The international avoirdupois pound is equal to exactly 453.59237 grams. The definition of the international pound was agreed by the United States and countries of the Commonwealth of Nations in 1958.
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 0·453 592 37 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.