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Metrication or metrification is the process of converting from the various other systems of units used throughout the world to the SI metric system. This process began in France in the 1790s and spread over the following two centuries to all but four countries, representing 95% of the world's population. The process was completed in most of the world in the 19th and early 20th centuries, replacing numerous historical weights and measures. The countries of the former British Empire (with the exception of the United States) completed metrication during the second half of the 20th century, with the Republic of Ireland recently completing metrication on January 20 2005. Today only the United States, Liberia, and Myanmar have not officially switched to the metric system (although Liberia and Myanmar use it in practice) and the United Kingdom is currently in the process of conversion. Only France, the United States, Canada, the United Kingdom, and Japan have seen significant popular opposition to metrication, the main objections being based on tradition, aesthetics, economical impact and distaste for measures viewed as foreign.

Before the metric system

For more details on this topic, see History of measurement.

Medieval trade was organized on a city-by-city basis by guilds, which set local laws on weights and measures. For example, the ell or elle was a unit of length commonly used in Europe, but its value varied from 40.2 cm in one part of Germany to 70 cm in The Netherlands to 94.5 cm in Edinburgh. A survey of Switzerland in 1838 revealed that the foot had 37 different regional variations, the ell had 68, there were 83 different measures for dry grain and 70 for fluids, and 63 different measures for "dead weights."[2] When Isaac Newton wrote his important work Philosophiae Naturalis Principia Mathematica in 1687, he quoted his measurements in Parisian feet so his readers could understand their size. Various efforts were made to have local intercity or national standards for measurements, such as a Scottish law of 1641 and the British standard Imperial unit system of 1845, which is still used for some things in the UK. However, revolutionary France was to produce the definitive International System of Units which has come to be used by most of the world today.

The desire for a single international system of measurement derives from growing international trade and the need to apply common standards to goods. For a company to buy a product produced in another country, they need to be sure that the product will arrive as described. The medieval ell was abandoned in part because its value could not be standardised. It can be argued that the Système International's (SI) primary advantage is simply that it is international, and the pressure on countries to conform to it grew as it became increasingly an international standard. SI is not the only example of international standardisation; several powerful international standardisation organisations exist for various industries, such as the International Organization for Standardization, the International Electrotechnical Commission, and the International Telecommunication Union.

Système International (SI)

See main article: International System of Units

Scientists, chiefly in France, had been advocating and discussing a decimal system of measurement based on natural units at least since 1640, but the first official adoption of such a system was after the French Revolution of 1789. The creators of the metric system tried to choose units that were non-arbitrary and practical, merging well with the revolution's official ideology of "pure reason". The original system started with the metre as the unit of distance, the gram as the unit of mass, and the second as the unit of time. Derived units are made from logical combinations of base units. For example, the speed of an object is defined by the number of metres it moves every second — m/s.

An object that is accelerating has a changing speed, so its m/s changes per second, thus the unit is m/s². The force exerted on an object can be described by its mass times the resulting acceleration of the object, thus—kg·m/s²—which is the newton (symbol N), named in honour of Isaac Newton. Further base units dealing with electricity, light and quantities of atoms were added later as these sciences became better understood.

The current version of this system was agreed upon in 1971 and is organised and maintained by the International Bureau of Weights and Measures. To avoid confusion over the precise value of base units, this organisation also maintains either an international prototype (in the case of a kilogram, a small object of platinum-iridium dubbed "Le Grand Kilo") or a precise recipe on how to recreate the unit, which is decided by the General Conference on Weights and Measures held every four years.

Time has resisted metrication. During the French revolution there was an attempt at a decimal time system with 100 seconds in a minute, 100 minutes in an hour and 10 hours in a day (100,000 seconds in a day as opposed to 86,400 currently—a metric second would be thus 14% shorter). The proposed system also included a ten-day week, which was probably its main reason for failure. The church and the people objected and the system was dropped. It is also interesting to note that the Chinese calendar had an almost identical decimal time system up until the 17th century.[1]

Conversion process

There are three common routes that nations take in converting from traditional measurement systems to the metric system. The first is a quick, so called "Big-bang" route which was used by India in the early 1960s and several other developing nations since then. The second two routes are both variations on the slower phase-in route that tends to be favoured by industrial nations.

The first, "Big-bang", route is to simultaneously outlaw the use of pre-metric measurement, metricise and reissue all government publications and laws, and change education systems to metric. India's changeover lasted from 1 April 1960, when metric became legal, to 1 April 1962, when all other systems were banned. The Indian model was extremely successful and was copied over much of the developing world.

The second possibility, and first phase-in route, is to pass a law permitting the use of metric units in parallel with traditional ones, followed by progressively banning the use of the older measures. This has generally been a slow route to metric. The British Empire permitted the use of metric in 1873, but the changeover was not completed until the 1970s and 1980s when government took an active role in the now-independent parts of the empire. Japan, too, followed this route and did not complete the changeover for 70 years.

A final possibility is to redefine traditional units in terms of metric values. These redefined units often stay in use long after metrication is said to have been completed. China followed this route, and thus while scientists in China know and use the kilogram, common people retain the jin, which now has a value of 500 g. (This route was once proposed for England with the pound to be redefined as 500 g, but the plan did not receive government support.) In the Netherlands, 500 g is informally referred to as a pond (pound) and 100 g as an ons (ounce), and in Germany and France 500 g is informally referred to respectively as ein Pfund and une livre (one pound). In Denmark, the re-defined pund (500 g) is occasionally used, particularly among older people and (older) fruit growers, since these were originally paid according to the number of pounds of fruit produced.

It is difficult to judge the degree to which ordinary people change to using metric in their daily lives. In countries that have recently changed, older segments of the population tend to still use the old, and more familiar to them, system. Also, local variations abound in what exactly becomes metricated and what does not. In Canada, for example, ovens and cooking temperatures are usually measured in Fahrenheit, and Canadians almost invariably use Fahrenheit for cooking; though this is not necessarily by choice but may instead be due to the overwhelming influence of the neighbouring and largely non-metricated United States. In the UK, which is still in the process of changing over, metric units are often used interchangeably with older measurements. Such countries could be said to be "semi-metric".


World map colour-coding year of metrication; Green is 1800, Red is most recent

The metric system, developed in France around the turn of the 19th century, was quickly taken up by Europe's scientists before spreading to traders and industrialists and finally to the common people. France's neighbour the Kingdom of the Netherlands (present The Netherlands, Belgium, and Luxembourg), changed in 1820. Spain and its former American colonies changed in the 1850s and 1860s. Italy and Germany went metric after their respective unifications in 1861 and 1871, followed shortly by Portugal, Norway, Sweden and Austria-Hungary. By 1900, 39 countries in Europe and Latin America were using the metric system.

The first Asian nations to convert were Mongolia (1918), Cambodia and Afghanistan (the 1920s). Japan began its slow conversion process in 1891 when it received a copy of the metre standard from the Institute in France. In 1924, the government decided to replace fully the traditional shaku-kan system within 10 years; however, public opposition delayed implementation. The U.S. occupation of the late 1940s briefly caused a de facto conversion to U.S. customary units. Metrication was completed in Japan by 1969, although some of the old units are still in informal use. India's conversion was far quicker, paradoxically helped by low popular literacy and the fact that there was previously no nationwide standard measurement system—British Imperial units were used by the upper class, while various regional systems were used by the poor. From 1956 to 1961, India simultaneously changed to metric and decimalised its currency.

China began conversion in the 1920s, but the process was not completed until communist times. China also decimalised their native measurement units and redefined them as even amounts of metric units. Thus jin was redefined to equal 500 grams. The Soviet Union changed from traditional units to metric in 1924.

Those Arab nations which were colonized by France adopted the system early: Algeria changed in 1840, Tunisia in 1890; this was extended to the other Arab countries after the conquest of the Ottoman Empire in 1918. Jordan, which had been a British mandate, was the last Arab nation to convert, in the 1950s. The German colonies of Rwanda and Burundi and the Belgian Congo (now Democratic Republic of Congo) were the first sub-Saharan African states to go metric in 1910. French territories in Africa were de facto metric while under French rule. On independence all gradually passed official metric weights and measures laws during the 1950s and 1960s. The last African states to go metric were the former British colonies of southern and eastern Africa.

Logo of Australian Metrication

Britain and its former colonies (with the notable exception of the United States) began their conversion process in the later part of the 20th century. South Africa began a ten-year process of metrication in 1967 with the creation of Metrication Advisory Board, a Metrication Department and a South African Bureau of Standards. Australia began work in 1969 with a publicity campaign involving lecture tours, theatrical advertisements and the free distribution of metric-sized items, including calendars, rulers and A4-sized leaflets. Public opposition was on points of detail only, and the process was declared completed in 1977. Canada and New Zealand followed similar plans in the 1970s. Ireland completed a very gradual changeover process on 20 January 2005 with the conversion of road speed limits to km/h. Ireland began metrication in 1970 when schools switched to teaching only the metric system.


There are three main exceptions to the metrication trend: the UK, the U.S., and global air and sea transport industry. Contrary to popular belief, the U.S. and the UK do not use a common system (see English unit, Imperial unit and U.S. customary units for details). With the exception of the length, with the yard standardised at exactly 0.9144 m by an international conference in 1958, most other units differ in value in the U.S. and UK. A gallon of liquid, for example, is 3.8 l in the U.S. but 4.5 l in the UK. The U.S. also has a dry gallon, which is 4.4 l.

United Kingdom

Logo of UK Metrication
See also: Metrication in the UK

The UK made initial moves to convert to metric as long ago as 1862, when the Select Committee on Weights and Measures favoured the introduction of metric weights and measures accompanied by decimalisation of the currency.[2] The country finally decimalised their currency over a century later in 1971. There is some public resistance to metrication, which some see as the imposition of a foreign system and to be connected with other unpopular ideas from mainland Europe, such as the Euro. The "metric martyrs" were shop owners in the UK who were fined for refusing to use metric units and for overcharging in response to metric customer requests.

The result has been a mixture of metric and non-metric units, although metric units have gradually been phased in. Metric units have been taught in UK schools since the late 1960s, 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 imperial sizes but with metric size descriptions, for example, as 13 mm (rather than as half-inch) thick plasterboard. However, draught beer and cider are still sold in pints, milk may be sold in pints in returnable containers, and the mile, yard, foot and inch are used for road signage and associated measurements. The acre is used for land registration (although any registration since 1995 has used metric measurements). These units are defined in terms of SI units. Other exceptions include aviation, shipping and rail transport. For example, the foot for aircraft altitude, nautical miles and knots ([3], [4]).

In August 2005, the European Commission announced it would require Britain to set a legal deadline for the completion of metrication.[5]

United States

Logo of U.S. Metrication
See also: Metrication in the United States and Fair Packaging and Labeling Act

The United States remains the main exception. Although Thomas Jefferson recommended metrication and currency decimalisation, and metrication is the official policy of the U.S. government since the Convention du Mètre (Metre Convention), and several laws encourage or require use of metric units in various contexts [6], the progress of metrication has been much slower in the United States than in the rest of the world. Non-metric units continue to be predominantly used in everyday life and in commerce, engineering, and aviation, although most scientific work in the U.S. is now conducted using metric units. However, change has occurred, with most products in the U.S. now required by law to be labeled with both metric and non-metric units, and a number of companies and government agencies switching to metric standards. Additionally, the metric system is taught in schools, in the context of the sciences.

One peculiar example of this is bottled soft drinks, commonly sold in units of two litres, and with units of 500 ml, one litre, and three litres being less, but increasingly, common. This is a result of the introduction of PET bottling technology coinciding with a particularly strong metrication push in the mid to late 1970s; consumers found that they could buy a two-liter plastic bottle of their favorite soft drink more cheaply than they could four one-pint glass bottles, and the convention stuck. Smaller units, however, continue to be sold more often in fluid ounces, such as 8 ounce (240 ml) and 12 ounce (355 ml) aluminium cans and 20 ounce (591 ml) PET bottles.

Some other products, notably toiletries such as shampoo and mouthwash, have begun to be sold in metric sizes, and PowerBars and similar products have always been sold in metric sizes.

The United States continues to use only miles for road distance signs, with the exception of Interstate 19 in Arizona, some roads in Hawaii and a stretch of the westbound New York State Thruway between I-81 and I-481 near Syracuse. Delaware Route 1 in Delaware between Dover Air Force Base and Interstate 95 uses a dual-measurement system in which distances are in miles but exit numbers are based on the kilometric distances from the road's beginning. Some states have experimented with dual-unit signs, particularly near the borders with Canada and Mexico, but there are as yet no plans for large-scale conversion. Originally, U.S. legislation set October 2000 as a deadline by which states must undertake construction work and statistics in metric for states to be eligible for federal funding, but that requirement has since been rescinded. There is presently little political or popular support for a comprehensive switch to the metric system.

Liberia and Myanmar

Liberia has used the U.S. system of weights and measures since its founding in the 19th century by freed slaves from the United States. In modern times instability and civil war has meant that measurement reform has not been a high priority and the country in fact uses a mixture of U.S., metric and local customary measures. Myanmar has not officially adopted "everyday use" of the metric system, but unofficial metrication has taken place and the Myanmar economy primarily operates using the metric system.

Air and sea transport

Some industries have resisted metrication. Non-metric measures in air and sea transport retain worldwide dominance. In these areas the nautical mile (1.852 km) is preferred over the kilometre, because it closely represents a minute of arc of the circumference of Earth. While the metre was also based on the Earth with 100 km equal to an arc of 1 grad, those units of angle have not seen widespread use, though they do appear on some maps.

The knot, which is nautical miles per hour, remains the prime unit of speed for maritime and air navigation. (However, before the 1960s, statute miles per hour—which bear no relationship to the Earth—were most often used for this purpose, and remained in fairly common use for some purposes into the 1970s and later.) For aviation, altitudes are usually estimated based on air pressure values and described in nominal feet rather than nominal metres. However, several countries and air forces (mainly, but not only former Warsaw Pact) use metres for altitude. Thus an individual pilot can sometimes operate with altitudes in metres and sometimes in feet. The policies of the International Civil Aviation Organization (ICAO) relating to measurement are:

  • there should be a single system of units throughout the world
  • the single system should be SI
  • the use of the foot for altitude is a permitted variation

Consistent with ICAO policy, aviation has undergone a lot of metrication over the years. For example, the United Kingdom and Ireland metricated runway length and many other measures several decades ago. The United States metricated temperature reports in 1996 and the US military has metricated some reports of visual range. Metrication is also gradually taking place in cargo weights/dimensions and fuel volume/weight.

Accidents and incidents

Confusion over units during the process of metrication can sometimes lead to accidents. One of the most famous examples is the Gimli Glider, a Boeing 767 that ran out of fuel in Canada in 1983 due, in large part, to confusion at Air Canada during Canada's metrication.

While not strictly an example of national metrication, the use of two different systems was the contributing factor in the loss of the Mars Climate Orbiter in 1998. NASA specified metric units in the contract. NASA and other organisations worked in metric units but one subcontractor, Lockheed Martin, provided thruster performance data to the team in pound force seconds instead of newton seconds. The spacecraft was intended to orbit Mars at about 150 km altitude but the incorrect data meant that it descended to about 57 km and probably burned up in the Martian atmosphere.


Main article: Metrication arguments and counter-arguments

Interestingly, considering it was the birthplace of the metric system, France experienced a particularly rough journey to metrication. The traditional French measuring system was chaotic, with size of units differing in each small town, and often even within towns. Lyon had two different values of pound in general use, one of 14 ounces, and another of 15 ounces, the latter only being used for measuring silk. The revolutionary government, which had ordered the creation of the metric system, first attempted a quick conversion, legalising metric in 1795 and, just four years later, banning the use of traditional units. Massive popular opposition led Napoleon, after he came to power, to roll back these reforms. He publicly denounced the previous government for "tormenting people with trifles". It appears that it was decimalisation that disturbed the people most — as, although Napoleon decreed that there should be "such fractions and multiples as were generally used", he redefined the old base units in metric terms. The original metric system was made law again in France in 1837.[2]

Few countries experience much popular opposition to metrication. Many, such as 19th century Europe, Russia, India and China, converted before most of their populations were literate, and so the initial conversion affected few people. For others, such as Ireland, the previous system was also seen as foreign and unloved. Japan also saw popular resistance to their 1920s metrication program, where opponents of the metric system believed that the adoption of a foreign measuring system would have a bad influence on national sentiment, cause dislocations in public life, needless expense to the nation, prove disadvantageous to foreign trade, and would hurt the national language and culture. In 1933, the government postponed the date of the first stage of conversion by five years, and the date of the second stage by ten years. The process was not finally completed until 1969.

See also



  1. Grierson, Philip (1971). 'The Stenton Lecture 1971';English Linear Measures: an essay in origins, University of Reading.
  2. McGreevy, Thomas (1995). The Basis of Measurement: Historical Aspects, Picton Publishing (Chippenham Ltd). ISBN 0-948251-82-4.
  3. McGreevy, Thomas (1995). The Basis of Measurement: Metrication and Current Practice, Picton Publishing (Chippenham Ltd). ISBN 0-948251-82-0.

External links

Websites supporting metrication:

Websites opposing metrication:


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