## ::Kilogram

### ::concepts

Kilogram::would    Which::units    Title::balance    Defined::national    Standard::constant    Weights::relative

{{#invoke:redirect hatnote|redirect}} {{ safesubst:#invoke:Unsubst||\$N=Use mdy dates |date=__DATE__ |\$B= }} {{#invoke:Infobox|infobox}} The kilogram or kilogramme (SI unit symbol: kg), is the base unit of mass in the International System of Units (SI) (the Metric system) and is defined as being equal to the mass of the International Prototype of the Kilogram (IPK).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The gram, 1/1000th of a kilogram, was originally defined in 1795 as the mass of one cubic centimeter of water at the melting point of water.<ref>Gramme, le poids absolu d'un volume d'eau pure égal au cube de la centième partie du mètre, et à la température de la glace fondante.; The term poids absolu was at the time used alongside masse for the concept of "mass" (which latter term had first been introduced in its strict physical sense in English in 1704). See e.g. Mathurin Jacques Brisson, Dictionnaire raisonné de toutes les parties de la Physique, Volland, 1787, p. 401.</ref> The original prototype kilogram, manufactured in 1799 and from which the IPK is derived, had a mass equal to the mass of 1.000028 dm3 of water at its maximum density at approximately 4 °C.{{ safesubst:#invoke:Unsubst||date=__DATE__ |\$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }}

The kilogram is the only SI base unit with an SI prefix ("kilo", symbol "k") as part of its name. It is also the only SI unit that is still directly defined by an artifact rather than a fundamental physical property that can be reproduced in different laboratories. Three other base units (Cd, A, mol) and 17 derived units (N, Pa, J, W, C, V, F, Ω, S, Wb, T, H, kat, Gy, Sv, lm, lx) in the SI system are defined relative to the kilogram, so its stability is important. Only 8 other units do not require the kilogram in their definition: temperature (K, °C), time and frequency (s, Hz, Bq), length (m), and angle (rad, sr).<ref name=SIbrochure8th/>:112–118

The International Prototype Kilogram was commissioned by the General Conference on Weights and Measures (CGPM) under the authority of the Metre Convention (1875), and is in the custody of the International Bureau of Weights and Measures (BIPM) who hold it on behalf of the CGPM. After the International Prototype Kilogram had been found to vary in mass over time relative to its reproductions, the International Committee for Weights and Measures (CIPM) recommended in 2005 that the kilogram be redefined in terms of a fundamental constant of nature. At its 2011 meeting, the CGPM agreed in principle that the kilogram should be redefined in terms of the Planck constant. The decision was originally deferred until 2014; in 2014 it was deferred again until the next meeting.<ref>http://www.bipm.org/utils/common/pdf/CGPM-2014/25th-CGPM-Resolutions.pdf</ref>

The International Prototype Kilogram (IPK) is rarely used or handled. Copies of the IPK kept by national metrology laboratories around the world were compared with the IPK in 1889, 1948, and 1989 to provide traceability of measurements of mass anywhere in the world back to the IPK.

The avoirdupois (or international) pound, used in both the Imperial system and U.S. customary units, is defined as exactly {{safesubst:#invoke:val|main}}, making one kilogram approximately equal to 2.2046 avoirdupois pounds. Other traditional units of weight and mass around the world are also defined in terms of the kilogram, making the IPK the primary standard for virtually all units of mass on Earth.

Kilogram sections
Intro  Name and terminology   Nature of mass    Kilogramme des Archives    International prototype kilogram    Proposed future definitions    SI multiples    Glossary    See also    Notes    References    External links

 PREVIOUS: Intro NEXT: Name and terminology << >>