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Recommendation 1 of the 11th CCQM (2005)

On the possible redefinition of the kilogram

The Consultative Committee for Amount of Substance: metrology in chemistry (CCQM),


  • the recent proposal to redefine the kilogram in terms of a fixed value for either the Planck constant or the Avogadro constant at the 23rd General Conference in 2007,
  • that the realization of a definition of the kilogram in these terms necessarily requires the permanent availability of measurement techniques that relate mass measurements to fundamental constants,
  • the advantages that such a redefinition would bring to the scientific community through the significant reduction in the uncertainties of the SI values of many fundamental constants,
  • that a redefinition based on a fixed value for the Avogadro constant would have further benefits for the scientific community, particularly for metrology in chemistry,

recalling 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 existing discrepancy of about 1 part in 106 between the results from watt balance and x-ray crystal density/molar mass measurements,
  • the fact that this discrepancy is significant at the practical level and is unlikely to be resolved by 2007,

recommends that

  • any decision on redefining the kilogram be deferred until the 24th General Conference in 2011, and should be based on consultation with all users and stakeholders that would be affected,
  • laboratories continue to make their best efforts to produce data for the 2010 CODATA adjustment of the values of the fundamental constants in order to support a possible redefinition of the kilogram at the 24th General Conference,
  • at that time, full consideration be given to fixing the values of either the Planck constant or the Avogadro constant, and the interests of the chemical metrology community in this choice,
  • any new definition of the kilogram should, if possible, be easily understood by the general public.

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