Resolution 12 of the 23rd CGPM

On the possible redefinition of certain base units of the International System of Units (SI)

The 23rd General Conference,


  • that, for many years, National Metrology Institutes (NMIs) as well as the International Bureau of Weights and Measures (BIPM) have made considerable efforts to advance and improve the International System of Units (SI) by extending the frontiers of metrology so that the SI base units could be defined in terms of the invariants of nature - the fundamental physical constants,
  • that, of the seven base units of the SI, only the kilogram is still defined in terms of a material artefact - the international prototype of the kilogram (2nd CGPM, 1889, 3rd CGPM, 1901) and that the definitions of the ampere, mole and candela depend on the kilogram,
  • Resolution 7 of the 21st General Conference (1999) which recommended that "national laboratories continue their efforts to refine experiments that link the unit of mass to fundamental or atomic constants with a view to a future redefinition of the kilogram",
  • the many advances, made in recent years, in experiments which relate the mass of the international prototype to the Planck constant h or the Avogadro constant NA,
  • initiatives to determine the value of a number of relevant fundamental constants, including work to redetermine the Boltzmann constant kB,
  • that as a result of recent advances, there are significant implications for, and potential benefits from, redefinitions of the kilogram, the ampere, the kelvin and the mole,
  • Recommendation 1 of the International Committee (C1-2005) at its meeting in October 2005, and various Recommendations of Consultative Committees on the subject of a redefinition of one or more of the base units of the SI,


  • that any changes in definitions of units of the SI must be constrained by self-consistency,
  • that it is desirable that definitions of the base units should be easily understood,
  • the work of the International Committee and the Consultative Committees,
  • the need to monitor the results of relevant experiments,
  • the importance of soliciting comments and contributions from the wider scientific and user communities, and
  • the decision of the International Committee in 2005 to approve, in principle, the preparation of new definitions of the kilogram, ampere, kelvin and the possibility of redefining the mole,

recommends that National Metrology Institutes and the BIPM

  • pursue the relevant experiments so that the International Committee can come to a view on whether it may be possible to redefine the kilogram, the ampere, the kelvin, and the mole using fixed values of the fundamental constants at the time of the 24th General Conference (2011),
  • should, together with the International Committee, its Consultative Committees, and appropriate working groups, work on practical ways of realizing any new definitions based on fixed values of the fundamental constants, prepare a mise en pratique for each of them, and consider the most appropriate way of explaining the new definitions to users,
  • initiate awareness campaigns to alert user communities to the possibility of redefinitions and that the technical and legislative implications of such redefinitions and their practical realizations be carefully discussed and considered,

and requests the International Committee to report on these issues to the 24th General Conference in 2011 and to undertake whatever preparations are considered necessary so that, if the results of experiments are found to be satisfactory and the needs of users met, formal proposals for changes in the definitions of the kilogram, ampere, the kelvin and mole can be put to the 24th General Conference.

reference The reader should note that the official version of this Resolution is the French text

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