The kilogram is the last remaining unit of the International System of Units (SI) which is still defined by a material artefact, the international prototype kept at the BIPM. The major disadvantage of this definition is that artefacts such as the international prototype age at a rate which often cannot be known with a high degree of precision.

The results of comparisons between the official copies and the international prototype show some divergence with time. The graph shows changes of about 5 x 10^–8, equivalent to 50 µg, in the mass of the standards since their first calibration, more than 100 years ago. All measurements are with respect to the international prototype. For this reason, the mass of the international prototype defines the x-axis of this graph.

Unknown changes in the mass unit would also influence the electrical units, because the definition of the ampere is related to the kilogram.

The 21st General Conference for Weights and Measures (CGPM) therefore recommended in its Resolution 7 that efforts continue to refine experiments linking the unit of mass to fundamental constants with a view to a future "quantum-based" redefinition of the kilogram. Any new definition would need to be consistent within some parts in 10⁸ with the present definition to avoid noticeable changes of mass values.

The existing experiments fall mainly into two classes, according to the fundamental constant to which the kilogram could be linked:

• The number of atoms in a weighed quantity of matter is determined (Avogadro project, ion accumulation), thus establishing a relationship between the kilogram and an atomic mass.
• A second class of electro-mechanical experiments (watt balance, magnetic levitation) links the kilogram to the Planck constant h.

Taking into account the required uncertainty mentioned above and the levels of performance reached up to now by the different techniques we believe that the watt balance is a very promising candidate for a future redefinition of the kilogram.

Within the context of the present preparations for an improved SI system, which might be approved by the CGPM in 2011, watt balances have three different roles:

• Before the redefinition can take place, the value of the Planck constant h needs to be measured in the present SI system with a sufficiently small uncertainty, to avoid any significant discontinuity between the present and the future kilogram definition.
• After the redefinition has taken place, a number of watt balances will be needed to realize the new kilogram definition in practice. Being at present the custodian of the international prototype of the kilogram, the BIPM sees its future role in permanently operating a watt balance for the realization of the new kilogram definition as a service for national metrology institutes.
• Together with a calculable capacitor, which can determine a value for the von Klitzing constant R_K, the watt balance allows determination of a value of the Josephson constant K_J, and provides a means of checking the veracity of the Josephson equation K_J = 2e/h, where e is the elementary charge.