The kilogram is the last unit of the International System of Units (SI) still defined by an artefact, namely the International Prototype of the Kilogram (IPK). Comparisons carried out over 100 years between IPK, its official copies and national prototypes have revealed a mass change (in relative term) of five parts in 108. After a brief history of the unit of mass, this paper underlines the need to redefine the kilogram with reference to a fixed numerical value of a fundamental constant. It explains why the Planck constant h was chosen and gives the last results obtained with watt balance experiments nowadays able to link h to a macroscopic mass to within a few parts in 108. Finally, it proposes a possible route for the national metrology institute for the mise en pratique of the kilogram after its redefinition.

Key words

unit of mass
redefinition of the kilogram
planck constant
watt balance