Metrication in the United Kingdom

Metrication is the act or process of converting to the metric system of measurement. The United Kingdom, through voluntary and mandated laws, has metricated most of government, industry, commerce, and scientific research to the metric system; however, the previous measurement system (Imperial units) is still used in society. Imperial units as of 2024 remain mandated by law to still be used without metric units for speed and distance road signs, and the sizes of cider and beer sold by the glass, returnable milk containers and precious metals, and in some areas both measurement systems are mandated by law.

Due to metrication many Imperial units have been phased out. However, the national curriculum requires metric units and imperial units that still remain in common usage to be taught in state schools. As such, the public is familiar with both metric and Imperial units, and may interchange measurements in conversation, for example: distance and body measurements.

Adopting the metric system was discussed in Parliament as early as 1818 and some industries and government agencies had metricated, or were in the process of metricating by the mid-1960s. A formal government policy to support metrication was agreed by 1965. This policy, initiated in response to requests from industry, was to support voluntary metrication, with costs picked up where they fell. In 1969, the government created the Metrication Board as a quango to promote and coordinate metrication. The treaty of accession to the European Economic Community (EEC), which the United Kingdom joined in 1973, obliged the United Kingdom to incorporate into domestic law all EEC directives, including the use of a prescribed SI-based set of units for many purposes within five years. In 1978, after some carpet retailers reverted to pricing by the square yard rather than the square metre to try to make the prices appear cheaper, government policy shifted, and they started issuing directives making metrication mandatory in certain sectors.

In 1980, government policy shifted again to prefer voluntary metrication, and the Metrication Board was abolished. By the time the Metrication Board was wound up, all the economic sectors that fell within its remit except road signage and parts of the retail trade sector had metricated, and most pre-packaged goods were sold using the prescribed units. Mandatory use of prescribed units for retail sales took effect in 1995 for packaged goods and in 2000 for goods sold loose by weight. The use of "supplementary indications" or alternative units (generally the traditional imperial units formerly used) was originally to have been permitted for only a limited period, that period being extended a number of times due to public resistance, until in 2009 the requirement to ultimately cease use of traditional units alongside metric units was finally removed.

British scientists, philosophers and engineers have been at the forefront of the development of metrication. In 1861 a committee from the British Association for Advancement of Science (BAAS), which members included James Prescott Joule, Lord Kelvin, and James Clerk Maxwell, defined several electrical metric units. In the 1870 the international prototype kilogram was manufactured by the British company Johnson, Matthey & Co.