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CCU Working Group on Angles and Dimensionless Quantities in the SI (CCU-WGADQ)
Having completed its mission, this group was closed by the CCU in October 2019.


The present treatment of so-called dimensionless quantities in the SI Brochure needs to be addressed:

  • The role of the radian and steradian, and units for angles in general, is ambiguous in the SI. This can lead to errors of 2π in calculations involving periodic phenomena.

  • The assignment of the unit s-1 to express counts and also to express frequencies with the special name "hertz", symbol Hz, in the SI can cause the hertz to be misused to denote counts.

  • The use of "items" as units in equations is excluded from the SI, although they are commonly used this way by the scientific community, including in physics, chemistry, bioscience and biotechnology.

  • Many methods of measurement in bioscience and biotechnology are based on counting, but there are neither examples nor guidance given in the SI Brochure on how to report such measurement results in a way that is compliant with the SI.

Terms of reference
  • To review the treatment of dimensionless quantities in the SI Brochure and identify potential issues such as those mentioned above.

  • To propose changes to be made to the language in the SI Brochure that would eliminate these issues, noting that the changes should be drafted in such a way as to not cause an undue burden for the scientific community.

  • To consider the need for further guidance documents on expressing the results of measurements of dimensionless quantities in the SI, notably in the fields of physics, chemistry, bioscience and biotechnology, and developing these as necessary.

  • To engage the CCU in a review and possible refinement of the suggestions, so the CCU could, in turn, present the changes to the CIPM for consideration and approval.


  • Dr Jörn Stenger  (PTB)


  • Dr Walter Bich (INRIM)
  • Dr Michael Krystek (PTB)
  • Dr Michal Máriássy (SMU)
  • Prof. Ian M. Mills FRS
  • Dr Peter Mohr (NIST)
  • Ms Helen Parkes (LGC)
  • Prof. William Phillips (NIST)
  • Prof. Joachim Ullrich (PTB)
  • Dr Robert I. Wielgosz (BIPM)

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20–24 September 2021: 25th meeting of the CCU and related meetings

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