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On the revision of the SI

In a landmark decision, the BIPM's Member States voted on 16 November 2018 to revise the International System of Units (SI), changing the world's definition of the kilogram, the ampere, the kelvin and the mole.

The decision, made at the 26th meeting of the General Conference on Weights and Measures (CGPM) in Versailles, France, means that all SI units will now be defined in terms of constants that describe the natural world. This will assure the future stability of the SI and open the opportunity for the use of new technologies, including quantum technologies, to implement the definitions.

In the revised SI four of the SI base units – namely the kilogram, the ampere, the kelvin and the mole – are redefined in terms of constants; the new definitions are based on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (k), and the Avogadro constant (NA), respectively. Further, the new definitions of all seven base units of the SI are also uniformly expressed using the explicit-constant formulation, and specific mises en pratique will be drawn up to explain the realization of the definitions of each of the base units in a practical way.

The new definitions will come into force on 20 May 2019.

Connect to the BIPM's YouTube channel to watch a recording of the live open session of the 26th General Conference on Weights and Measures which took place on 16 November 2018:

In the revised SI all units are defined in terms of a set of seven reference constants, to be known as the "defining constants of the SI", namely the caesium hyperfine splitting frequency, the speed of light in vacuum, the Planck constant, the elementary charge (i.e. the charge on a proton), the Boltzmann constant, the Avogadro constant, and the luminous efficacy of a specified monochromatic source.

This results in a simpler and more fundamental definition of the entire SI, and dispenses with the last of the definitions based on a material artefact – the international prototype of the kilogram.

Resolution 1 (2018): On the revision of the International System of Units (SI)

On 16 November 2018 the CGPM at its 26th meeting adopted a revision of the SI in which:

    • the unperturbed ground state hyperfine transition frequency of the caesium 133 atom capital DeltanuCs is 9 192 631 770 Hz,

    • the speed of light in vacuum c is 299 792 458 m/s,

    • the Planck constant h is 6.626 070 15 × 10–34 J s,

    • the elementary charge e is 1.602 176 634 × 10–19 C,

    • the Boltzmann constant k is 1.380 649 × 10–23 J/K,

    • the Avogadro constant NA is 6.022 140 76 × 1023 mol–1,

    • the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, is 683 lm/W,


    • the hertz, joule, coulomb, lumen, and watt, with unit symbols Hz, J, C, lm, and W, respectively, are related to the units second, metre, kilogram, ampere, kelvin, mole, and candela, with unit symbols s, m, kg, A, K, mol, and cd, respectively, according to
      Hz = s–1, J = kg m2 s–2, C = s A, lm = cd m2 m–2 = cd sr, and W = kg m2 s–3,

    • the numerical values of h, e, k, and NA are based on the most recent CODATA adjustment.

The SI may alternatively be defined by statements that explicitly define seven individual base units: the second, metre, kilogram, ampere, kelvin, mole, and candela. These correspond to the seven base quantities time, length, mass, electric current, thermodynamic temperature, amount of substance, and luminous intensity. All other units are then obtained as products of powers of the seven base units, which involve no numerical factors; these are called coherent derived units.

For full details the reader is referred to the formal texts of the adopted Resolutions:

See also:

    The revision of the SI will ensure that the SI continues to meet the needs of science, technology, and commerce in the 21st century.

    Of the seven base units of the SI, only the kilogram is still defined in terms of a material artefact, namely the international prototype kept at the BIPM. The major disadvantage of the present definition of the kilogram is that it refers to the mass of the artefact which, by its very nature, we know cannot be absolutely stable.

    click for more details about the periodic verifications

    The results of comparisons between the official copies and the international prototype show some divergence with time. The graph opposite shows changes of about 5 parts in 108, equivalent to 50 µg, in the mass of the standards since their first calibration more than 100 years ago. Note that this graph shows only the relative changes from the mass of the international prototype (corresponding to the zero value of the y-axis). The drift in the mass of the international prototype itself since 1889 cannot be shown but it must certainly be present. The rate of change of its mass can be determined only by absolute experiments which up to now are of insufficiently high precision.

    Unknown changes in the mass unit also influence the electrical units, because the definition of the ampere is related to the kilogram. Similarly, the definitions of the mole and candela also depend on the kilogram.

    At its 21st meeting (1999) the 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 ensure continuity of mass values.

    When the definition of the kilogram is based on an invariant of nature instead of a material artefact, it will be possible to realize the SI unit of mass at any place, at any time and by anyone (see also What is a mise en pratique?). Resolution 1 (2011) also highlights the following advantages:

    • The uncertainties of all SI electrical units realized directly or indirectly by means of the Josephson and quantum Hall effects together with the SI values of the Josephson and von Klitzing constants KJ and RK would be significantly reduced if the kilogram were redefined so as to be linked to an exact numerical value of h, and if the ampere were to be redefined so as to be linked to an exact numerical value of the elementary charge e.

    • The kelvin is currently defined in terms of an intrinsic property of water that, while being an invariant of nature, in practice depends on the purity and isotopic composition of the water used. The kelvin would be better defined if it were linked to an exact numerical value of the Boltzmann constant kB.

    • Redefining the mole so that it is linked to an exact numerical value of the Avogadro constant NA would have the consequence that it is no longer dependent on the definition of the kilogram even when the kilogram is defined so that it is linked to an exact numerical value of h. This would thereby emphasize the distinction between the quantities "amount of substance" and "mass".

    • The uncertainties of the values of many other important fundamental constants and energy conversion factors would be eliminated or greatly reduced if h, e, kB and NA had exact numerical values when expressed in SI units.

The following roadmap sets out the likely timescale to redefinition of the units in 2018:

In October 2017 at its 106th meeting, the CIPM noted (Decision CIPM/106-10) that the agreed conditions for the redefinition are now met and decided to submit draft Resolution A to the 26th meeting of the General Conference on Weigths and Measures (CGPM) and to undertake all other necessary steps to proceed with the planned redefinition of the kilogram, ampere, kelvin and mole.

The 26th meeting of the CGPM will take place the 13-16 November 2018 in Versailles.

After intense communication with all stakeholders, the draft of the 9th edition of the SI Brochure was endorsed by the CIPM at its 106th meeting and is now being edited by the BIPM.

[dated 6 February 2019]

The CIPM Consultative Committees prepared draft mises en pratique for the new definitions of the units:

[version 11.0]

These documents will remain as drafts until the new definitions come into force on 20 May 2019.

See also:

Resolution 1 "On the revision of the International System of Units (SI)" was adopted by the CGPM at its 26th meeting on 16 November 2018.

At its meeting in 2014 the CGPM encouraged:

    • continued effort in the NMIs, the BIPM, and academic institutions to obtain data relevant to the determination of h, e, kB, and NA with the requisite uncertainties;

    • the NMIs to continue acting through the CCs to discuss and review this data;

    • the CIPM to continue developing a plan to provide the path via the Consultative Committees and the CCU for implementing Resolution 1 adopted by the CGPM at its 24th meeting (2011); and

    • continued effort by the CIPM, together with its Consultative Committees, the NMIs, the BIPM, and other organizations such as the International Organization of Legal Metrology (OIML), to complete all work necessary for the CGPM at its 26th meeting to adopt a resolution that would replace the current SI with the revised SI, provided the amount of data, their uncertainties, and level of consistency are deemed satisfactory.

For more information please see: