Metrication in the United Kingdom

Metrication in the United Kingdom
The metric and imperial systems of measurement are used side by side in the United Kingdom: Eurostar's design speed is cited both as "300 km/h"[1] and as "186 mph".[2]

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

Proposals that the British people should use a decimal system date back to at least the 1668 paper of John Wilkins - the earliest known such proposal in history. The adoption of metric units has also been discussed intermittently by Parliament since 1818. A formal policy of metrication started in 1965. As of 2011, metrication in the UK is partial, with imperial units remaining in common and widespread use.

In its accession treaty to the European Economic Community in 1973, the United Kingdom 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 permitted for a limited period only, but that period was extended a number of times. In 2007, rather than extend the cut-off date for the use of supplementary units again, the EU announced that supplementary units could be used indefinitely.[3]

As of 2011, imperial units are still used to describe, amongst other things: body measurements, journey distances and vehicle speeds. Vehicle fuel economy is described in terms of "miles per gallon" even though fuel has not been sold in gallons since the early 1980's. The pint is used when referring to beer or cider consumption, and in association with milk usage. At school, pupils are taught metric units in subjects, and in mathematics they are also taught "rough metric equivalents of imperial units still in daily use", and conversion between metric and imperial as a mathematical exercise. Metrication has been opposed and even resisted by members of the British public.



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.[4] 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.

Gunter's chain - one of Britain's earliest decimal-based measuring devices, each link being 0.001 furlongs, greatly simplified the measurement of land area

This period marked the Age of enlightenment when people started using the power of reason in order 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.[5]

A decimal system of measurement was part of Wilkin's philosophical language published in 1668

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.[6][7] 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 fundemental 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.[8] 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 Parliamt, 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.[9] 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.


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 [10] 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 and having a diameter of 28 millimetres.

In 1854 Wrottesley set up the "Decimal Association" in order 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).[11] 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.[12]

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.[13] 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.

Joule's Heat Apparatus, 1845

While the politicians were discussing whether or not to adopt the 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.[14]

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)[15] 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)[16] 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.[17][18]

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.[18][19][20] The CGS system became the basis for scientific work for the next seventy years.

Danish prototype kilogram: British firm Johnson Matthey is the principal supplier of such prototypes.

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[21] 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.[22] 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.[23][24] 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.[25]

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 £100m alone.

1945 to 1973

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.[26] 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).

Metrication logo of the Board of Trade

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.[27]

Metric Britain logo, Metrication Board

A Commons debate in 1970[28] 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.[29]

1973 onwards

Before the UK joined the European Community in 1973, the only units of measure that were legally defined were those used in trade - length, area, volume, "mass or weight" [sic] and electrical units. Scientists and engineers were free to use whatever units they saw fit.[30]

When the UK joined the European Community (now the European Union) it was obliged to accept into its national law within five years all EEC directives that were then in force. This included directive 71/354/EEC.[31] This directive catalogued units of measure that might be used and for "economic, public administration, public health or public safety purposes". It also catalogued those units whose use was permitted only until the end of 1977. For many EU countries, these directives meant dropping the local equivalent of the pound in favour of "500 g", but in the case of the United Kingdom and the Republic of Ireland it meant the completion of their metrication programmes which in the case of the United Kingdom had been under way since 1965. It also required the United Kingdom 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), power (watt) and so on.

The UK government renegotiated this date, first to 31 December 1989, then 1994, 1999 and 2009. 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.[32] 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.[32]

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[citation needed]. 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.[33] 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"[34]; 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.[35]

Legal requirements

An example of metrication of UK consumer products. Two of the four items are in rounded metric amounts. The milk is labelled "1.136 litres (2 pints)" and the sausages are labelled "340 g (12oz)"

Since 1 January 2000, most loose goods sold by weight, volume or length must, by law, be measured and sold by metric units, with the display of imperial units being permitted as "supplementary indications", on the condition that they are written in characters no larger than the metric measurement.[36]

Non-metric units, allowed by UK law for economic, public health, public safety or administrative use from 1 January 2000, are limited to:

  • the mile (1.609 km), yard (91.44 cm), foot (30.48 cm) and inch (2.54 cm) for road traffic signs, distance and speed measurement,
  • the pint (568 ml) for the dispensing of draught beer and cider, and for the sale of milk in returnable containers,
  • the troy ounce (~31 g) for transaction in precious metals.[36]

Goods and services sold by a description 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 x 4 inch photograph frame, but a pole sold with the price described as "50 pence per linear foot", with no accompanying metric price, would be illegal.

There are no plans to change road signs and road speed measurement to metric units. On 23 February 2006 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.[37]

Current usage

Many aspects of life have been metricated either totally or partially; including industry, building, education and some sports such as rugby union. Many remain without visible evidence of metrication where Imperial units are used or even mandated[citation needed], including road signs, estate agents' advertisements and the non-specialist media.[citation needed] Trade is substantially metric.[38]


Most radio and television weather reports give temperatures in degrees Celsius, sometimes with approximate conversions into degrees Fahrenheit. Snow and rainfall are generally reported in metric units, again sometimes with the inch equivalents, particularly for extreme weather events. Wind speeds are generally given in miles per hour or knots.


Many wholesale markets are almost entirely metric: the meat industry trade press quotes prices in metric units,[39] agricultural products are traded by the tonne or the kilogram[40] lot sizes at both the London-based NYSE Euronext Futures and Derivatives market[41] and the London Metal Exchange[42] are denominated in tonnes rather than imperial units, but others such as the oil industry use US dollars per barrels (42 US gallons/158.99 litres).[43]


There is a legal distinction between

  • price marking - in general - where a unit is used as part of the price calculation (e.g. £1 per kg), it must be metric. Other units are permitted subject to a few constraints.
  • container labels - in general - where a unit is used to indicate container size, it must be metric. Other units are permitted subject to a few constraints.
  • container (or delivery) size - in general - where a unit is used to indicate container size, it must be metric. Other units are permitted subject to a few constraints.

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, 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 certain produce in round imperial measures again.[44][45] 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".[46] 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.[47]


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 13 pint (190 ml) (rarely encountered), 12 pint (284 ml) and multiples of the latter.[36][48] Bottled and canned beer is most often sold in 250ml, 330ml, 440ml and 500ml containers, the uneven metric 568ml (1 pint) containers are becoming rare now.


Cosmetics and toiletries are often labelled both in metric units (grams or millilitres) and US customary units (ounces or US fluid ounces); this is standard practice throughout the world 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).


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.


Since 1974, the metric system has been the primary system of measurement taught in schools.[49] 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 everday 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.[50]

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.[51]

Public administration

The civil service is bound by law to follow EU directives relating to public administration.[52]


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.[53] A metric National Grid was used as the basis for maps published by the Ordnance Survey since World War II.[53] A metric grid was used by War Office maps from 1920 onwards.[54]

News media

Journalists are not bound by any legal requirements over use of measurement systems and newspapers styles vary. While The Guardian'[55], The Times[56] and The Independent[57] prefer metric units in most circumstances, and 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 The Guardian and The Times 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 Daily Telegraph stylebook uses "imperial measures except where for accuracy's sake - as in some scientific or foreign story, or one detailing the calibre of armaments - metric is appropriate."[58] 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".[59]

Road transport

Road system

A British 50 mph speed limit sign
Driver location sign, and post, both indicating 415.0 kilometres

Road signs in Great Britain are regulated by Traffic Signs Regulations and General Directions (TSRGD)[60] which specifies the design and the units of measure for the signs. Distance signs are specified with miles or yards as the allowable units, height limit and width limit signs are specified to use feet and inches with metres allowed as optional supplementary indicators and vehicle length signs allow just feet and inches. Weight limits are expressed in tonnes and the legislation permits either "t" (the SI symbol for tonnes[61]) or "T" (clashing with the SI symbol for teslas[61] to be used as the abbreviation for "tonnes" on road signs. 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.[61] When SI units are used, such as metre on height and width 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) in order to comply with the TSRGD requirement for yards to be used on such road signs.[62]

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. 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,[63] 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." [64]

As of September 2011, no legislation resulting from that part of the consultation that deals with metric signs has been put to Parliament.

Since the late 1960s, British roads have been designed using metric units. Location marker posts are erected at 100-metre intervals [65] 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.[66][67] 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.[citation needed] Fuel consumption is still commonly quoted in miles-per-imperial gallon. Legislation requires that the offical fuel economy guide from which advertisers may quote must catalogue "fuel consumption ... in [either] 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".[68]

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).[69][70]

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.[71]

Rail transport

An 1845 Act of Parliament[72] fixed British rail gauges at 4 ft 8½ in and Irish rail 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[73]) - 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.[73] (The British loading gauge is specific to Britain). However, track distances of most of Britain’s rail network are shown in miles and chains and speeds and speed limits are in miles per hour, though metric units are used on the London Underground,[74] Channel Tunnel Rail Link, Tyne and Wear Metro, and Croydon metro.

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.[75][76]


The metric system is now used in the majority of industries.[citation needed] 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):

  • The UK Metric Association campaigns for the complete replacement of the imperial measurement system with the metric system in the United Kingdom.[81]


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[82] 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.[83] In contrast, the United Kingdom Metrication Association, in a report published in 2006[84] and using a model based on the Irish road sign metrication program[85] 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 reproted recouping their costs within a year as a result of improved production.[86] 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 [87]), 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


  1. ^ "Channel Tunnel Rail Link gets Royal Assent". Railway Gazette Intenational. 1 February 1997. Retrieved 17 October 2011. 
  2. ^ "High-speed rail line switched on". BBC. 8 January 2007. Retrieved 3 April 2010. 
  3. ^ Metric martyrs take a mile, The Guardian 9 May 2007.
  4. ^ "Weights & Measures". Scotlands People (A partnership between General Register Office for Scotland, the National Archives of Scotland and the Court of the Lord Lyon). Retrieved 26 October 2011. 
  5. ^ Watson, C. M. (1910), British Weights and Measures, London: John Murray, pp. 77–81, .
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