63).

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 (or those they were based upon) were independently defined before dimensional analysis was understood.

In 1861, a committee of the British Association for Advancement of Science (BAAS) including William Thomson (later Lord Kelvin), James Clerk Maxwell and Joule among its members was tasked with investigating the "Standards of Electrical Resistance". In their first report (1862), 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 (1863), they introduced the concept of a coherent system of units whereby units of length, mass and time were identified as "fundamental units" (now known as base units). All other units of measure could be derived (hence derived units) 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 (centimetre-gram-second) 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 (Metric System) Act 1897 (60 & 61 Vict. c. 46), 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 (former Empire) 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.

Metrication in the UK (1962–1980)

The British Standards Institution (BSI) chose to stimulate discussion about metrication in May 1962 by issuing a short statement on the subject. The introduction of the metric system was a topic at the Fifth Commonwealth Standards Conference in Sydney in October 1962. Also in October 1963, the BSI, based on the results of inquiries by its committees, stated that their view was that changes in the field of measurement were inevitable. They also stated that they thought these changes should be channelled towards the metric system becoming the primary weights and measures system for the UK as soon as possible.

In 1965, the then Federation of British Industry 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. There would be minimal legislation as the programme was to be voluntary and costs were to be borne where they fell.

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 using metric units. This approach was used where any radical changes would have been impractical.
• Revision of measurement techniques were revised 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" (described by Lord Brown as being "based on a test that involves the burning of a given quantity of gunpowder").

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.

The White Paper on Metrication was published in February 1972, and it set out the case for metrication and refuted the charge of metrication by stealth as metric units had been lawful for most purposes since 1897. It also reported that metrication would be necessary for the UK to join the European Common Market and that as British industry was exporting to all parts of the world they would benefit. It also reiterated the previous government's policy that metrication should be voluntary and hoped metrication would be mostly complete within ten years. The expectation was also expressed that with both the imperial and metric systems coexisting for many years, that consumers would gradually become familiar and comfortable with the metric system.

Progress

Shortly after the publication of the White Paper, the Minister of Transport announced postponement of the metrication of speed limits, which had been scheduled for 1973. The rest of the metrication programme continued, with the following completion dates:

• 1970 Electric Cable Makers Confederation, British Aerospace Companies Limited drawing and documentation, London Metal Exchange, flat glass
• 1971 Paper and board, National Coal Board designs, pharmaceuticals
• 1972 Paint industry, steel industry, building regulations
• 1974 Textile and wool transactions, leading clothing manufacturers adopt dual units
• 1975 Retail trade in fabrics and floor coverings, post office tariffs, medical practice
• 1976 Bulk sales of petroleum, agriculture and horticulture
• 1977 Livestock auctions
• 1978 Solid fuel retailing, cheese wholesaling, bread, London Commodity Market

Yet the target of completion by 1975 "in concert with the Commonwealth" was not achieved; Australia, New Zealand and South Africa all completed their metrication processes by 1980.

Education

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In England and Wales, unlike Scotland, education was controlled at county council level rather than at national level. In 1967 the Department for Education alerted all local education authorities to the need to adapt to the metric system. In 1968 all bodies that had an interest in the examination system were invited to contribute to the discussion of both metrication and decimalisation in education. In science subjects, this meant a conversion from the cgs system to SI, in geography from the imperial system to SI while in mathematics it meant discarding the teaching of mixed unit arithmetic, a topic that took up a significant part of the time allocated in primary schools to arithmetic/mathematics and 7% of total time allocated to all subjects.

In Scotland, virtually all examinations set from 1973 onwards used SI, especially those connected with science and engineering. In England, each examination board had its own timetable: the Oxford Delegacy of Local Examinations, for example, announced a change to SI in 1968, with examinations in science and mathematics using SI by 1972, geography in 1973 and home economics and various craft subjects were converted by the end of 1976. Pupils were hampered by a revolution in teaching methods that was taking place at the same time and a lack of coordination at the national level.

In 1974 the Department of Education and Science issued advice (which still stands) to schools that teaching should be conducted principally in metric terms while maintaining general familiarity with imperial units.

According to a report in 1982, children were taught the relationship between decimal counting, decimal money and metric measurements, with time being the only quantity whose units were manipulated in a mixed-unit manner.

Wholesale, retail and consumer industries

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The retail industry proved difficult for the Metrication Board. The sector saw little benefit in metrication – competition was fierce and margins low. The opinions of the trade organisations with which the Metrication Board could negotiate were fragmented.

Many sectors of the industry did agree to a programme coordinated by the Metrication Board, with metrication of pre-packaged goods being introduced on a commodity by commodity basis. In 1977 when a carpet retailing chain reneged on an industry-wide agreement to use metric units (carpeting at £8.36 per square yard looked more appealing in price to the customer than carpeting at £10.00 per square metre), it became necessary for the first time to use legislation to enforce metrication rather than to rely on a voluntary adoption of the system.

Much of the retail industry was metricated during 1977 and 1978 by means of statutory orders.

Other sectors

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Before the Hodgson Committee, the metrication process was already in operation. One example was the Ordnance Survey, the national mapping agency for Great Britain, which initiated the Retriangulation of Great Britain in 1936, using metric measures. A metric National Grid was used as the basis for maps published by the Ordnance Survey from World War II onwards; War Office maps had had a metric grid since 1920. The Ordnance Survey decided on full metrication in 1964. The one inch to the mile (1:63,360) range of maps started being replaced with the 1:50000 range in 1969. The metrication of Admiralty Charts began in 1967 as part of a modernisation programme. As of 2020, road and street maps with primary scales in miles per inch are being marketed under the A-Z brand.

Another example was the Met Office, which began publishing temperatures in both Celsius and Fahrenheit in 1962 and stopped using Fahrenheit in their official reports in 1970.

Many other sectors metricated their operations in the late 1960s or early 1970s. This was not visible to the general public, though the financial pages of newspapers displayed metric prices, e.g. in the principal London commodity markets (the London Metal Exchange, and the various agricultural markets, but not the oil industry).

Costs

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The basis of the British metrication programme as announced in 1966 was a voluntary adoption of the metric programme, with the costs being absorbed where they fell. As a result, the costs of and savings from metrication in the United Kingdom have not been comprehensively determined, and studies have tended to focus on specific programmes. As the programme was voluntary, industry was free to take the most cost-efficient approach. In many cases this meant installing equipment calibrated in metric units as part of an ongoing maintenance cycle rather than as part of a specific metrication programme. Such an approach was taken by the gas industry: all newly installed meters record usage in cubic metres, but many older installations still measure in cubic feet.

A 1970s study by the United Kingdom chemical industry estimated costs at £6m over seven years, or 0.25% of expected capital investment over the change period. Other estimates ranged from 0.04% of a large company's turnover spread over seven years to 2% of a small company's turnover for a single year. Many companies reported recouping their costs within a year as a result of improved production.

Approximately, 85% of United Kingdom exports go to metric countries, with the majority of the non-metric exports going to the United States. Though the USA accepted metricated goods they weren't always compatible with domestic production. Further to ensure understanding by their population, all goods were required to be labelled in US customary units only as per the Fair Packing and Labelling Act (updated in 1992, with all imports requiring dual units from 1994).

There are real costs to business of manufacturing goods in two units of measurement. These costs have been estimated at 3% of annual turnover by the Institute of Production Engineers, and at £1.1 billion (1980) per annum by the CBI.

European Economic Community

In 1973 the United Kingdom joined the European Economic Community, and though the current metrication program had been under way for eight years, the current weights and measures legislation in the United Kingdom only applied to trade, however the directive EEC directive 71/354/EEC, which related to weights and measures, required the United Kingdom to formally define in law a number of units of measure, hitherto formally undefined in law, including those for electric current (ampere), electric potential difference (volt), temperature (degree Celsius and kelvin), pressure (pascal), energy (joule) and power (watt).

In the late 1970s, the UK government asked the EEC to postpone the deadlines for the introduction of metric units. The result was the repeal of directive 71/354/EEC and the introduction of directive 80/181/EEC.

• A number of units that had been proscribed under Directive 71/354/EEC could continue to be used until the end of 1985.
• A number of imperial units including the pound, ounce, yard, foot, inch, gallon and pint could continue to be used until the end of 1989.
• The mile, yard, foot and inch could be used on road traffic signs, distance and speed measurement, pints could be used for the sale of milk in returnable containers and for the measurement of draught beer and cider, acres could be used for purposes of land registration and troy ounces could be used when dealing with precious metals until a date to be determined by the states in question.
• Supplementary units were permitted until the end of 1989, provided that the supplementary indicator was not the dominant unit and that it was "... expressed in characters no larger than those of the corresponding indication ...".

Metrication in the UK (post-1980)

Metrication Board final report

In its final report , the Metrication Board wrote "Today metric units are used in many important areas of British life – including education; agriculture; construction; industrial materials; much of manufacturing; the wholesaling of petrol, milk, cheese and textiles; fatstock markets and many port fish auctions, nearly all the principal prepacked foods; posts and telecommunications: most freight and customs tariffs; all new and revised Ordnance Survey maps; and athletics. Nevertheless, taken as a whole, Britain is far from being wholly metric." The report identified two major sectors that had not yet been metricated being: retailing of weighed out foods and many sales by length, volume or area and speed limits and road distance, height and weight signs .

Retail industry

By the beginning of 1980, 95% of the "basic shopping basket" of foods were sold in metric quantities, with only a few products not being sold in prescribed metric quantities. The final report of the Metrication Board catalogues dried vegetables, dried fruit, flour and flour products, oat products, cocoa and chocolate powder, margarine, instant coffee, pasta, biscuits, bread, sugar, corn flakes, salt, white fats, dripping and shredded suet as being sold by prescribed metric quantities while no agreement had been reached with the industry regarding jam, marmalade, honey, jelly preserves, syrup, cereal grain and starches.

The changeover to selling of petrol by the litre rather than by the gallon took place after the Board was wound up. It was prompted by a technical shortcoming of petrol pump design: pumps (which were electro-mechanical) had been designed to be switchable between metric and imperial units, but had no provisions for prices of £2 or more per unit of fuel. Once the price of petrol rose above £1 per gallon, the industry requested that they be permitted to sell fuel by the litre rather than the gallon, enabling them to reduce the unit price by a factor of about 4.5 and so to extend the lives of existing pumps.

The Weights and Measures Act 1985 removed from the statute book many imperial units that had fallen into disuse as a result of the completed elements of the metrication programme. The units of measure removed from the statute book were the furlong, chain, square mile, rood, cubic yard/foot/inch, bushel, peck, fluid drachm, minim, ton, hundredweight, cental, quintal, stone, dram, grain, pennyweight, apothecaries ounce, drachm, scruple and the phrase "metric ton"

1995

On 1 October 1995 the following were removed from the list of allowable units for general use, though their continued use was permitted in specified circumstances: yard, therm, inch, foot, fathom, mile, acre, fluid ounce, gill, pint, quart, gallon, ounce (troy), ounce (avoirdupois), pound.

At the same time, regulations were passed prescribing metric quantities by which the remaining pre-packaged retail commodities not yet defined in metric terms could be sold. From the beginning of 1995, pre-packed coffee, coffee mixtures and coffee bags had to be sold in the prescribed quantities of 57 grams (2 oz), 75 grams (2.6 oz), 113 grams (4 oz), 125 grams (4.4 oz), 227 grams (8 oz), 250 grams (8.8 oz), 340 grams (12 oz), 454 grams (1 lb), 500 grams (1.10 lb), 680 grams (1.50 lb), 750 grams (1.65 lb) or a multiple of 454 grams (1 lb) or of 500 grams (1.10 lb); and honey, jam and marmalade other than diabetic jam or marmalade, jelly preserves and molasses, syrup and treacle in quantities of 57 grams (2 oz), 113 grams (4 oz), 227 grams (8 oz), 340 grams (12 oz), 454 grams (1 lb), 680 grams (1.50 lb) or a multiple of 454 grams (1 lb).

In 1995 the alcoholic drinks for consumption on the premises the 1⁄4, 1⁄5 and 1⁄6 gill measures for spirits (whisky, gin, rum and vodka) were replaced by 25 ml and 35 ml measures on 1 January 1995, and wine can only be sold in 125 ml, 175 ml or 250 ml glasses; prior to 1995, the size of wine glasses was unregulated. (From 2006, the legal measures for pints are 1⁄3 pint (189 ml), 1⁄2 pint (284 ml), 2⁄3 pint (379 ml), and subsequent multiples of the half-pint.)

2000s

On 1 October 1999, with effect from 1 January 2000, the following were removed from the list of allowable units for general use: fathom (used for marine navigation), pint (used for the sale of waters, lemonades and fruit juices in returnable containers), gills (sale of spirit drinks), ounce and pound (goods sold loose in bulk) and therm (gas supply). A direct result of the changes that were effective from 1 January 2000 was the requirement that most loose goods sold by weight, volume or length (for example, potatoes or tomatoes that were sold loose, or cheese or meat that was cut or weighed in front of the customer) must be priced and measured using metric units.

Some traders continued to sell produce from their market stalls using imperial-only scales from 2000. They were variously prosecuted for using unlawful scales, giving short measure and failing to display unit price per kilogram. Five traders, who became known as the Metric Martyrs, appealed unsuccessfully to the High Court, were refused appeal to the House of Lords, and appealed unsuccessfully to the European Court of Human Rights (ECHR).

During the 1990s, a series of statutory instruments relating to weighing devices and to the sale of pre-packaged goods were issued to ensure that United Kingdom law on metrology was harmonised with that of its EEC partners. In line with EEC practice, the meaning of weights displayed on pre-packaged goods was changed in 1980 to show the average weight of each item in the batch rather than the guaranteed minimum weight of each individual item. The EU Measuring Instruments Directive (Directive 2004/22/EU) which was intended to create a common market for measuring instruments across the countries of the EU came into force on 30 October 2006 with a ten-year transition period.

The regulations that came into force on 1 January 2000 regarding the sale of loose goods effectively made it mandatory to use metric units in the retail industry for most products, though supplementary indicators using certain imperial units were permitted. The units permitted as supplementary indicators under The Weights and Measures (Packaged Goods) Regulations 2006 are the gallon, quart, pint, fluid ounce, pound and ounce.

The provision of the EEC's directive 80/181/EEC that any unit of measure could be followed by a "supplementary indicator" was initially to have expired in 1989, but it was extended first to 1999 and then to 2009. During the 2007 consultations on the revision of the directive, strong representations were made to retain this provision, as its removal would impede trade with the United States. When the directive was revised in 2009, the "sunset clause" was removed from the text.

Various price-marking orders prescribed the sizes in which products could be marketed. Some of these restrictions, such as wine being sold in 750 ml bottles, were derived from EU directives, while others, such as the production of bread in 400 g or 800 g loaves, were applicable to the UK only. The principle of the Internal Market, backed up by a judgment of the European Court of Justice, required that any product that was legally produced anywhere in the European Union could, in most cases, be sold anywhere in the EU. Thus a 500 g packet of rye bread, legally manufactured in Germany, could be sold in the United Kingdom even though it was not lawful under British law for a British baker to produce an identical 500 g packet of bread.

A consultation by the EU aimed at bypassing this impasse was launched in 2004. The outcome was Directive 2007/45/EC, which deregulated prescribed packaging of most products, leaving only wines and liqueurs subject to prescribed EU-wide pre-packaging legislation. While this effectively undid much of the work done by the Metrication Board by deregulating prescribed sizing for over 40 products, the law relating to labelling of products has remained unchanged.

The EU non-automatic weighing instrument directive (directive 2009/23/EC), which came into force in 2009 and was superseded by directive 2014/31/EU, codified existing regulations regarding the harmonisation of non-automatic weighing devices used for trade, medical purposes or in the preparation of evidence to be heard in court. The directive identified four classes of weighing device ranging from Class I (having a minimum accuracy of 1 part in 50,000) to class IIII (sic) (having a minimum accuracy of 1 part in 100). Devices that fall within the scope of the directive are required to be recalibrated at regular intervals and to have an output showing SI units, except for those used for weighing precious metals or stones. Secondary indications may be shown, provided that they cannot be mistaken for primary indications. The impact of this directive in the United Kingdom is that most traders cannot legally use weighing devices calibrated in units other than SI units.

In its initial form, the scope of directive 80/181/EEC was restricted to "economic, public health, public safety and administrative" purposes only. An outcome of the 2007 consultations was a proposal by the EU Commission to extend the scope of the directive to include "consumer protection" and "environmental issues". This was implemented by removing the phrase limiting the scope of the directive, thereby extending it to all matters that come under the ambit of the Internal Market Chapter of the EU Treaty. The directive specifically excluded units of measurement used in international treaties relating to rail traffic, aviation and shipping such as expressing aircraft altitude in feet.

The United Kingdom's legislation of 2009 that implemented these changes made no reference to the extension of the directive's scope. The use of the acre as the primary unit for land registration was officially replaced by the hectare on 1 January 2010, under an EU ruling. The acre is still used as a supplementary unit alongside the hectare for land registration.

A LACORS report published in March 2010 highlighting widescale use of inappropriate scales in hospitals, sometimes of domestic quality, recommended that on safety grounds NHS hospitals should use Class III (or better) metric-only scales. A Department of Health alert was subsequently sent to all NHS trusts endorsing these recommendations.

As of December 2023, the UK Government indicated that there will be no change to the Government’s current systems of measurement. The UK Government sought public opinion on changing the current systems of measurement when buying or selling goods. The results of the analysis were released December 2023, and showed that of the over 100,000 respondents nearly 99% were in favour of using metric units when buying or selling products, either as the primary unit of sale (maintaining status quo) or as the sole unit of sale (purely metric). Just over 1% of respondents were in favour of increased use of imperial units when buying or selling products, either by increased choice between using metric and imperial units or by moving to a purely imperial system. Thus there was no change to the current laws on measurement.

Road and rail transport

Transport infrastructure standards were metricated using soft conversions, as part of the general metrication of the engineering industry. The standard railway track gauge, fixed at 4 feet 8+1⁄2 inches (1,435.1 mm) in 1845, was redefined as 1,435 mm – a nominal decrease of 0.1 mm but within the engineering tolerances.

Motorway marker posts used by road maintenance teams and emergency services demarcate locations in multiples of 100 m. Standards relating to the design and building of new road and rail vehicles have been metric since the engineering changeover in the 1970s. Imperial units have been retained for both road and railway signage except on new railways such as the Channel Tunnel Rail Link, and the Tyne and Wear Metro and London Tramlink which along with all other modern British tram systems also operate in metric. The Cambrian Line has also changed to metric units with the change to ERMTS signalling. London Underground has converted to using metric units for distances but not for speeds.

In 1978 the cost of converting road signs from miles to kilometres in the United Kingdom was estimated to be between £7.5 million and £8.5 million. In 2005 The Department for Transport (DfT) costed the replacement of all of the United Kingdom's road signs in a short period of time at between £565 million and £644 million. In 2007, £760 million was set for the metrication of traffic signs (speed and distance), however this lapsed when EU Directive 2009/3/EC came into force on 27 May 2009 amending Council Directive 80/181/EEC on the approximation of the laws of the Member States relating to units of measurement.

Apart from the sale of fuel, which was metricated in the 1980s, motorists have seen little metrication. Speedometers and mandatory information on car advertisements such as fuel consumption are given in both metric and imperial units. The 1994 TSRGD permitted the use of metric units alongside imperial units for width and height warning signs and dual metric/imperial signs became mandatory from March 2015. Distances and speed restrictions are shown only in imperial units.

Assessment of the British metrication programme

After the UK government's White Paper on metrication was published in February 1972, the journal New Scientist reported the lack of urgency in the minister's handling of the issue and described how the government refused to use its purchasing power to advance the metrication process. It quoted one (unnamed) metricationalist as saying " is not firming things up at all. It will turn us into a dual country".

Studies of the British metrication programme included two by US government agencies: NASA in October 1976 and the National Bureau of Standards in April 1979. Both reports noted that the British metrication programme lacked leadership from government. This manifested itself in many ways including:

1. The failure to appoint the Metrication Board at the start of the metrication programme meant that industry had to take the lead in a programme that affected everybody and did not have the machinery to implement metrication in, especially, the retail sector.
2. The failure of government to provide funding – much of the initial work was funded by industry itself.
3. The failure to provide a "champion" for metrication – such a role fell outside the remit of the Metrication Board.
4. The belief that the programme could be accomplished purely by voluntary means – both reports highlighted the need for appropriate legislation to keep the programme on track.

These sentiments were echoed in the final report of the Metrication Board.

The involvement of the European Commission led metrication to be linked in public debate with Euroscepticism, and traditionally Eurosceptic parts of the British press often exaggerated or invented the extent of enforced metrication. Example stories include the Daily Star, which on 17 January 2001 claimed that beer would soon have to be sold by the litre in pubs, something not demanded in any EU directive.

Reaction to the UK Metric Association report A Very British Mess (2004), the executive summary of which was published in Science in Parliament, was mixed: the Daily Telegraph suggested that the UKMA's assertion of hostility or indifference by the British public to the metric system was due to the lack of cultural empathy rather than it being "foreign or European", while the Economist said that retreat was impossible and the current impasse costly.

Public surveys

2007 telephone survey

An Ipsos MORI telephone survey conducted in September 2007 for The Sun newspaper, entitled "Northern Rock, Metric Measurements and the EU Constitutional Treaty" found significant opposition to metrication in response to the question, "How strongly would you support and oppose Britain switching to use entirely metric measurements, rather than continuing to use traditional units?": The greatest variation in opinion was between tabloid and broadsheet readers rather than by age, social class or voting intention.

2013 public survey of understanding and use

The UK Metric Association (UKMA) commissioned YouGov to carry out a survey to investigate "public understanding and use of metric and imperial units and of public support for completing the metric changeover". The UKMA executive summary of results of the September and November 2013 survey, published in 2014, presents the following points as the key results:

• Half of respondents were opposed to completing metrication, with a quarter supportive and a fifth indifferent or non-committal.
• Younger generations were more supportive than the older but 36% of the 18–24 age group were opposed (with 33% supportive and 22% indifferent or non-committal).
• Where there are specific practical reasons for using metric units, the majority of the population prefer to use them.
• Where parental, peer and media pressures are strongly in favour of imperial units, all age groups continue to use imperial – including for personal weighing (89% of the over 60s and 64% of the 18–24s).
• There was a definite association between age and acceptance/use of metric units but there was still either a majority or a large minority of younger people who habitually use imperial rather than metric units for various everyday functions.
• Despite opposition to metrication, it was not likely to affect voting intentions in the next general election (when asked to choose 4 issues out of 17 as the most important when deciding how to vote only 1% selected converting from imperial to metric measurements).

The sample size was 1,978 adults in September and 1,878 in November. The results were weighted and are said by YouGov to be representative of all GB adults (aged 18+).

2022 YouGov survey of usage by general public

In 2022 YouGov conducted a survey, published the following year, on the systems of measurement preferred by the general public, split by age group under six different circumstances. Key results included:

• Younger generations tended to be more in favour of metric units, however they still preferred imperial units for measuring a person's height, speed and long distances.
• Younger generations preferred metric units for measuring short distances and for weighing goods.
• 18–29 year olds were almost evenly divided on how to weigh a person, with 47% using stones and pounds and 44% using kilograms.

Current status

Since 1 January 2010, the remaining non-metric units, allowed by United Kingdom law without supplementary indicators for economic, public health, public safety or administrative use, are limited to:

• the mile, yard, foot and inch for road traffic signs, for distance and speed measurement,
• the imperial pint for the dispensing of draught beer and cider, and for the sale of milk in returnable containers,
• the troy ounce for transaction in precious metals.

Goods and services sold by a description, as opposed to a price per unit quantity, are not covered by weights and measures legislation; thus, a fence panel sold as "6 foot by 6 foot" is legal, as is a 6 × 4 inch photograph frame, but a pole sold as "50 pence per linear foot", with no accompanying metric price, would be illegal.

Supplementary indicators

Supplementary indicators are permitted provided that they are not the dominant unit and that they are "... expressed in characters no larger than those of the corresponding (metric) indication ..." Under the Weights and Measures (Packaged Goods) Regulations 2006, these indicators are restricted to the imperial units of the gallon, quart, pint, fluid ounce, and pound.

There are no restrictions on the units that consumers can use when asking for goods, and the use of supplementary indicators and dual measure weighing scales (provided these have been calibrated in metric) means that a consumer can see an imperial price, request an imperial quantity and be supplied with the imperial quantity, provided that the seller legally weighs out and sells the metric equivalent.

Imperial packaging sizes

Imperial packaging weights converted to metric units are still common in the UK. For example, jars of jam, packs of sausages, and tins of golden syrup are sold as 454g (which is one pound).

Mandatory dual measurements

For cars sold in the UK the speedometers, and information on car advertisements such as fuel consumption, are must be stated in both metric and imperial units.

For width and height warning signs 1994 TSRGD had permitted the optional use of metric units, however dual metric/imperial signs became mandatory from March 2015.

Post-Brexit review of domestic trading units

As part of a larger post-Brexit review of laws inherited from the European Union (EU), the Brexit minister, David Frost, announced in September 2021 that the government would be reviewing the laws requiring traders to use metric units for the sale of most packaged or loose goods in the UK. The inherited laws allowed the display prices by imperial units, but they were not allowed to "stand out more" than the equivalent prices by metric units. Since the "Metric Martyrs" case in 2001, in which a group of market traders were convicted for using only imperial units, this law has not been rigorously enforced. Ahead of the 2019 General Election, the prime minister, Boris Johnson, pledged to change the rules (if elected to push Brexit through), saying, "We will bring back that ancient liberty. I see no reason why people should be prosecuted."

The UK government started a consultation on 3 June 2022 on the choice of units of measurement for consumer transactions. However at the time some of the British public believed that the consultation was biased in favouring imperial units. The result of the consultation was published on 27 December 2023. The report showed that 98.7% of 100,938 responses preferred the metric system; 17.6% wanted a purely metric system whereas 81.1% opted for the status quo (being as per the current legislation, which is imperial units can be displayed only as optional, less prevalent, supplementary units). Only 0.4% would prefer to return to a purely Imperial system. It was reported that the prime minister Rishi Sunak had abandoned Johnson's proposal to allow the sale of goods in Imperial units.

On 27 December 2023 the Government updated its website to further promote the use of the optional imperial units as supplementary units. Further, the Government advised that it would introduce a new directive in 2024 to allow wine to be sold by the imperial pint (568ml), however industry advised that companies were unlikely to adopt the pint unit.

Advocacy groups

A number of advocacy groups exist to promote either the metric or the imperial system. The groups include:

• Active Resistance to Metrication, founded by Eurosceptic politician Tony Bennett, is best known for its direct action campaign against metric signs.
• The British Weights and Measures Association campaigns for the retention of imperial measurements in the United Kingdom.
• Metric martyrs, a group that campaigns to be able to sell goods in any chosen measurement system.
• The UK Metric Association campaigns for the complete replacement of the imperial measurement system with the metric system in the United Kingdom.

See also

• 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

Notes

1. A sheet of A4 paper has an area of 0.0625 m (i.e. 1⁄16), A3 an area of 0.125 m (i.e. 1⁄8) ... and A0 an area of 1 m.

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• Metrication in the United Kingdom