The United States and countries of the Commonwealth of Nations agreed upon common definitions for the pound and the yard. Since 1 July 1959, the international avoirdupois pound (symbol lb) has been defined as exactly 0.45359237 kg.
In the United Kingdom, the use of the international pound was implemented in the Weights and Measures Act 1963.
The yard or the metre shall be the unit of measurement of length and the pound or the kilogram shall be the unit of measurement of mass by reference to which any measurement involving a measurement of length or mass shall be made in the United Kingdom; and- (a) the yard shall be 0.9144 metre exactly; (b) the pound shall be 0.45359237 kilogram exactly.
— Weights and Measures Act, 1963, Section 1(1)
An avoirdupois pound is equal to 16 avoirdupois ounces and to exactly 7,000 grains. The conversion factor between the kilogram and the international pound was therefore chosen to be divisible by 7, and an (international) grain is thus equal to exactly 64.79891 milligrams.
In the UK, the process of metrication and European units of measurement directives were expected to eliminate the use of the pound and ounce, but in 2007 the European Commission abandoned the requirement for metric-only labelling on packaged goods there, and allowed for dual metric–imperial marking to continue indefinitely. When used as a measurement of body weight the UK practice remains to use the stone of 14 pounds as the primary measure e.g. "11 stone 4 pounds", rather than "158 pounds" (as done in the US), or "72 kilograms" as used elsewhere.
The US has not adopted the metric system despite many efforts to do so, and the pound remains widely used as one of the key United States customary units.