Recommended values of standard frequencies

The CIPM has charged the CCL-CCTF Frequency Standards Working Group (CCL-CCTF-WGFS) with maintaining this list of recommended values of standard frequencies for applications including the practical realization of the definition of the metre and secondary representations of the definition of the second.

All the radiations in the list (apart from the ⁸⁷Rb 6.8 GHz and ¹⁷⁶Lu⁺ 353.6 THz transition) may be used for the practical realization of the definition of the metre.

The information is available in PDF format below or in machine-readable form from our API service.

API service

Please also find the data associated with the 2021 and 2025 adjustments of standard frequencies.

Supplementary documentation for the 2021 calculation

Supplementary documentation for the 2025 calculation

The radiations marked [SRS] may be used as secondary representations of the second.

Graphical representation

CCL-CCTF-WGFS Realization of the definition of some important SI units

Related publications

• Metrologia 61 (2024) 035005

  TThe CIPM list 'Recommended values of standard frequencies': 2021 update

  Authors:

  Margolis H.S., Panfilo G., Petit G., Oates C., Ido T., Bize S.

  Data associated: 10.59161/StdFreq202301

• Metrologia 58 (2021) 052002

  The new mise en pratique for the metre – a review of approaches for the practical realization of traceable length metrology from 10⁻¹¹ m to 10¹³ m

  Authors:

  Schödel R., Yacoot A., Lewis A.

• Metrologia 55 (2018) 188-200

  The CIPM list of recommended frequency standard values: guidelines and procedures

  Authors:

  Riehle F., Gill P., Arias F., Robertsson L.

• Metrologia 53 (2016) 1272-1280

  On the evaluation of ultra-high-precision frequency ratio measurements: examining closed loops in a graph theory framework

  Authors:

  Robertsson L.

• Metrologia 52 (2015) 628-634

  Least-squares analysis of clock frequency comparison data to deduce optimized frequency and frequency ratio values

  Authors:

  Margolis H.S., Gill P.