Overview
While the CIA Factbook in 2006 stated that the "US is the only industrialized nation that does not mainly use the metric system", (in addition to non-industrialised Burma/Myanmar, Liberia), the actual situation, however, is more complicated than the CIA Factbook would suggest - in the United Kingdom, for example, although metric is the official system for most regulated trading by weight or measure purposes, the pint is a permitted unit for milk in returnable bottles and for draught beer and cider in British pubs, and miles, yards and feet remain the official units for road signage, but not road design.
Some sources identify Liberia as metric while reports from Burma suggest that that country is planning to adopt the metric system. However, they all have adopted metric measures to some degree through international trade and standardisation for example, Sierra Leone switched to selling fuel by the litre in May 2011. The United States mandated the acceptance of the metric system in 1866 for commercial and legal proceedings, without displacing their customary units.
In 1971 the National Bureau of Standards completed a 3-year study of the impact of increasing worldwide metric use on the U.S. The study ended with a report to the Congress entitled A Metric America - A Decision Whose Time Has Come. In the last few years metric use has been increasing rapidly in the U.S., principally in the manufacturing and educational sectors. Public Law 93-380, enacted 21 Aug 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.
Both Liberia and Myanmar are substantially metric countries, trading internationally in metric units. Visiting advocates of metrication also claim that they use metric units for many things internally with exceptions such as old petrol pumps in Myanmar, calibrated in British Imperial gallons.
A number of jurisdictions have laws mandating or permitting other systems of measurement in some or all contexts, such as the United Kingdom, which still uses 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. 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. Antigua, also 'officially' metric, is moving toward total implementation of the metric system, but slower than expected. The government of Antigua and Barbuda have announced that they have plans to convert their country to the metric system by the first quarter of 2015. Other Caribbean countries such as Saint Lucia are officially metric but are still in the process toward full conversion.
In the European Union, the European Council (of Ministers) 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. During these negotiations, 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 (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.
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.
In January 2007 NASA agreed to use metric units for all future moon missions due to pressure from other space agencies.
The United States and the United Kingdom have some active opposition to metrication, particularly where attempts are made to stamp out the use of their indigenous systems of measurements. Other countries, like France and Japan, that once had significant popular opposition to metrication now have complete acceptance of metrication.
Read more about this topic: Metrication