Metrication in the United Kingdom

[3] 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.

[5] Having difficulties in communicating with German scientists, the Scottish inventor James Watt, in 1783, called for the creation of a global decimal measurement system.

[6] 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.

[11] 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.

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".

[25] 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 favoured the board being made up of part-time members drawn from commerce and industry, with government, education and consumer interests also being represented.

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.

[41] 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.[30]: para.

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.

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.

[56] 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.

21 , 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 its final report [1980], 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.

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.[31]: para.

2.14-2.15 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.

During the 1990s, a series of statutory instruments relating to weighing devices and to the sale of pre-packaged goods were issued[70] to ensure that United Kingdom law on metrology was harmonised with that of its EEC partners.

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.

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.

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.

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.

[78] 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.

[82] Due to no longer being bound by the regulations of the EU common market, the UK Government sought public opinion on changing the current systems of measurement when buying or selling goods.

[86] It was reported that the prime minister Rishi Sunak had abandoned Johnson's proposal to allow the sale of goods in Imperial units.

[105] Studies of the British metrication programme included two by US government agencies: NASA in October 1976[106] and the National Bureau of Standards in April 1979.

[107] 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.

[107] Reaction to the UK Metric Association report A Very British Mess (2004),[108] the executive summary of which was published in Science in Parliament,[109] 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",[110] while the Economist said that retreat [to the imperial system] was impossible and the current impasse costly.

[114] 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: Since 1 January 2010, UK law currently requires metric units to be used for all trade purposes with only limited exceptions, the remaining non-metric units, allowed by UK law without supplementary indicators [116] for economic, public health, public safety or administrative use, are limited to: 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.