Metrication of British transport

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The InterCity 225, shown here in the livery of the Flying Scotsman, was so named because its design speed is 225 km/h (140 mph).[1]

A metrication programme for the United Kingdom was announced by the government in 1965 at the behest of the engineering industry and received government approval on condition that its implementation was voluntary, that it was implemented on an industry-by-industry basis and that costs were absorbed where they fell. The metrication of transport related engineering and operational standards, including those used in vehicle design, took place over the following decades, though metrication of certain related sectors had already occurred before this.

Not all of British transport operational standards have been metricated – in the railway industry, although metric units and distances are used on many new systems, most pre-existing systems have retained imperial units, especially for speed and distance. For roads, the engineering of new roads will be to metric standards, but all road signs except those related to vehicle weight limits, use imperial units only, or for those related to vehicle dimension limits may use dual imperial/metric units. Nautical and aviation operational practice is to retain the use of the knot for speed, the nautical mile for distance and, in the case of aviation, the foot for altitude.


Historically, British industry opposed metrication on the ground that most of British exports went to countries that used the imperial or US customary systems of units. By the 1960s, changing trade patterns meant that this was no longer the case, and in 1965 the Federation of British Industry (now the Confederation of British Industry) initiated a change to the metric system.[2] The government agreed on condition that the changeover was voluntary on a sector-by-sector basis, that costs would be absorbed where they fell and that there would be minimal legislation.[3] In some cases it was appropriate to use hard metrication and in other cases soft metrication – soft metrication being defined as "the conversion of ... non-SI measurements to equivalent SI units, within the established measurement tolerances. In general, 'soft metric' products will not differ physically from analogous [non-SI] products.[4]

Until 1969, when the Metrication Board was set up, the Royal Society and the British Standards Institution took the lead. When the Metrication Board was set up, much of the groundwork had been done, and metrication of many engineering sectors, including transport, was under way. From the early to mid-1970s, the lack of compulsion slowed the process of metrication, so a draft order was prepared to complete the process. However, the Government, which had a very small majority, declined to proceed with the order.[5]

When Britain joined the European Economic Community in 1973, it was obliged to adapt local law to accommodate EEC directives that were in place.[6] One of these directives, 71/354/EEC, required a harmonisation of units of measure "for economic, public health, public safety or administrative purposes". By the late 1970s the British metrication programme had run out of steam and, at Britain's request, Directive 71/354/EEC was superseded by Directive 80/181/EEC, which among other things permitted the use of miles, yards, feet, inches and fathoms in the United Kingdom, and knots until 1989 (subsequently extended to 1994). As from 1 January 1990, the use of miles, yards, feet and inches was restricted to "Road traffic signs, distance and speed measurement[s]" only. The directive did not apply to areas of transport that were subject to governmental international agreements.

Road system[edit]

Road signs[edit]

A road sign warning of a low bridge with its distance given in imperial units and its height given in dual imperial/metric units

The regulations for road signs in Great Britain are published in Traffic Signs Regulations and General Directions (TSRGD) which specifies the design and the units of measure for the signs.[7]

Distance signs are specified with miles or yards as the only allowable units. Vehicle height, width, and length limit signs are required to use feet and inches, with equivalent dimensions in metres, typically to one decimal place, allowed as optional secondary information. In 2014, following a consultation by the DfT, it was announced that the use of dual units on height and width restriction signs would become mandatory as of March 2015.[8] The values on new weight limit signs are expressed in tonnes and since the introduction of regulation amendments in 2011, require the character "t" to be used as the symbol for tonnes, in accordance with the SI convention ("18 t" for example). Earlier regulations also allowed the use of the character "T" ("7.5 T" for example), so older signs using this notation are also in use; this notation conflicts with the SI symbol for the tesla. Speed limits are in miles per hour, most commonly with no units shown on the signs.

Advance-warning signs display distances in yards or miles, and can use the character "m" as an abbreviation for miles (conflicting with the SI use of "m" to denote the metre) and "yds" for yards.[9] When metres are used, such as on width and height restriction signs, the SI symbol "m" is also used. Advance-warning signs for road works and other temporary road obstructions may actually be positioned at multiples of 100 metres from the feature to which they refer, but with the distances indicated in round numbers of 100 yards as a 10% tolerance on distances is allowed in the regulations – since a yard is approximately 0.91 metres, such measurements fall within this 10% tolerance.[10]

In TSRGD 1994 the regulations allowed metric units to be given as secondary information for many, but not all, height-limit warning and prohibition signs. In Schedules 16.1 and 16.2 of the TSRGD 2002 catalogue, the signs that may display metric units in addition to imperial units are maximum headroom warning signs and height, width and length prohibition signs.

On 23 February 2006 the then 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.[11]

In late 2009 and early 2010, the UK Department for Transport (DfT) proposed modifying the legislation to make it mandatory to use dual-units signs for height and width limit warning and restriction signs,[12] as it was believed that this would reduce bridge strikes by vehicles originating outside the UK, because these vehicles are not generally required to carry in-cab signs informing the driver of their height in imperial units.[13] The analysis stated 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."[14] In December 2011, some amendments to legislation resulting from that part of the consultation that dealt with metric signs were put to Parliament in TSRGD Amendments 2011. This came into force in late January 2012.[15] At the same time, the DfT reconfirmed that there were no plans at all to replace miles with kilometres on distance signs.[13]

Road design[edit]

Road and bridge design are regulated by the Design Manual for Roads and Bridges (DMRB).[16] In the DMRB, design speeds for rural roads are quoted in kilometres per hour while road design for urban areas uses the actual posted speed limit in miles per hour.[16] All other road and bridge design standards use metric units.

Road maintenance and incident management[edit]

Location marker posts (small roadside posts giving location definition data for maintenance and incident management use) are installed on motorways and many trunk roads at 100-metre intervals.[17] These posts give a unit-less location index (actually the distance in kilometres from a defined reference point for the road). The digits on these posts are barely visible to motorists. The same location index information may also appear on roadside furniture such as lampposts and signal cabinets and be encoded into the emergency roadside telephones.

Increased use of personal mobile phones in emergency situations, rather than fixed, location index encoded, roadside telephones, meant that another way of pinpointing the location of motorists was required. For this purpose, large roadside driver location signs were installed at 500 m intervals[Note 1] on motorways and some trunk roads in England during the period 2007 to 2010.[18][19] These signs replicate the unit-less location indexes shown on the smaller location marker posts. As of 2012 these have not been installed on Welsh roads.[20]

Motor vehicles[edit]

Although motor fuel has been retailed in litres since the 1980s,[21] fuel consumption is still commonly quoted in miles per [imperial] gallon.[22]

Legislation requires that:[23]

  1. The fuel economy guide published by the government, which is a list of all new passenger car models available for purchase within the United Kingdom, must express "fuel consumption ... in [either] litres per 100 kilometres (l/100 km) 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".
  2. The fuel economy label, that dealers must place on or near each new car, must show the vehicle's fuel consumption in both miles per imperial gallon (mpg) and litres per 100 kilometres (L/100 km).
  3. The poster or display, that car dealers must exhibit in a prominent position, must display the value of the official fuel consumption expressed in miles per imperial gallon (mpg) and either in litres per 100 kilometres (L/100 km), kilometres per litre (km/L), or an appropriate combination of these.
  4. Where fuel consumption is shown on promotional literature for new cars, it must express the figures in miles per imperial gallon (mpg) and in either litres per 100 kilometres (L/100 km), or kilometres per litre (km/L) or an appropriate combination of these.

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).[24][25]

HMRC legislation requires any vehicle imported from the EU to have an odometer reading below 6,000 km (3,730 mi) in order to be registered as a new vehicle in the UK.[26]

Sign on a van in the UK showing a speed limiter set to 100 km/h

In line with EU directives, most new vehicles in the UK weighing over 3.5 t must be equipped with a speed limiter. Speed limiters across Europe are set to speeds in kilometres per hour. Warning signs placed on such vehicles in the UK may show the limited speed only in miles per hour. In addition, digital tachographs are legally required to be fitted to many categories of heavy vehicle; these instruments typically record distances and speeds in kilometres and kilometres per hour respectively.[27] [28]

Metric units — kilowatts (kW) for power, kilometres per hour (km/h) for speed, kilograms (kg) for weight and cubic centimetres ("cc", although the correct SI symbol is "cm3") for engine capacity — are used in legislation relating to motorcycle driving licences.[29]

Road transport[edit]

The regulations controlling lorry sizes in the UK use metric units. The general maximum gross vehicle weight (GVW) is 44 tonnes (lorry, fuel and load). The general maximum allowed vehicle length for rigid vehicles is 12 metres (39 feet 4 inches). For articulated lorries the maximum allowed length is 16.5 metres (54 feet 2 inches) (truck and trailer). Road trains can be up to 18.75 metres (61 feet 6 inches) long. The general maximum width for any vehicle is 2.55 metres (8 feet 4 inches). Vehicles with a height of 3 metres (9 feet 10 inches) or above must have a notice in the cab showing their full height.[30]

Freight containers are sized in feet; three common lengths are 20 feet (6.1 metres), 40 feet (12 metres) and 45 feet (14 metres). They are usually 8 feet (2.4 metres) wide and 8 feet 6 inches (2.6 metres) high.

Rail transport[edit]

Speed restrictions in km/h for the Tyne and Wear Metro (hexagonal sign) below mainline speed restriction signs in mph (round signs)

The main push towards using the metric system took place during the early 1970s.[31] In respect of the railway industry, the engineering aspects of the railways were metricated at that time, for example the Class 58 diesel-electric locomotive which entered service in 1982 and changes to the loading gauge resulting from the installation of 25 kV AC overhead line in the 1970s were specified using metric units.[32] However, at the time there was little or no change in the operational aspects of the railways.

Railway infrastructure[edit]

Metrication of the United Kingdom's railway engineering standards was a "soft metrication" exercise since anything else would have required a gauge conversion. An 1846 Act of Parliament fixed the track gauge for Great Britain at 4 ft 812 in and for Ireland at 5 ft 3 in.[33] The 4 ft 812 in gauge, now known as standard gauge, was the basis of 60% of the world's railways[34] and is now expressed as 1,435 mm[35] – a decrease of 0.1 mm, which, with a slightly tighter engineering tolerance, keeps track spacing within the original tolerance band.[Note 2] The Irish gauge is now expressed as 1,600 mm gauge[34] giving a difference between the imperial and metric values of exactly 0.2 mm and thus also well within engineering tolerances.[Note 2]

The specifications of loading gauge and other engineering parameters for rolling stock and line-side structures were converted to metric units during the 1970s,[36] and subsequently updated many times.[35]

Railway operations[edit]

Before metrication, track distances on Britain's national rail network were shown in miles and chains, with speed limits in miles per hour. During the metrication program of the 1970s these units were retained on the British Rail network,[37] but London Transport remeasured the London Underground network in kilometres using Ongar as its zero point.[38][39] Changes in operational procedures have not always been synchronised with changes in technology – for example, in 2010 it was reported that drivers of freight trains operating on single track in the West Highlands had to report the lengths of their trains verbally to the signalling staff using feet even though the computer generated information available to the driver was in metres.[40]

Some newer railway systems, however, make some use of metric units: for example, one map showing part of the route of the proposed London Birmingham High Speed Link is published in metric units.[41] This document uses the word "chainage", but the running distances are actually shown in metres.

Speed limit sign for tram drivers on public roads shown in kilometres per hour[42]

Britain's tramway systems, apart from the Blackpool line, were decommissioned in the decades immediately following the Second World War, culminating with the closure of the Glasgow Corporation Tramways in 1962. From 1980 onwards,[43] a number of light railways (including tramways) that operate entirely to metric standards have been built in various metropolitan areas.[44] Speed limit signs for trams that run on public roads quoted in kilometres per hour have a distinctive black-and-white diamond shape,[42] although km/h speed limit signs on dedicated rights of way can have different shapes such as the hexagonal signs used on the Tyne and Wear line. New light railways built in Britain that operate using km/h since 1980 include:

The operations manual of the Blackpool tramway (1885) used mph until 2011 when it was upgraded to light rail specifications.

Pan-European signalling systems[edit]

An ETCS driver machine interface panel showing a speed of 39 km/h and a hook (maximum permitted speed) of 98 km/h[54]
A Eurobalise that communicates ETCS positional information to passing trains

The European Rail Traffic Management System (ERTMS) is an initiative backed by the European Union to enhance cross-border interoperability and the procurement of signalling equipment by creating a single Europe-wide standard for train control and command systems. Its main components are European Train Control System (ETCS) and GSM-R communications system.[55] ETCS is a standard for track-train radio communications using balises (Eurobalises) and associated in-cab train control[56] while GSM-R is the GSM mobile communications standard for railway operations. ERTMS can operate at different levels depending on specific local requirements.[57][58][Note 3] Under ERTMS speeds are displayed in the driver's cab in km/h and at Level 2, lineside speed indicators are optional.[59]

In 2007 the British Government published its response to a European Union directive[60][61] requiring the use of ERTMS on High Speed (TEN-R)and Conventional Trans-European Railway Network (TEN) routes. The response proposed a roll-out plan of ERTMS equipment on existing lines that would be completed by 2044 though the actual timing of the programme will depend on changing circumstances – new trains will be ordered with ERTMS equipment on board and ERTMS would be installed during any electrification programs.[62] In 2009, the Rail Safety and Standards Board confirmed that km/h would be used on ERTMS lines in the United Kingdom.[63] With the ongoing introduction of ERTMS, it is foreseen that the metrication of British rail transport will be completed over the next few decades.[64]

The Uff/Cullen inquiry (2001) following the Southall and Ladbroke Grove rail crashes identified a need for in-cab signalling on high-speed trains and recommended that ERTMS should be installed onto all of Britain's high-speed lines by the year 2010. However, this timescale was not viable because of the time required to develop the technology.[65]

A standard feature of the speedometers used by ERTMS/ETCS systems is the use of the metric system.[66] At a Railway Conference in 2002, it was argued that a changeover to using metric units for speed in advance of the introduction of ERTMS was unlikely to be financially viable unless the decision is taken to adopt Level 2 ERTMS without lineside signalling. There would however still be a need to handle dual both mph and km/h.[59] A 2010 voluntary standards document published by the Rail Safety and Standards Board addressed this issue when it recommended that the speedometer of a ETCS system be designed so that it switches automatically between mph and km/h depending on the route being traversed. The speedometer would display "mph" when the speedometer was displaying "miles per hour", otherwise would display nothing. Its graduations would be chosen such that the angle of the needle would not change when the system switched from one scale to the other. The conversion between metric and imperial units would be a function of the speedometer, not of any other on-board equipment.[67] In 2012 a technical specification matching this proposal was published.[68]

The Cambrian Line, a low volume 215 km (134 mi) rail link between Shrewsbury in the east and Aberystwyth and Pwllheli in the west, was chosen as Britain's first ERTMS line. This line was chosen as its signalling system had reached the end of its useful life, and because it is a low capacity line almost separate from the national network, making it an ideal site on which to gain ERTMS experience.[62][69] All speeds in the Cambrian Line Rule book are in km/h."[54][70] ERTMS will be rolled out on the Great Western Route as part of the electrification and resignalling work[71] which is expected[Note 4] to reach Oxford and Newbury by 2016 and Cardiff by 2017.[72] Other early mainline conversions to the ERTMS standards are expected to include[62] the 251 km (156 mi) London (Kings Cross) to Doncaster route by 2020 and the 158 km (98 mi) London (St Pancras) to Leicester route by 2022[73][74] while the specification for the proposed High Speed 2 (HS2) link from London to Manchester, Leeds and beyond assumes a minimum of Level 2 ERTMS control and signalling for the "day one service".[75] The Crossrail project in London will also implement ERTMS technology.[76]

Channel Tunnel and its London link[edit]

The Class 395 (Javelin) trains use km/h on the CTRL and mph on domestic routes.

The Channel Tunnel links the British and French rail networks and entered commercial service in 1994. It was designed using metric units.[77] Although both networks use a 1435 mm (standard) rail gauge, they differ in many ways: different loading gauges, different working practices and different units of measure. Initially the link to London used the existing 750 V DC Southern Region third rail system and the British AWS and TPWS signalling systems[78] with a maximum speed of 100 mph (160 km/h).[77] Although the French TVM 430 signalling system (which uses km/h) and KVB train protection system[79][80] was used in the tunnel itself, the Eurostar trains switched over to using the British AWS/TPWS system (and to mph)[81] on entering the Southern Rail link.[82] The Eurotunnel Shuttle, which ferries motor vehicles between Folkestone and Calais, uses km/h throughout.[77]

The first part of the 109-kilometre (68 mi) Channel Tunnel Rail Link (CTRL) was opened in 2003 and the remaining section in 2007. This link which uses TVM 430 signalling (and km/h) along all but the final few hundred metres of its course (where train protection is provided by KVB)[83] enables trains between the Channel Tunnel and London to reach speeds of up to 300 km/h (186 mph).[78] The line is also used by the Class 395 (Javelin) for domestic services to Kent. These trains are equipped with both TVM 430 for use on the CTRL, KVB for use in the St Pancras terminal and AWS/TPWS for use on domestic lines.[84]

Water transport[edit]

Units of measure related to British water transport can be divided into several categories including: those related to navigation, those related to transport operation, those related to the engineering of waterways and those related the engineering of the vessels. Many of these units of measure used are dictated by industrial standards and international conventions rather than by United Kingdom and EU legislation;[Note 5] with UK law itself appearing not only in Acts of Parliament and Statutory Instruments but also in Merchant Shipping Notices (MSNs) containing the technical details of Statutory Instruments.[85]

Units of measure used in navigation[edit]

Depth gauge at Salcombe harbour marked in metres

In the 18th century, principal units of "sea measure" in the UK included:

  • the fathom, a measure derived from an "arms-reach" – the amount of rope that a sailor could pull with a single "reach" of his arms,[86][87] variously reckoned to be 5 ft (1.52 m), 5 ft 6 in (1.68 m) or 6 ft (1.83 m), depending on the class of ship.[88]
  • the cable, equivalent to the standard length of a rope cable, typically 100 to 115 fathoms (183 to 210 m)[89]
  • the nautical or sea mile, equivalent to one minute of arc measured along the meridian at the latitude of measurement and which varied between 6,048.8 and 6,107.8 ft (1,843.7 and 1,861.7 m).[90]
  • the knot, a measure of speed equal to one nautical mile per hour (approximately 1.85 km/h, varying with the definition of the nautical mile).

In the years following the publication the Admiralty Chart in 1795, nautical units of measure were standardised. The popularity[Note 6] of British charts in comparison with those of other countries led to the adoption in 1884 of Greenwich as the prime meridian.[91] By 1967, when the changeover to metric units on Admiralty charts was announced,[92] the fathom had been standardised at 6 feet (1.8288 m); sea bed depths of less than 30 feet (9.1 m) were expressed in feet with fathoms being used for greater depths. The nautical mile was standardised at 6080 feet and the cable at one tenth of a nautical mile.[90][93] In 1929, the Extraordinary Hydrographic Conference in Monaco defined the international nautical mile as exactly 1852 m, as opposed to the metric equivalent of the British nautical mile (1853.184 m).[94]

There was a three-pronged approach to metrication of Admiralty charts:[95]

  • Charts that had sufficient changes as to warrant redrawing rather than updating would be redrawn in metric units.
  • Charts that were produced in collaboration with international partners, for example Australia and New Zealand, would be metricated in conjunction with those partners.
  • Charts that did not fall into either of these categories would be metricated as part of an ongoing cycle of upgrades.

By 2009, 86% of the charts had been metricated.[96] Completion of metrication is ongoing: for example, a metric replacement for Chart 373 (Progreso and Terminal Remota [on the Mexican coast]) was published in March 2011.[97]

Although metrication of admiralty charts began in 1967, the Royal Navy did not immediately switch to metric units. Reports from the 1982 Falklands War (made public in 2011) reported that HMS Sheffield "... sank in 1000 fathoms"[98] and that "... divers visited the wreck [of HMS Ardent] which is about 4 cables South West ...".[99] The HMS Sheffield report did however mention that the swell was "about 2 m". Likewise, a 2005 Marine Accident Investigation Branch report cited both cables and metres for lengths and distances, though all depths were given in metres.[100]

The Met Office uses a mixture of metric and imperial units in its weather forecasts. Both metric and nautical units of measure are used in the definition of visibility,[Note 7] gale warnings use the Beaufort scale with names attached to Beaufort forces 8 to 12 while the height of waves is defined using metric units.[Note 8][101]

Units of measure used in shipbuilding[edit]

The standard load line marks on a vessel, calibrated in this case in decimetres

Ship design is subject to a number of laws relating to safety, including various Acts of Parliament, Statutory Instruments and Maritime (M) Notices[102] In addition, notice must be taken of various international conventions such as the International Convention for the Safety of Life at Sea (SOLAS).

Until the end of the 19th century, the British Government had passed few laws about safety at sea. The Merchant Shipping Act made the Plimsoll line or load line compulsory in 1876, but it was not until 1894 that the positioning of the mark was fixed in law. It was the Lloyds Register (the insurance institution) who in 1835 originally recommended freeboards as a function of the depth of the hold (three inches per foot of depth, equivalent to 25 cm/m).[103]

In 1907 a parliamentary bill to make the metric system compulsory within the United Kingdom failed. The engineering industry was amongst the opponents of the bill, citing the expense of the conversion with no visible benefit. Archibald Denny, a shipbuilder, was one of the prominent supporters of the British Weights and Measures Association, an organisation set up in 1904 to oppose metrication.[104] Half a century later metrication of the engineering industry, fuelled by the need for international compatibility,[105] started in 1965. By 1970 the conversion to metrication for new projects in most branches of engineering including shipbuilding were, according to John Eden, then Minister of Technology, largely completed.[106] Although the basics had been completed by 1970, it was still several years before metrication in the industry was completed. For example, Yarrow Shipbuilders in Glasgow were advertising for staff specifically to help complete the metrication process in 1974.[107]

Before 1983, legislation about health and safety in dockyards and shipyards and maritime construction used imperial units. In that year the Docks, Shipbuilding etc. (Metrication) Regulations 1983 replaced the imperial units of measure in five pieces of legislation[Note 9] with metric units was published.[108] In 2012, when the government proposed removing this piece of legislation as part of its "removal of red-tape campaign", the Chartered Institute of Environmental Health responded that such removal could only be justified if the underlying legislation (Shipbuilding and Ship-repairing Regulations 1960) was also removed.[109]

The loss of the Titanic on 15 April 1912 led to the first International Conference on Safety of Life at Sea being convened in London in November 1913 and the signing of the International Convention on Safety of Life at Sea (SOLAS) in January the following year.[110][111] A large part of the convention was devoted to the size of watertight compartments in ships and the number and design of lifeboats. Relevant dimensions were given in both metric and imperial units. The convention was updated in 1929, 1948, 1960 and 1974. Since 1974 the convention has had many minor updates.

When the convention was first signed, Britain had the largest merchant navy fleet in the world, owning half the world's shipping tonnage in 1890, which had reduced to 22% by 1948.[112] During the same period the United States increased her proportion of the world's maritime tonnage to become a major player. Both countries used the imperial units of measure. The role of the imperial system in international affairs is shown in the text of the 1948 version of the International Convention for the Safety of Life at Sea[113] Although the English and the French texts are given equal prominence, the conversion factors show that the treaty was drawn up using imperial units which were then converted to metric units: for example the metric equivalent of "10 feet" is shown as 3.05 metres in both texts. When the convention was updated in 1972 it was significantly enlarged. Metric units were used throughout the document with imperial units following in brackets.[114]

In 1974, the conversion to the use of metric units by the British shipbuilding industry was all but complete, and the United States was preparing to pass the Metric Conversion Act. That year, when the International Conference on Safety of Life at Sea updated the convention, they also invited the Maritime Safety Committee to study the "Système international d'unités" (SI System) with a view to its use in the International Convention, bearing in mind the desirability of expressing values in the metric system of units only. After reviewing the report, the 1975 Conference decided to rewrite the existing convention using SI units only.[115]

As of 2012, law on the design of British ships and registration of ships flying the British flag uses metric units: the Merchant Shipping (Registration of Ships) Regulations 1993 require that the ship's overall length, registered length, overall breadth, registered breadth, gross tonnage, net tonnage and engine power be recorded in metres, tonnes and kilowatts as appropriate;[116] while the Maritime and Coastguard Agency (MCA) standards and policies which are used by marine surveyors and which are partly derived from legislation use only metric units.[117][Note 10]


Units of measure used in aviation can be split into two groups – those used during aircraft design and construction and those used during operations.

Aircraft design[edit]

Concorde – the Anglo-French supersonic airliner that was designed in a mixture of imperial and metric units

Before the Second World War aircraft in design in both the United Kingdom and in the United States used imperial or customary units of measure. Each country used its own gallon in the design of fuel tanks, feet and inches were used for length, horsepower for engine power, pounds per square inch for pressure but the British used degrees Celsius[Note 11] for oil temperature whereas the Americans used degrees Fahrenheit.[118][119][120]

Since the advent of flight the United Kingdom has had an independent aircraft industry. Notable post-war British aircraft include the English Electric Lightning, De Havilland Comet, Vickers VC10, the V bomber family, Hawker Siddeley Harrier, Hawker Siddeley Hawk, British Aerospace 146 and the BAE HERTI – the earlier of which had been designed and built using imperial units. From the 1960s onwards, British aircraft manufacturers entered into many joint ventures with European aircraft manufacturers.

Imperial units were used in 1960 in the design of the British Aircraft Corporation (BAC) supersonic Bristol 198. The proposed aircraft would have been very expensive and BAC entered into a joint venture with the French firm Sud-Aviation who were also designing a supersonic passenger aircraft. The resultant aircraft was Concorde which, after many expensive design overruns, entered commercial service in 1976 .[121] Imperial units were used for the sections designed by the British team and metric units for those designed by the French team.[122][123]

Airbus 380 — as of 2016 the largest passenger aircraft in operation — was designed using metric units.

The Jaguar program began in the early 1960s, in response to a British requirement (Air Staff Target 362) for an advanced supersonic jet trainer and a French requirement (ECAT or École de Combat et d'Appui Tactique, "Tactical Combat Support Trainer") for a cheap, subsonic dual role trainer and light attack aircraft. A Memorandum of Understanding was signed in May 1965, for the two countries to develop two aircraft, a trainer based on the ECAT, and the larger AFVG (Anglo-French Variable Geometry)[124] In the same year that the Memorandum of Understanding was signed, the then Federation of British Industry informed the British Government that its members favoured the adoption of the metric system. The Jaguar was designed using metric units[123] and entered service in the British and French air forces in 1974 and 1973 respectively and as of July 2012 was still in service in the Indian Air Force and the Royal Air Force of Oman. Subsequent military aircraft designed and built by Pan-European teams included the Panavia Tornado[125] and the Eurofighter Typhoon, both multirole combat aircraft.[126] The Saab Gripen was designed and built by an Anglo-Swedish team and military aircraft designed and built by Anglo-American teams include the McDonnell Douglas AV-8B Harrier II and the Lockheed Martin F-35 Lightning II.

Airbus Industrie, the makers of the Airbus began in 1967 as a consortium of French, German and British aviation firms to compete with American companies such as Boeing, McDonnell Douglas, and Lockheed.[127] It was formally established as a Groupement d'Interet Économique (Economic Interest Group or GIE) on 18 December 1970.[128] The consortium produced a series of successful aircraft including the A300 the group's first aircraft which entered service in 1974, the A320 and variants which first entered service in 1988 and which in 2005 ranked as the world's fastest-selling jet airliner family and in 2007 the world's largest passenger jet, the A380 with a seating capacity of up to 853 passengers. By 2010 the consortium was equal in size to Boeing, and the A380 has been compared by many commentators with the Boeing 787 Dreamliner, an aircraft whose development was beset with problems. One commentator suggested that one of the underlying problems for the Dreamliner delays was Boeing's use of customary units which hampered the sub-contracting of precision work to Far Eastern suppliers – Airbus' use of metric units did not cause such problems.[129]

Air traffic control[edit]

In its early days, most of British aviation used imperial units. With the advent of metrication, the industry started using metric units and as of 2012 a significant proportion of the information supplied in the United Kingdom Aeronautical Information Publication (AIP) is in metric units, though a few measurements are still given in imperial units. The following items are catalogued in the AIP for each airfield in the United Kingdom:[130]

  • Runway and clearway length and width – metres
  • Visibility – metres or kilometres as appropriate
  • Distances from airfield – nautical miles
  • Altitude – feet above sea level or flight level (FL = multiples of 100 ft)
  • Fuel capacity/delivery – litres and litres/minute
  • Temperature – degrees Celsius

The publication Airport data, published by the Liaison Group of UK Airport Consultative Committees follows the units of measure used by the CAA apart from runway lengths which are given in feet,[131] though the Air Pilots Manual notes that the use of feet for runway lengths denotes either an old publication or a US-oriented publication.[132]


  1. ^ Legislation permits signs to be erected at 300 m or 400 m intervals where appropriate.
  2. ^ a b Typically railway gauge tolerances are two orders of magnitude greater than this value; for example
    "All Track Gauges". Rapid Rail: The Railway & Mining Asset Trading Platform. Retrieved 13 July 2012. 
  3. ^ Three levels have been identified for ERTMS:
    • Level 1 – ERTMS is added to or overlaid on lineside signals and train detectors. Communication is via balises (Eurobalises) of an existing railway system.
    • Level 2 – ERTMS uses balises to communicate with the driver, thereby dispensing with lineside signalling equipment. The location of each block is fixed (as with traditional railway systems).
    • Level 3 – Level 3 is an enhancement on Level 2 whereby the block will move with the train.
  4. ^ As of 2012, the rebuilding of bridges to accommodate the overhead wires was in progress
  5. ^ EU directive 80/181/EEC specifically excludes cases that "have been laid down in international conventions or agreements binding the Community or the Member States"
  6. ^ In 1884 ships carrying 72% of the world's tonnage used British charts (Clark Blaise pg 200).
  7. ^ The Met Office definitions of visibility are:
    • Very poor visibility – Less than 1,000 metres
    • Poor visibility – Between 1,000 metres and 2 nautical miles
    • Moderate visibility – Between 2 and 5 nautical miles
    • Good visibility – More than 5 nautical miles
  8. ^ Sea state is defined as follows:
    • Smooth Wave  – height less than 0.5 m
    • Slight Wave – height of 0.5 to 1.25 m
    • Moderate Wave – height of 1.25 to 2.5 m
    • Rough Wave – height of 2.5 to 4.0 m
    • Very rough Wave – height of 4.0 to 6.0 m
    • High Wave – height of 6.0 to 9.0 m
    • Very high Wave – height of 9.0 to 14.0 m
    • Phenomenal Wave – height more than 14.0 m
  9. ^ These were:
    • The Docks Regulations 1925
    • The Docks Regulations 1934
    • The Shipbuilding and Ship-repairing Regulations 1960
    • The Shipbuilding (Lifting Appliances etc. Forms) Order 1961
    • The Docks Certificates (No. 2) Order 1964
  10. ^ These design requirements include:
  11. ^ In 1948 "degrees Centigrade" were renamed "degrees Celsius".


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