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Redefinition in terms of the kilogram

The British Weights and Measures Act 1878 (41 & 42 Vict. c. 49) said that contracts worded in terms of metric units would be deemed by the courts to be made according to the Imperial units defined in the Act, and a table of metric equivalents was supplied so that the Imperial equivalents could be legally calculated. This defined, in UK law, metric units in terms of Imperial ones. The equivalence for the pound was given as 1 lb = 453.592 65 g or 0.45359 kg, which made the kilogram equivalent to about 2.204 6213 lb.

In 1883, it was determined jointly by the standards department of the British Board of Trade and the Bureau International that 0.453 592 4277 kg was a better approximation, and this figure, rounded to 0.453 592 43 kg was given legal status by an Order in Council in May 1898.

In 1959, based on further measurements and international coordination, the International Yard and Pound Agreement defined an "international pound" as being equivalent to exactly 0.453 592 37 kg. This meant that the existing legal definition of the UK pound differed from the international standard pound by 0.06 milligrams. To remedy this, the pound was again redefined in the United Kingdom by the Weights and Measures Act 1963 to match the international pound, stating: "the pound shall be 0.453 592 37 kilogramme exactly", a definition which remains valid to the present day.

The 2019 revision of the SI means that the pound is now defined precisely in terms of fundamental constants, ending the era of its definition in terms of physical prototypes.