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Metrication in the United Kingdom is the process of introducing the metric system of measurement in place of imperial units in the United Kingdom.

The adoption of metric units has been discussed intermittently by Parliament since 1818. A formal policy of metrication started in 1965. As of 2012, metrication in the UK is partial, with imperial units remaining in common and widespread use. Most regulated selling by weight or measure is conducted in metric units.

In its accession treaty to the European Economic Community (EEC) in 1973, the UK was obliged within five years to incorporate into domestic law all EEC directives including the use of metric units for many purposes. By 1980 most pre-packaged goods were sold by metric measure, but the mandatory use of metric units for packaged goods only took effect in 1995. Mandatory metric measures for goods sold loose or from bulk began in 2000. The use of "supplementary indications" (Imperial units given alongside the metric) was originally to be permitted for a limited period only, but that period was extended a number of times and eventually permission was granted indefinitely.

In a survey in 2007, 56% opposed a change to complete metrication, 19% supported it and 22% said they neither opposed nor supported it. Imperial units are still commonly used to describe body measurements, journey distances and vehicle speeds, and vehicle fuel economy is described in terms of "miles per gallon" even though fuel has not been sold in gallons since the early 1980s. The pint is used when referring to draught beer or cider-by-the-glass consumption, and often in association with bottled milk usage. At school, pupils are taught metric units and rough metric equivalents of imperial units that are still in daily use .

History

Before 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 pint and gallon being more than twice the size of their English counterparts. This situation continued until 1707 when, under the Act of Union, the Parliaments of England and Scotland were merged and the English units of measure became the standard for the United Kingdom.

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 (one tenth of a furlong) in length and consisted of 100 links, making each link 0.001 furlongs. 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 decimals of an acre.

In 1670, John Wilkins, the first president of the Royal Society, published his proposal for a decimal system of measure in his work An Essay towards a Real Character and a Philosophical Language. His proposal envisaged a system of units in which the base unit of length was defined by a pendulum that had a period of one second and that the base unit of mass was defined by a cube of rainwater having sides equal to the base unit of length. He recycled the existing names of units of measure so that there were 10 lines in an inch, 10 inches in a foot and so on. A century later his concept of defining unit mass in terms of a cube of water with edges of unit length was one of the fundamental concepts of the metric system.

In finding difficulties in liaising with German scientists, the British 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 proposed by Wilkins) 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 Système International d'Unités and is the sole measurement system for most of the world.

1799–1945

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In 1799 the French adopted the metre and the kilogram as their new units of length and mass. As use of the new system, originally called the "Decimal System", grew through Europe, pressure grew in the UK for decimalisation. The issue of decimalisation of measurement was intertwined in the UK with decimalisation of currency. The idea was first discussed by a Royal Commission that reported in 1818 and again in Parliament by Sir John Wrottesley in 1824. Another Royal Commission was set up 1838 by Chancellor of the Exchequer Thomas Spring Rice and it reported in 1841 that decimal coinage was required first. A third commission advocated in 1853 decimal coinage in the form £1 : 10 florin : 100 cent : 1000 mil. The first florins (one tenth of a pound sterling) were struck in 1849 as silver coins weighing 11.3 grams (0.40 ounces) and having a diameter of 28 millimetres (1.1 inches).

In 1854 Wrottesley set up the "Decimal Association" to lobby for decimalisation of both measurement and coinage. An early supporter of the Decimal Association was the mathematician Augustus de Morgan whose articles supporting the metric system had been published in the Penny Cyclopeadia (1833) and The Companion to the Almanac (1841). A few days later Wrottesley met with Gladstone, then Chancellor of the Exchequer, but was unable to win him over to the idea. 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 the House of Commons passed a law by 110 votes to 75 mandating the use of the metric system throughout the Empire, but due to lack of parliamentary time the bill was not debated in the House of Lords and so did not become 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. However, ambiguous wording of the 1864 law meant that traders who possessed metric weights and measures were still liable to arrest under Acts 5 and 6 William IV c63.

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 James Prescott Joule published a paper in which he 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.

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 tasked with "the Selection and Nomenclature of Dynamical and Electrical Units" recommended using the CGS (centimetre-gram-second) system of units. The committee also recommended the names of "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 which resulted in the setting up of the 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.

In 1896 Parliament passed the Weights and Measures (Metric System) Act, 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 opposition declared that decimalisation of coinage would cost £100 million alone.

1945 to 1973

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The matter was dropped in the face of wars and depression, and would not be again raised until the 1951 Hudgson Report, the result of yet another Select Committee which unanimously recommended compulsory metrication and currency decimalisation within 10 years. It said "The real problem facing Great Britain is not whether to adhere either to the Imperial or to the metric system, but to maintain two legal systems or to abolish the Imperial." The report also recommended that the change should be done in concert with the Commonwealth (former Empire) and the USA. It also pointed out that metric standards were more accurate than Imperial ones, and that the yard and pound should be pegged to definite metric values. This was done by international agreement in 1959 and currently the yard is defined as 0.9144 metres exactly, and the pound as 0.453 592 37 kg exactly. Agreement could not be reached on the pint (and gallon), and this value still differs between the UK and US (the only countries that maintain legal definitions of these units).

In 1965 the Board of Trade and the Confederation of British Industry declared their full support for metrication and decimalisation. Currency decimalisation finally took place on Decimal Day, 15 February 1971, although £1 did not change in value. The Metrication Board was set up in 1969. Unlike its South African and Australian counterparts which had mandatory powers, it only had an "advisory, educational and persuasive role". Metric units have been taught in UK schools since the late 1960s (and exclusively since 1974), and certain industries also converted or largely converted decades ago. For example the paper industry converted in 1970, and the construction industry between 1969 and 1972 – although certain products continue to be produced to with reference to Imperial trade names but made using metric dimensions in the factory; for example, a 13 mm thick plasterboard is still often called 'half-inch', even though the measurement is rounded to a convenient metric size and so is now only approximately half an inch thick.

A Commons debate in 1970 on the introduction of compulsory metrication ended in farce. The governing Labour party was then unpopular and the opposition Conservatives revolted on the issue. Examples include these Conservative MPs' speeches:

• Robert Redmond: "When I have travelled abroad and particularly on the Continent, I have noticed that people have on their desks calculating machines while we in Britain do the same sums in our heads." (In 1970 the United Kingdom still used pounds, shillings and pence).
• Henry Kerby: "This metric madness, this alien academic nonsense, introduced secretly through the back door by a bunch of cranks and the big business tycoons... and put into clandestine operation."
• Carol Mather: "I am led to the conclusion that comprehensive universal metrication is a bit of a nonsense... there is a gap between the millimetre and the metre, there is no centimetre.... The kilo is too heavy for the housewife to carry and we know that in France and Denmark they use the old system of the pound."

The press reports on the debate, particularly those of The Daily Telegraph and The Times, were very favourable to the opinions of the Conservatives. Following the debate the projected deadlines for the phased metrication steps were delayed one by one. The original intention of metrication "in concert with the Commonwealth" backfired; Australia, New Zealand and South Africa all completed their metrication processes by 1980, the year that the Metrication Board was abolished as a cost-cutting measure. (In contrast, the situation of metrication in Canada resembles that of the UK, except that all road signs were converted in the 1970s.) The last laws which restricted the sale of metricated goods were only removed in 1995; though it is still illegal to sell draught beer in metric units, which in 2002 led to an Austrian-themed pub being asked to stop selling beer by the half litre traditional German steins.

1973 onwards

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Although in the United Kingdom a metrication process had already been committed to in 1965, before 1973, the year that the UK joined what was then the European Economic Community (EEC), the only units of measure that were legally defined were those required to be used in trade - length, area, volume, mass or weight and electrical units. For all other purposes, including in science and in engineering, there was a free choice of which units to use.

As a condition of joining the EEC, the UK had agreed to introduce into its national law, within five years, all EEC directives that were then in force. This included directive 71/354/EEC; the directive that catalogued units of measure that should be used for "economic, public administration, public health or public safety purposes". It also catalogued other units which could only continue to be used until the end of 1977. For many EEC countries, these directives only meant dropping the local equivalent of the pound (usually a named unit defined as equal to 500 g), but for the United Kingdom and Ireland it meant replacing many traditional and customary units with metric units for certain specified applications. Membership of the EEC also required the UK to formally define a number of other units of measure including those for electric current (ampere), electric potential difference (volt), temperature (degree Celsius and kelvin), pressure (pascal), energy (joule) and power (watt). In susequent years, the UK government renegotiated the 1977 cut-off date, getting it changed first to 31 December 1989, then 1994, 1999, 2009 and finally in 2007 they got it abolished altogether.

The involvement of the European Commission led metrication to be linked in public debate with Euroscepticism, and traditionally Eurosceptic parts of the British press have taken a dim view of the process, often exaggerating or inventing 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.

The European Union Units of Measurement Directive as amended by Directive 89/617/EEC required the UK government to pass laws in 1994 finally permitting the sale of goods using metric labelling, while permitting dual measurement. Public reaction to these regulations was negative . Such suspicion of externally imposed change has long traditions; as Philip Grierson notes, the town of Lincoln paid "lavish" fines in 1201 rather than use government-imposed reformed weights and measures. Steve Thoburn applied to the European Court of Human Rights on the basis that his human rights had been violated but the court decided that no violation had occurred. George Gardiner of the Federation of Small Businesses called (without apparent response) for a civil disobedience campaign. In 1999 further laws were brought metricating the sale of, among other things, fresh fruit. The "Metric Martyrs" were shop owners that were fined for refusing to use metric units.

In August 2005, the European Commission announced it would require Britain to set a legal deadline for the completion of metrication. In January 2007 the Department of Trade and Industry announced that "the Government intends to support the continued use of supplementary indications after 2009 for an indefinite period"; on 9 May 2007 European Commission Industry Commissioner Günter Verheugen announced that the Commission had dropped its plans to enforce the abolition of Imperial measures from 2010, so that 'supplementary' imperial indications could continue to be used alongside, but not instead of metric units. On 10 September 2007 the European Commission published a proposed amendment to EU Directive 80/181/EEC that would permit "supplementary indicators" to be used indefinitely.

Legal requirements

For most activities and in most situations, there are no legal restrictions on, or legal requirements for, which specific system of units of measurement is used in the UK. For certain trading activities (the selling of certain loose goods priced by weight or measure for example) and for certain public administration activities (the wording of new laws and regulations for example), units from a specified system are legally required. Where the unit system is specified it is usually the metric system (for selling pre-packaged food sold by weight, for the selling of loose vegetables priced by weight and in the wording of new laws and regulations for example), although in some cases the imperial system is specified (for the dispensing of draught beer for example). Even when the unit system is specified, units from the other system can be used concurrently as so-called "supplementary indications" (pre-packaged sausages, which require metric system units at least to be used, can be marked "454 g (1 lb)" for example).

Popular opinion

A September 2007 Ipsos MORI telephone survey conducted for The Sun newspaper, entitled "Northern Rock, Metric Measurements And The EU Constitutional Treaty" found, of the sample questioned, that 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?":

• 11% strongly supported a switch to entirely metric measurements
• 8% tended to support a switch to entirely metric measurements
• 22% neither supported nor opposed
• 14% tended to oppose a switch to entirely metric measurements
• 42% strongly opposed a switch to entirely metric measurements

Detailed results in this survey showed net opposition by all demographic groups; with more than 50% net opposition by the following: Sun readers, aged 65+, Conservative voting intentions, and tabloid newspaper readers.

The Times reported in 2007 that the results of a survey by the British Weights and Measures Association (a group whose stated aim is "to protect and promote British weights and measures, and to oppose compulsory use of the metric system") returned the following results:

• 80% of people prefer imperial to metric
• 70% (including 18-24 year olds) can make sense of weights only in imperial measurements

Current usage

The United Kingdom is unique within Europe in that it has retained the imperial system of measurements. The population continue to resist metrication and the traditional imperial measures are preferred by the majority and continue to be in widespread common use. Many aspects of business, commercial and government and associated administrative activities have metricated either totally or partially; including manufacturing and building industries and education. The sport of rugby union has also metricated. Many activities remain without visible evidence of metrication where imperial units are used or even mandated, including road signs, estate agents' advertisements and the non-specialist media. Trade is substantially metric.

Weather forecasts

The BBC weather forecast website allows UK users to choose the units of measure to be used. The choices are between degrees Celsius and Fahrenheit for temperature, and between miles per hour and kilometres per hour for wind speed. Pressure is always expressed in millibars.

The move from Fahrenheit to Centigrade (as it was then referred to) was the subject of a Parliamentary question in 1971, and the reply referred to the increasing use and acceptance of the Centigrade scale since it had first been introduced into public weather forecasts issued by the Met Office in 1962.

In a February 2006 article in The Times, the writer suggests that the British tend to use centigrade values for low temperatures and Fahrenheit values for higher temperatures. The rationale being that -6°C sounds colder than 21°F and 94°F sounds more impressive than 34°C.

Commodities

The principal London commodity markets, apart from oil, are metric: the London Metal Exchange having gone metric in 1970, the various agricultural markets which completed their metrication program in 1970 as is the commodities derivatives market LIFFE which was founded in 1982.

The oil industry however uses US dollars per barrels, which is 42 US gallons (159.0 litres; 35.0 imperial gallons).

Energy

Domestic gas is billed in kilowatt hours (kWh) - although gas meters only record the volume of gas used, which is later converted to kWh. All newly-installed gas meters record usage in cubic metres, but many older installations will measure in cubic feet.

Retail

The extent to which various retailing activities have been metricated in the UK varies from not at all, as in the case of draught beer and cider, for example, to partial (metric measures must be used and imperial measures may be used concurrently), as for example, in the case of pre-packaged food sold by weight.

Groceries

While several food items continue to be packaged and sold in sizes based on Imperial units, many now display only the uneven metric equivalent value on the label (such as 454 or 907 grams for 1- or 2-pound packages). Items include jam, marmalade, honey, dates, strawberries, sausages, beefburgers, fresh coffee, malt vinegar, and Christmas puddings.

Major supermarket chains continue to sell own-labelled cow's milk in 1-, 2-, 4-, and 6-pint plastic bottles with the uneven metric equivalent value always shown before the Imperial volume (if that is shown at all). However these same chains sell own-labelled other milks (goat milk, buffalo milk etc) plus most of the dairy-labelled cow's milk in litre-based units, and have been doing so for many years. However, they are gradually introducing more litre-based bottles (e.g. 1, 2, or 3 litres) under their own label as well. In other shops, such as newsagents and convenience stores, milk is usually sold only in litre-based units.

In May 2011, the Asda supermarket chain stated that consumer research had shown that 70% of their customers found metric confusing and would prefer products to be labelled in imperial units. As a result, they were beginning to experiment with selling strawberries by the pound again – and to do the same with other fruit and vegetables if there was enough shopper demand.

In 2000, the Tesco supermarket chain began selling produce in imperial, stating that their survey of 1,000 customers had shown that 90% of their customers "still used imperial measures in their heads". Tesco's use of imperial units over metric, with prices per pound displayed more prominently that those per kilo, was identified in a 2004 Which? magazine report criticising supermarket pricing tactics, as a possible means of appearing cheaper than its rivals.

Alcohol

In the United Kingdom, draught beer and cider are the only goods that may not be sold in metric units; the only legal measures for these drinks when sold on draught are 1⁄3 pint (190 ml) (rarely encountered), 1⁄2 pint (284 ml) and multiples of the latter. Bottled and canned beer is most often sold in 250 ml, 330 ml, 440 ml and 500 ml containers, the uneven metric 568 ml (1 pint) containers are becoming rare now.

Cosmetics

Cosmetics and toiletries are labelled in metric units (grams or millilitres) but some are additionally labelled with US customary units (ounces or US fluid ounces); this is standard practice for goods intended for importation to the US, where dual labelling is compulsory. The US fluid ounce is 4% larger than its imperial counterpart (29.6 ml as against 28.4 ml).

Clothes

Clothing is usually sold and marketed in inches and UK sizes, with the centimetre dimensions and continental size increasingly shown alongside the Imperial, with equal prominence. Shoes are most often seen with traditional British sizes (though the Paris point sizes (exclusively) are not rare).

Consumer electronics

Dual measures are often seen in the home entertainment and computer markets, for describing television, digital camera and monitor screen sizes. (The imperial size given for CRTs is typically that of the tube, whereas the metric measure – tagged 'vcm' – is that of the visible screen excluding the bezel). Products that may appear to be Imperial are actually manufactured to metric specifications, using metric drawings and made on metric machines, even if references to Imperial units persist in some areas.

Education

Since 1974, the metric system has been the primary system of measurement taught in schools. In the National Curriculum for England, metric is the principal system of measurement and calculation. However, pupils are expected to know how to convert between metric and imperial units still in everyday use, specifically citing "pounds, feet, miles, pints and gallons", and conversion between the two systems is given as an example of numerical problems students should be able to solve.

National Health Service

On 25 February 2010, concern was expressed in the House of Lords that 30% of scales used in National Health Service hospitals and facilities for weighing patients were switchable between metric and imperial units and that 10% were permanently set to imperial units. Since drug doses are worked out using patients' weights in kilograms, the use of Imperial or switchable scales risks giving the patient the wrong dose. The Government announced it was taking steps to remedy this situation and insist that all NHS facilities complied with the requirement that all weighing scales were properly calibrated and maintained and displayed only metric units.

Information dissemination

The principal channels for dissemination of information are the press and government agerncies. The approach taken to metrication by writers in various disseminators of information varies considerably. The civil service is bound by law to follow EU directives relating to public administration while journalists are not bound by such restrictions.

Government Organisations

Government disseminators of information include the Office for National Statistics and the Ordnance Survey office both of whom, being government departments, use metric units in their work. The first Ordnance Survey maps with metric values and scales to be produced were the large scale maps which were required by the construction industry following its committment to metrication, and were introduced from 1969 onwards. A metric National Grid was used as the basis for maps published by the Ordnance Survey since World War II. A metric grid was used by War Office maps from 1920 onwards.

Journalism

Newspapers styles vary. While both The Guardian' and The Times prefer metric units in most circumstances, and both provide exceptions where imperial units are preferred, they differ on some details.

The Times specifies that heights and weights put Imperial measures first while the Guardian's examples are from metric to Imperial. Similarly, while both give first place to hectares, the Guardian prefers square kilometres (with square miles in brackets) while the Times prefers square miles. Both retain the preference for the mile in expressing distances. In contrast, The Economist prefers metric units for "most non-American contexts," except for the United States section where "you may use the more familiar measurements." However, The Economist also specifies "you should give an equivalent, on first use, in the other units".

Unit of measure inconsistencies

These rules of style have led to inconsistencies between administrative documents and the resulting news reports. Examples include:

• On 18 March 2005 Johnson Beharry was awarded the Victoria Cross for valour while serving in Iraq. The official citation included the text "...drive the vehicle through the remainder of the ambushed route, some 1500m long". The BBC, in paraphrasing the citation, used the expression "He guided the column through a mile of enemy ground".
• The Channel Tunnel Rail Link carries traffic from the Channel Tunnel to London. Government reports cited the design speed on the link as being 300 km/h while the BBC cited speeds of 186 mph.

Mapping

The Ordnance Survey, the UK's national mapping agency for Great Britain, initiated the Retriangulation of Great Britain in 1936, using metric measures from the start. A metric National Grid was then used as the basis for maps published by the Ordnance Survey from World War II onwards. A metric grid had been used for War Office maps from 1920 onwards.

The first Ordnance Survey maps with metric values and scales to be produced were the large-scale maps which were required by the construction industry following its commitment to metrication, and were introduced from 1969 onwards. Ordnance Survey completed the replacement of its best selling 1 inch to the mile range of maps with the 1:50000 (two centimetres to the kilometre) range in 1974.

Road transport

Road system

Road signs in Great Britain are regulated by Traffic Signs Regulations and General Directions (TSRGD) which specifies the design and the units of measure for the signs. Distance signs are specified with miles or yards as the only allowable units. Height limit, width limit and vehicle length limit signs are required to use feet and inches, but with metres allowed as optional supplementary indicators. Weight limits are expressed in tonnes: the 2011 legislation now correctly requires "t" as the symbol to be used for tonnes on road signs ("18 t" for example). Earlier legislation had also allowed the use of "T" to represent "tonnes" ("7.5 T" for example), so older signs using this notation are also in use. Speed limits are in miles per hour with no units shown on the signs. Advance-warning signs display distances in miles often using the character "m" as an abbreviation (clashing with the SI use of "m" as the symbol for metre. When SI units are used (such as metres on height, width and length restriction signs if the optional metric-measurement is given) the SI symbol "m" is correctly used.

Advance-warning signs for road works and other temporary road obstructions are generally positioned at multiples of 100 metres from the feature to which they refer, with the distances indicated in yards - to the nearest 100 yards (which is within the 10% tolerance allowed) to comply with the TSRGD requirement for yards to be used on such road signs.

In TSRGD 1994 the legislation included the allowance of metric units as "supplementary indications" for many (but not all) height limit warning and prohibition signs. Schedules 16.1 and 16.2 of the TSRGD 2002 catalogue the signs that may display metric units in addition to imperial units: maximum headroom warning signs and height, width and length prohibition signs. On 23 February 2006 the Secretary of State for Transport Alistair Darling said on the BBC Question Time programme that the Government had abandoned its previously long-standing plans to convert the UK's 2 million road signs to metric, due to the cost.

In late 2009 and early 2010, the DfT proposed modifying the legislation to make it mandatory to use dual units signs for height and width limit warning and restriction signs, as it was believed that this would reduce bridge strikes. The analysis noted that

"approximately 10–12% of bridge strikes involved foreign lorries. This is disproportionately high in terms of the number of foreign lorries on the road network."

In December 2011, some amendments to legislation resulting from that part of the consultation that dealt with metric signs have been put to Parliament in the shape of the TSRGD Amendments 2011. This came into force late January 2012.

Since the late 1960s, British roads have been designed using metric units. Location marker posts are erected at 100-metre intervals on the hard shoulder giving the distance from a notional reference point in kilometres to enable maintenance workers, emergency services and the like to pinpoint specific points on the motorway. The digits on these posts were barely visible to motorists. This number was also encoded into the emergency phones that could be used by stranded motorists. The advent of the mobile phone meant that the location of motorists could no longer be pinpointed by reference to the emergency telephone that they were using. To enable such motorists to communicate with the emergency services, driver location signs were erected at approximately 500-metre intervals in England during the period 2007 to 2010. These signs replicate the distances shown on the smaller location marker posts though no units are shown, but don't appear (yet) on Welsh motorways.

Motor vehicles

Motor fuel has been retailed in litres since the 1980s with news media commonly referring to fuel prices in pounds-per-gallon. Fuel consumption is still commonly quoted in miles-per-imperial gallon. Legislation requires that the official fuel economy guide from which advertisers may quote must catalogue "fuel consumption ... in litres per 100 kilometres (l/100km) or kilometres per litre (km/l), and quoted to one decimal place, or, to the extent compatible with the provisions of Council Directive 80/181/EEC ... in miles per gallon".

Almost all motor vehicles first used on public roads on or after 1 April 1984 are required to have speedometers fitted which can display speeds in both miles per hour and kilometres per hour (simultaneously or separately).

Metric units (kW for power, km/h for speed, kg for weight and cc for engince capacity) are used in legislation relating to driving licences.

Metric units are used in legislation relating to vehicle emissions (grammes of CO₂ per km), which affects vehicle taxation bands, and entry requirements to low emissions zones.

Rail transport

An 1845 Act of Parliament fixed British track gauges at 4 ft 8½ in and Irish track gauges at 5 ft 3 in. The 4 ft 8½ in gauge was the basis of 60 % of the world's railways, but is expressed as 1435 mm (including the United Kingdom) - a decrease of 0.1 mm, but well within the engineering tolerances. The Irish 5 ft 3 in gauge is now referred to as a 1600 mm gauge – the difference between the metric and imperial values being 0.2 mm, again well within engineering tolerances.

Metric units are used throughout for engineering purposes and rolling stock is designed using metric units as it is required to meet the loading gauge requirements (most of which are specific to Britain). Track distances of most of Britain's rail network are shown in miles and chains, with speed limits in miles per hour, although lineside signs and in-cab computer displays are now metric on routes where the latest 'ERTMS' signalling system has been installed and on the Channel Tunnel Rail Link. Metro and light-rail systems such as the London Underground, Tyne and Wear Metro and Croydon metro also operate using metric.

Air and shipping

Aviation uses the foot for aircraft altitude and both aviation and shipping use nautical miles and knots for distance and speed respectively.

Industry

The metric system is now used in the majority of industries. The coopers' trade is one of the exceptions to this rule. The print industry works with a wide variety of measurements, including paper size, thickness and weight, typographical measurements, pitch and size of holes, which may be imperial, metric or other.

Advocacy groups

A number of advocacy groups exist to promote the metric system at the expense of the imperial system and vice versa. The groups include (in alphabetic order):

• 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.

• The UK Metric Association campaigns for the complete replacement of the imperial measurement system with the metric system in the United Kingdom.

Costs

The costs of metrication in the UK have not been reliably calculated. True scientific calculations of the potential costs have been fairly rare, and tend to refer to specific proposals.

A 2005 report pointed to the metrication of the UK's 2 million road signs as the major cost of completing the United Kingdom's metrication program. The Department for Transport (DfT) costed the replacement of all of the United Kingdom's road signs in a short space of time at between £565 million and £644 million In 2008–09, before the outcome of the consultations that led to the EU directive 2009/3/EC was known, the DfT had a contingency of £746 million for the metrication of roads signs. In contrast, the United Kingdom Metrication Association, in a report published in 2006 and using a model based on the Irish road sign metrication program estimated the cost of converting road signs to be £80 million, spread over 5 years (or about 0.25% of the annual roads budget).

A 1970s study by the UK 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. Some 90% of UK exports go to metric countries (as only Liberia, Burma and the United States have not adopted the International System of Units ), and there are costs to business of maintaining two production lines (one for exports to the US in Imperial, and the other for domestic sales and exports to the rest of the world in metric). These have been estimated at 3% of annual turnover by the Institute of Production Engineers, and at £1100 million (1980) per annum by the CBI. Regardless of UK metrication, goods produced in the UK for export to the US would have still been labelled in non-metric units to comply with the US Fair Packaging and Labelling Act.

See also

• International System of Units
• Metrication opposition
• Imperial system

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Bibliography

1. Grierson, Philip (1995). English Linear Measures: an essay in origins. 'The Stenton Lecture 1971', University of Reading.
2. McGreevy, Thomas (1995). The Basis of Measurement: Historical Aspects. Chippenham, Picton Pub. ISBN 0948251824.
3. McGreevy, Thomas (1997). The Basis of Measurement: Metrication and Current Practice. Chippenham, Picton Pub. ISBN 0948251840.

• Use dmy dates from August 2011
• Metrication by country
• Economy of the United Kingdom