Overview

Since 2006, three countries formally do not use the metric system as their main standard of measurement: the United States, Myanmar, and Liberia.^[3] In the United Kingdom metric is the official system for most regulated trading by weight or measure purposes, but some imperial units remain the primary official unit of measurement. For example, miles, yards, and feet remain the official units for road signage – and use of imperial units is widespread.^[4][5] The Imperial pint (expressed in metric units) also remains a permitted unit for milk in returnable bottles and for draught beer and cider in British pubs.^[4] Imperial units are also legal for use alongside metric units on food packaging and price indications for goods sold loose, and may be used exclusively where a product is sold by description, rather than by weight/mass/volume. E.g. Television screen and clothing sizes tend to be denominated in inches only, but a piece of material priced per inch would be unlawful unless the metric price was also shown.
Some sources now identify Liberia as metric, and the government of Myanmar has stated that the country would metricate with a goal of completion by 2019.^[6][7] Both Myanmar and Liberia are substantially metric countries, trading internationally in metric units. Visiting advocates of metrication have stated that they use metric units for many things internally with exceptions such as old petrol pumps in Myanmar, calibrated in British Imperial gallons. These and other countries have adopted metric measures to some degree through international trade and standardization^[8] for example, Sierra Leone switched to selling fuel by the litre in May 2011.^[9] The United States mandated the acceptance of the metric system in 1866 for commercial and legal proceedings, without displacing their customary units.^[10] The United Kingdom followed suit in 1897 and did not make the use of metric units compulsory for most purposes until 1995.
In 1971 the National Bureau of Standards completed a three-year study of the impact of increasing worldwide metric use on the U.S. The study concluded with a report to Congress entitled A Metric America – A Decision Whose Time Has Come. Since then metric use has increased in the U.S., principally in the manufacturing and educational sectors. Public Law 93-380, enacted 21 August 1974, states that it is the policy of the U.S. to encourage educational agencies and institutions to prepare students to use the metric system of measurement with ease and facility as a part of the regular education program. On 23 December 1975, President Gerald Ford signed Public Law 94-168, the Metric Conversion Act of 1975. This act declares a national policy of coordinating the increasing use of the metric system in the U.S. It established a U.S. Metric Board whose functions as of 1 October 1982 were transferred to the Dept of Commerce, Office of Metric Programs, to coordinate the voluntary conversion to the metric system.^[11]
Most countries have adopted the metric system officially over a transitional period where both units are used for a set period of time. Some countries such as Guyana, for example, have officially adopted the metric system, but have had some trouble over time implementing it.^[12] Antigua and Barbuda, also "officially" metric, is moving toward total implementation of the metric system, but slower than expected. The government had announced that they have plans to convert their country to the metric system by the first quarter of 2015.^[13] Other Caribbean countries such as Saint Lucia are officially metric but are still in the process toward full conversion.^[14]
The European Union used the Units of Measure Directive to attempt to achieve a common system of weights and measures and to facilitate the European Single Market. Throughout the 1990s, the European Commission helped accelerate the process for member countries to complete their metric conversion processes. Among them is the United Kingdom where laws in some or all contexts mandate or permit many imperial measures, such as miles and yards for road-sign distances, road speed limits in miles per hour, pints of beer, and inches for clothes.^[15] The United Kingdom secured permanent exemptions for the mile and yard in road markings, and (with Ireland) for the pint (Imperial) of draught beer sold in pubs^[16] (see Metrication in the United Kingdom). In 2007, the European Commission also announced that (to appease British public opinion and to facilitate trade with the United States) it was to abandon the requirement for metric-only labelling on packaged goods, and to allow dual metric–imperial marking to continue indefinitely.^[16]
Other countries using the imperial system completed official metrication during the second half of the 20th century or the first decade of the 21st century. The most recent to complete this process was the Republic of Ireland, which began metric conversion in the 1970s and completed it in early 2005.^[17]
In January 2007 NASA decided to use metric units for all future moon missions, in line with the practice of other space agencies.^[18]
The United States, the United Kingdom, and Canada have some active opposition to metrication, particularly where updated weights and measures laws would make obsolete historic systems of measurement. Other countries, like France and Japan, that once had significant popular opposition to metrication now have complete acceptance of metrication.^{[citation needed]}