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Mass and related quantities: Unit of mass (kilogram)
Unit of mass (kilogram)
SI Brochure, Section

    The international prototype of the kilogram, an artefact made of platinum-iridium, is kept at the BIPM under the conditions specified by the 1st CGPM in 1889 when it sanctioned the prototype and declared:

      This prototype shall henceforth be considered to be the unit of mass.

    The 3rd CGPM (1901), in a declaration intended to end the ambiguity in popular usage concerning the use of the word "weight", confirmed that:

      The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

    The complete declaration appears here.

    It follows that the mass of the international prototype of the kilogram is always 1 kilogram exactly, m(grand K) = 1 kg. However, due to the inevitable accumulation of contaminants on surfaces, the international prototype is subject to reversible surface contamination that approaches 1 µg per year in mass. For this reason, the CIPM declared that, pending further research, the reference mass of the international prototype is that immediately after cleaning and washing by a specified method (PV, 1989, 57, 104-105 and PV, 1990, 58, 95-97). The reference mass thus defined is used to calibrate national standards of platinum-iridium alloy (Metrologia, 1994, 31, 317-336).

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Decisions relating to the base units of the SI
SI Brochure, from Appendix 1


    1st CGPM, 1889 sanction of the prototype kilogram
    3rd CGPM, 1901 declaration on distinguishing mass and weight, and on the conventional value of gn
    CIPM, 1967 declaration on applying prefixes to the gram
    21st CGPM, 1999 future redefinition of the kilogram
    23rd CGPM, 2007 on the possible redefinition of certain base units of the International System of Units (SI)
    24th CGPM, 2011 on the possible future revision of the International System of Units, the SI
    25th CGPM, 2014 on the future revision of the International System of Units, the SI
Mass and related quantities
Unit of mass (kilogram)
Committee structure
Strategic plan
International equivalence
Calibration and measurement capabilities (CMCs)
International comparisons
At the BIPM
BIPM laboratory work
Calibration services
Recent BIPM publications in the scientific literature
BIPM contact persons