View source for Kilogram

{{Short description|Metric unit of mass}}
{{Redirect|kg||KG (disambiguation){{!}}KG}}
{{Use dmy dates|date=October 2024}}
{{Infobox unit
| name          = kilogram
| image         = Poids fonte 5 kg à 2 hg 02.jpg
| caption       = A series of 5, 2, 1, 0.5 and 0.2 kilogram weights, made of [[cast iron]]
| standard      = [[SI]]
| quantity      = [[mass]]
| symbol        = kg
| units1        = [[Avoirdupois]]
| inunits1      = {{Ublist|≈ {{val|2.2046}} [[pound (mass)|pounds]]<ref group="Note">The avoirdupois pound is part of both [[United States customary units|United States customary system of units]] and the [[Imperial units|Imperial system of units]]. It is [[International yard and pound|defined as exactly]] {{val|0.45359237|u=kilograms}}.</ref>|≈ {{val|35.274|ul=oz}}}}
| units2        = [[British Gravitational System|British Gravitational]]
| inunits2      = ≈ {{val|0.0685}} [[slug (unit)|slugs]]
| units3        = Unified atomic mass units
| inunits3      = {{val|6.02214076|e=26|ul=Da}}
}}

The '''kilogram''' (also spelled '''kilogramme'''<ref name=":1" />) is the [[SI base unit|base unit]] of [[mass]] in the [[International System of Units]] (SI), equal to one thousand grams. It has the unit symbol '''kg'''.<ref name=":1" /> The word "kilogram" is formed from the combination of the [[metric prefix]] [[kilo-]] (meaning one thousand) and [[gram]];<ref>{{Cite web |title=Kilogram |url=https://www.collinsdictionary.com/dictionary/english/kilogram |access-date=14 October 2024 |website=Collins Online Dictionary}}</ref> it is commonly [[Shortening (linguistics)|shortened]] to "'''kilo'''" (plural "kilos").<ref>[https://www.merriam-webster.com/dictionary/kilo Merriam-Mebster definition of Kilo]</ref>

The kilogram is an SI [[Base unit (measurement)|base unit]], defined ultimately in terms of three [[International System of Units#SI defining constants|defining constant]]s of the SI, namely [[Caesium standard|a specific transition frequency]] of the [[Isotopes of caesium|caesium-133]] atom, the [[speed of light]], and the [[Planck constant]].<ref name="SIBrochure9thEd"/>{{rp|131}}  A properly equipped [[metrology]] laboratory can calibrate a mass measurement instrument such as a [[Kibble balance]] as a primary standard for the kilogram mass.<ref>{{Cite web |date=July 7, 2021 |title=Mise en pratique for the definition of the kilogram in the SI |url=https://www.bipm.org/documents/20126/41489673/SI-App2-kilogram.pdf/5881b6b5-668d-5d2b-f12a-0ef8ca437176?version=1.9&t=1637237674882&download=false |access-date=February 18, 2022 |website=BIPM.org}}</ref>

The kilogram was originally defined in 1795 during the [[French Revolution]] as the mass of one [[litre]] of [[properties of water|water]] (originally at 0&nbsp;[[Celsius|°C]], later changed to the temperature of its [[maximum density]], approximately 4&nbsp;°C). The current definition of a kilogram agrees with this original definition to within 30 [[parts per million]] (0.003%). In 1799, the [[platinum]] ''[[Grave (unit)#Kilogramme des Archives|Kilogramme des Archives]]'' replaced it as the standard of mass. In 1889, a cylinder composed of [[Platinum–iridium alloy|platinum–iridium]], the [[International Prototype of the Kilogram]] (IPK), became the standard of the unit of mass for the metric system and remained so for 130 years, before the current standard was [[2019 revision of the SI|adopted in 2019]].<ref name="vox" />

== Definition ==
The kilogram is defined in terms of three defining constants:<ref name="SIBrochure9thEd">{{citation |title=The International System of Units (SI) |author=International Bureau of Weights and Measures |author-link=New SI |date=20 May 2019  |edition=9th |isbn=978-92-822-2272-0 |url=https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf| archive-url = https://web.archive.org/web/20211018184555/https://www.bipm.org/documents/20126/41483022/SI-Brochure-9.pdf/fcf090b2-04e6-88cc-1149-c3e029ad8232 |archive-date=18 October 2021 |url-status=live}}</ref>  
* a specific atomic transition frequency {{math|[[Caesium standard|Δ''ν''<sub>Cs</sub>]]}}, which defines the duration of the [[second]],
* the [[speed of light]] in [[vacuum]] {{mvar|c}}, which when combined with the second, defines the length of the [[metre]],
* and the [[Planck constant]] {{mvar|h}}, which when combined with the metre and second, defines the [[mass]] of the kilogram.
The formal definition according to the [[General Conference on Weights and Measures]] (CGPM) is:
<!-- this is an exact quote. Do not change it.-->{{Blockquote|The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant {{mvar|h}} to be {{val|6.62607015|e=-34}} when expressed in the unit J⋅s, which is equal to kg⋅m<sup>2</sup>⋅s<sup>−1</sup>, where the metre and the second are defined in terms of {{mvar|c}} and {{math|Δ''ν''<sub>Cs</sub>}}.|source= CGPM<ref name="draft-resolution-A">
{{citation
 |title=Draft Resolution A "On the revision of the International System of units (SI)" to be submitted to the CGPM at its 26th meeting (2018)
 |url=https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN.pdf
 |archive-url=https://web.archive.org/web/20210402142630/https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN.pdf
 |archive-date=April 2, 2021
 |url-status=live
}}</ref><ref>[http://www.bipm.org/en/committees/cipm/meeting/105.html Decision CIPM/105-13 (October 2016)]. The day is the 144th anniversary of the [[Metre Convention]].</ref>}}
Defined in term of those units, the kg is formulated as:<ref>[https://www.bipm.org/en/publications/si-brochure/ SI Brochure: The International System of Units (SI)]. BIPM, 9th edition, 2019.</ref>

{{block indent|1=kg = {{math|{{sfrac|({{val|299792458}}){{sup|2}}|({{val|6.62607015|e=-34}})({{val|9192631770}})}}{{sfrac|{{gaps|''h''|Δ''ν''<sub>Cs</sub>}}|''c''{{sup|2}}}}}}}}
{{block indent|1=
≈ {{math|({{val|1.4755214|e=40}}){{sfrac|{{gaps|''h''|Δ''ν''<sub>Cs</sub>}}|''c''{{sup|2}}}}}}.}}

This definition is generally consistent with previous definitions: the kilogram remains within 30 [[Parts-per notation|parts per million]] (0.003%) of the mass of one [[litre]] of [[water]] at the temperature of its [[maximum density]] (approximately 4&nbsp;[[Celsius|°C]]), with the [[density of water]] at that temperature very close to 1&nbsp;kg/L.<ref>The density of water is {{val|0.999972|u=g/cm3}} at {{val|3.984|u=°C}}. See {{cite book |last=Franks |first=Felix |title=The Physics and Physical Chemistry of Water |url=https://books.google.com/books?id=5f_xBwAAQBAJ&pg=PA376 |year=2012 |publisher=Springer |isbn=978-1-4684-8334-5}}</ref>

=== Timeline of previous definitions ===
[[File:International prototype of the kilogram aka Le Grand K.jpg|thumb|right|The [[International Prototype of the Kilogram]], whose mass was defined to be one kilogram from 1889 to 2019.]]
* 1793: The [[grave (unit)|grave]] (the precursor of the kilogram) was defined as the mass of 1 [[litre]] (dm<sup>3</sup>) of [[water]], which was determined to be 18841 [[Grain (unit)|grains]].<ref>{{cite book |title=Annales de chimie ou Recueil de mémoires concernant la chimie et les arts qui en dépendent |date=1792|url=https://books.google.com/books?id=FufDNJHvgFEC&q=18841+grains+grave&pg=RA1-PA278 |location= Paris |publisher= Chez Joseph de Boffe |page= 277 |volume = 15–16| last1=Guyton| last2=Lavoisier| last3=Monge| last4=Berthollet| display-authors=etal|author-link1=Louis-Bernard Guyton de Morveau| author-link2=Antoine Lavoisier| author-link3=Gaspard Monge| author-link4=Claude Louis Berthollet}}</ref> 
* 1795: the [[gram]] ({{Fraction||1|1000}} of a kilogram) was provisionally defined as the mass of one cubic [[centimetre]] of water at the [[melting point]] of ice (0&nbsp;[[Celsius|°C]]).<ref>{{lang|fr|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}}</ref>
* 1799: The [[Kilogramme des Archives]] was manufactured as a prototype. It had a mass equal to the mass of 1&nbsp;dm<sup>3</sup> of water at the [[temperature]] of its [[maximum density]] (approximately 4&nbsp;°C).<ref name="Zupko">{{cite book |last=Zupko |first=Ronald Edward| author-link =Ronald Edward Zupko|date=1990 |title=Revolution in Measurement: Western European Weights and Measures Since the Age of Science |url=https://archive.org/details/bub_gb_uYCNFkRgXCoC |location=Philadelphia |publisher=American Philosophical Society |isbn=978-0-87169-186-6}}</ref>
* 1875–1889: The [[Metre Convention]] was signed in 1875, leading to the production of the [[International Prototype of the Kilogram]] (IPK) in 1879 and its adoption in 1889.<ref>{{cite encyclopedia|url = http://www.britannica.com/EBchecked/topic/378767/Treaty-of-the-Metre|title = Treaty of the Metre|encyclopedia = [[Encyclopædia Britannica]]|access-date = 18 July 2023|year = 2023}}</ref>
* 2019: The kilogram was [[2019 revision of the SI|defined]] in terms of the [[Planck constant]], the [[speed of light]] and [[Caesium standard|hyperfine transition frequency of <sup>133</sup>Cs]] as approved by the [[General Conference on Weights and Measures]] (CGPM) on 16 November 2018.<ref name="vox"/>

== Name and terminology ==
The kilogram is the only [[SI base unit|base SI unit]] with an [[SI prefix]] (''kilo'') as part of its name. The word ''kilogramme'' or ''kilogram'' is derived from the [[French language|French]] {{lang|fr|kilogramme}},<ref name=OED/> which itself was a learned coinage, prefixing the [[Koine Greek|Greek]] stem of {{lang|grc|χίλιοι}} {{Transliteration|grc|khilioi}} "a thousand" to {{lang|la|gramma}}, a Late Latin term for "a small weight", itself from Greek {{lang|grc|γράμμα}}.<ref>
{{cite book
 |title = The Concise Oxford Dictionary
 |year = 1964
 |last1 = Fowlers |first1 = HW
 |last2 = Fowler |first2 = FG
 |publisher = The Clarendon Press
 |location = Oxford
}} Greek {{lang|grc|γράμμα}} (as it were {{lang|grc|[[:wikt:γράφω|γράφ]]-[[:wikt:-μα|μα]]}}, Doric {{lang|grc|γράθμα}}) means "something written, a letter", but it came to be used as a unit of weight, apparently equal to {{sfrac|1|24}} of an [[ounce]] ({{sfrac|1|288}} of a {{lang|la|[[Pound (mass)#Roman libra|libra]]}}, which would correspond to about 1.14 grams in modern units), at some time during Late Antiquity. French {{lang|fr|gramme}} was adopted from Latin {{lang|la|gramma}}, itself quite obscure, but found in the {{lang|la|Carmen de ponderibus et mensuris}} (8.25) attributed by [[Remmius Palaemon]] (fl. 1st century), where it is the weight of two {{lang|la|[[obolus|oboli]]}} (Charlton T. Lewis, Charles Short, ''A Latin Dictionary'' [https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0059%3Aentry%3Dgramma2 s.v. "gramma"], 1879).
Henry George Liddell. Robert Scott. ''[[A Greek-English Lexicon]]'' (revised and augmented edition, Oxford, 1940) [https://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.04.0057:entry=gra/mma s.v. γράμμα], citing the 10th-century work ''[[Geoponica]]'' and a 4th-century papyrus edited in L. Mitteis, ''Griechische Urkunden der Papyrussammlung zu Leipzig'', vol. i (1906), 62 ii 27.</ref> 
The word {{lang|fr|kilogramme}} was written into French law in 1795, in the ''Decree of [[French Republican Calendar|18 Germinal]]'',<ref>
{{cite web
 |url = http://mjp.univ-perp.fr/france/1793mesures.htm
 |title = Décret relatif aux poids et aux mesures du 18 germinal an 3 (7 avril 1795)
 |language = fr
 |trans-title=Decree of 18 Germinal, year III (April 7, 1795) regarding weights and measures
 |website = Grandes lois de la République
 |publisher = Digithèque de matériaux juridiques et politiques, Université de Perpignan
 |access-date = November 3, 2011
}}</ref>
which revised the provisional system of units introduced by the French [[National Convention]] two years earlier, where the {{lang|fr|gravet}} had been defined as weight ({{lang|fr|poids}}) of a cubic centimetre of water, equal to 1/1000 of a {{lang|fr|[[Grave (unit)|grave]]}}.<ref>{{lang|fr|Convention nationale, décret du 1<sup>er</sup> août 1793, ed. Duvergier, ''Collection complète des lois, décrets, ordonnances, règlemens avis du Conseil d'état, publiée sur les éditions officielles du Louvre''|italic=unset}}, vol. 6 (2nd ed. 1834), [https://books.google.com/books?id=0mYZAAAAYAAJ&pg=PA70 p. 70].
The ''metre'' ({{lang|fr|mètre}}) on which this definition depends was itself defined as the ten-millionth part of a quarter of Earth's [[Meridian (geography)|meridian]], given in [[Units of measurement in France before the French Revolution|traditional units]] as 3 {{lang|fr|[[foot (unit)|pieds]]}}, 11.44 {{lang|fr|lignes}} (a {{lang|fr|ligne}} being the 12th part of a {{lang|fr|pouce}} (inch), or the 144th part of a {{lang|fr|pied}}.</ref> In the decree of 1795, the term {{lang|fr|gramme}} thus replaced {{lang|fr|gravet}}, and {{lang|fr|kilogramme}} replaced {{lang|fr|grave}}.<ref name="Zupko"/>

The French spelling was adopted in Great Britain when the word was used for the first time in English in 1795,<ref>
{{cite journal
 |url = https://books.google.com/books?id=24QCAAAAYAAJ&q=kilogramme+weights&pg=PA557
 |title = Paris, during the year 1795
 |author = Peltier, Jean-Gabriel
 |journal = Monthly Review |date=1795 |volume=17|pages=556|access-date = August 2, 2018
}} Contemporaneous English translation of the French decree of 1795</ref><ref name=OED>
{{cite web
 |url = http://www.oed.com/viewdictionaryentry:showfullentry/true?t:ac=Entry/103396
 |archive-url = https://archive.today/20130130180039/http://www.oed.com/viewdictionaryentry:showfullentry/true?t:ac=Entry/103396
 |url-status = dead
 |archive-date = 30 January 2013
 |website = Oxford English Dictionary
 |publisher = Oxford University Press
 |title = Kilogram
 |access-date = November 3, 2011
}}</ref> with the spelling ''kilogram'' being adopted in the United States. In the United Kingdom both spellings are used, with "kilogram" having become by far the more common.<ref name=":1">
{{cite web
 |url          = http://english.oxforddictionaries.com/definition/kilogram
 |title        = Kilogram
 |website      = Oxford Dictionaries
 |access-date  = November 3, 2011
 |url-status   = dead
 |archive-url  = https://archive.today/20130131014115/http://english.oxforddictionaries.com/definition/kilogram
 |archive-date = January 31, 2013
 |df           = mdy-all
}}</ref> UK law regulating the units to be used when [[Weights and Measures Acts of the United Kingdom|trading by weight or measure]] does not prevent the use of either spelling.<ref>
{{cite web
 |url = http://www.legislation.gov.uk/ukpga/1985/72/section/92
 |title = Spelling of "gram", etc
 |website = [[Weights and Measures Acts of the United Kingdom|Weights and Measures Act 1985]]
 |publisher = [[Office of Public Sector Information|Her Majesty's Stationery Office]]
 |date = October 30, 1985
 |access-date = November 6, 2011
}}</ref>

In the 19th century the French word {{lang|fr|kilo}}, a [[Clipping (morphology)|shortening]] of {{lang|fr|kilogramme}}, was imported into the English language where it has been used to mean both kilogram<ref>
{{cite encyclopedia
 |year=1989
 |edition = 2nd
 |title = kilo (n1)
 |encyclopedia = [[Oxford English Dictionary]]
 |publisher = Oxford University Press
 |location = Oxford
 |url = http://www.oed.com/viewdictionaryentry/Entry/103394
 |access-date = November 8, 2011}}</ref> and kilometre.<ref>{{cite encyclopedia
 |year = 1989
 |edition = 2nd
 |title = kilo (n2)
 |encyclopedia = [[Oxford English Dictionary]]
 |publisher = Oxford University Press
 |location = Oxford
 |url = http://www.oed.com/viewdictionaryentry/Entry/103395
 |access-date = November 8, 2011
}}</ref> While ''kilo'' as an alternative is acceptable, to ''[[The Economist]]'' for example,<ref>{{cite news |url=http://www.frzee.com/Education/The%20Economist%20Style%20Guide.pdf |title=Style Guide |newspaper=[[The Economist]] |date=January 7, 2002 |archive-url=https://web.archive.org/web/20170701053545/http://www.frzee.com/Education/The%20Economist%20Style%20Guide.pdf |archive-date=July 1, 2017 |url-status=dead |access-date=November 8, 2011}}</ref> the Canadian government's [[Termium Plus]] system states that "SI (International System of Units) usage, followed in scientific and technical writing" does not allow its usage and it is described as "a common informal name" on Russ Rowlett's Dictionary of Units of Measurement.<ref>
{{cite web |url=https://www.btb.termiumplus.gc.ca/tpv2guides/guides/wrtps/index-eng.html?lang=eng&lettr=indx_catlog_k&page=96vUJlKx4UCA.html |website=Termium Plus |publisher=Government of Canada |title=kilogram, kg, kilo |date=October 8, 2009 |access-date =May 29, 2019 }}</ref><ref>
{{cite web |url=http://www.unc.edu/~rowlett/units/dictK.html |title=kilo |website=How Many? |access-date=November 6, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20111116205434/http://www.unc.edu/~rowlett/units/dictK.html |archive-date=November 16, 2011}}</ref> When the [[United States Congress]] gave the metric system legal status in 1866, it permitted the use of the word ''kilo'' as an alternative to the word ''kilogram'',<ref>
{{cite web
 |url=http://lamar.colostate.edu/~hillger/laws/metric-act-bill.html
 |title=H.R. 596, An Act to authorize the use of the metric system of weights and measures
 |author=29th Congress of the United States, Session 1
 |date=May 13, 1866
 |url-status=dead
 |archive-url=https://web.archive.org/web/20150705015307/http://lamar.colostate.edu/~hillger/laws/metric-act-bill.html
 |archive-date=July 5, 2015
}}</ref> but in 1990 revoked the status of the word ''kilo''.<ref>
{{cite journal
 |journal      = [[Federal Register]]
 |volume       = 63
 |issue        = 144
 |date         = July 28, 1998
 |page         = 40340
 |url          = http://physics.nist.gov/cuu/pdf/SIFedReg.pdf
 |title        = Metric System of Measurement:Interpretation of the International System of Units for the United States; Notice
 |quote        = '''Obsolete Units''' As stated in the 1990 Federal Register notice, ...
 |access-date  = November 10, 2011
 |url-status   = dead
 |archive-url  = https://web.archive.org/web/20111015081850/http://physics.nist.gov/cuu/pdf/SIFedReg.pdf
 |archive-date = October 15, 2011
 |df           = mdy-all
}}</ref>

The SI system was introduced in 1960 and in 1970 the [[International Bureau of Weights and Measures|BIPM]] started publishing the [[International System of Units#SI Brochure|''SI Brochure'']], which contains all relevant decisions and recommendations by the [[General Conference on Weights and Measures|CGPM]] concerning units. The ''SI Brochure'' states that "It is not permissible to use abbreviations for unit symbols or unit names ...".<ref>{{SIBrochure8th|page = 130}}</ref><ref group = Note>The French text (which is the authoritative text) states "{{lang|fr|Il n'est pas autorisé d'utiliser des abréviations pour les symboles et noms d'unités ...}}"</ref>

For use with east Asian  character sets, the SI symbol is encoded as a single Unicode character, {{unichar|338f|SQUARE KG}} in the [[CJK Compatibility]] block.

== Redefinition based on fundamental constants ==
[[File:Unit relations in the new SI.svg|thumb|upright=1.35|The [[International System of Units|SI system]] after the 2019 redefinition: the kilogram is now fixed in terms of the [[second]], the [[speed of light]] and the [[Planck constant]]; furthermore the [[ampere]] no longer depends on the kilogram]]
[[File:Watt balance, large view.jpg|thumb|upright=1.35|A [[Kibble balance]], which was originally used to measure the [[Planck constant]] in terms of the IPK, can now be used to calibrate secondary standard weights for practical use.]]
{{main|2019 revision of the SI}}

The replacement of the [[International Prototype of the Kilogram]] (IPK) as the primary standard was motivated by evidence accumulated over a long period of time that the mass of the IPK and its replicas had been changing; the IPK had diverged from its replicas by approximately 50&nbsp;micrograms since their manufacture late in the 19th century. This led to [[Alternative approaches to redefining the kilogram|several competing efforts]] to develop measurement technology precise enough to warrant replacing the kilogram artefact with a definition based directly on physical fundamental constants.<ref name="vox">{{cite news |last1=Resnick |first1=Brian |title=The new kilogram just debuted. It's a massive achievement. |url=https://www.vox.com/science-and-health/2019/5/17/18627757/kilogram-redefined-world-metrology-day-explained |access-date=May 23, 2019 |publisher=vox.com |date=May 20, 2019}}</ref>

The [[International Committee for Weights and Measures]] (CIPM) approved a [[2019 revision of the SI|revision]] in November 2018 that defines the kilogram by defining the [[Planck constant]] to be exactly {{val|6.62607015|e=−34|u=kg⋅m<sup>2</sup>⋅s<sup>−1</sup>}}, effectively defining the kilogram in terms of the second and the metre. The new definition took effect on 20 May 2019.<ref name="vox"/><ref name=draft-resolution-A /><ref>
{{cite news
 |url= https://www.bbc.com/news/science-environment-46143399
 |title= Kilogram gets a new definition
 |author= Pallab Ghosh
 |date= November 16, 2018
 |journal= BBC News
 |access-date= November 16, 2018
}}</ref>

Prior to the redefinition, the kilogram and several other SI units based on the kilogram were defined by a man-made metal artifact: the ''[[Kilogramme des Archives]]'' from 1799 to 1889, and the IPK from 1889 to 2019.<ref name="vox"/>

In 1960, the [[metre]], previously similarly having been defined with reference to a single platinum-iridium bar with two marks on it, was redefined in terms of an invariant physical constant (the wavelength of a particular emission of light emitted by [[krypton]],<ref>{{SIbrochure8th|page=112}}</ref> and later the [[speed of light]]) so that the standard can be independently reproduced in different laboratories by following a written specification.

At the 94th Meeting of the CIPM in 2005, it was recommended that the same be done with the kilogram.<ref>
{{cite conference
 |url=https://www.bipm.org/cc/CIPM/Allowed/94/CIPM-Recom1CI-2005-EN.pdf
 |conference=94th meeting of the International Committee for Weights and Measures
 |title=Recommendation 1: Preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole in terms of fundamental constants
 |date=October 2005
 |page=233
 |access-date=February 7, 2018
 |url-status=live
 |archive-url=https://web.archive.org/web/20070630011658/https://www.bipm.org/cc/CIPM/Allowed/94/CIPM-Recom1CI-2005-EN.pdf
 |archive-date=June 30, 2007
}}</ref><!-- Original URL: http://www.bipm.org/utils/en/pdf/CIPM2005-EN.pdf -->

In October 2010, the CIPM voted to submit a resolution for consideration at the [[General Conference on Weights and Measures]] (CGPM), to "take note of an intention" that the kilogram be defined in terms of the [[Planck constant]], {{math|''h''}} (which has dimensions of energy times time, thus mass × length{{sup|2}} / time) together with other physical constants.<ref>{{cite journal|url=https://www.nist.gov/pml/wmd/20101026_si.cfm |title=NIST Backs Proposal for a Revamped System of Measurement Units |journal=NIST |date=October 26, 2010 |publisher=Nist.gov |access-date=April 3, 2011}}</ref><ref name="draft">
{{cite web
 |url = http://www.bipm.org/utils/en/pdf/si_brochure_draft_ch2.pdf
 |title = Draft Chapter 2 for SI Brochure, following redefinitions of the base units
 |author = Ian Mills
 |publisher = CCU
 |date = September 29, 2010
 |access-date =January 1, 2011
}}</ref> This resolution was accepted by the 24th conference of the CGPM<ref>
{{cite conference
 |url= http://www.bipm.org/utils/en/pdf/24_CGPM_Resolution_1.pdf
 |title= Resolution 1 – On the possible future revision of the International System of Units, the SI
 |conference= 24th meeting of the General Conference on Weights and Measures
 |location = Sèvres, France
 |date = October 17–21, 2011
 |access-date =October 25, 2011
}}</ref> in October 2011 and further discussed at the 25th conference in 2014.<ref name=":0">{{Cite web|url=http://www.bipm.org/en/CGPM/db/25/1/|title=BIPM – Resolution 1 of the 25th CGPM|website=www.bipm.org|access-date=March 27, 2017}}</ref><ref>
{{cite press release
 | url = http://www.bipm.org/utils/en/pdf/Press_release_resolution_1_CGPM.pdf
 | title = General Conference on Weights and Measures approves possible changes to the International System of Units, including redefinition of the kilogram.
 | publisher = [[General Conference on Weights and Measures]]
 | location = Sèvres, France
 | date = October 23, 2011
 | access-date = October 25, 2011
}}</ref> Although the Committee recognised that significant progress had been made, they concluded that the data did not yet appear sufficiently robust to adopt the revised definition, and that work should continue to enable the adoption at the 26th meeting, scheduled for 2018.<ref name=":0" /> Such a definition would theoretically permit any apparatus that was capable of delineating the kilogram in terms of the Planck constant to be used as long as it possessed sufficient precision, accuracy and stability. The [[Kibble balance]] is one way to do this.<ref>{{cite journal |doi=10.1088/0026-1394/53/5/A46 |title=The watt or Kibble balance: A technique for implementing the new SI definition of the unit of mass |journal=Metrologia |volume=53 |issue=5 |pages=A46–A74 |year=2016 |last1=Robinson |first1=Ian A. |last2=Schlamminger |first2=Stephan |pmid=35023879 |pmc=8752041 |bibcode=2016Metro..53A..46R |doi-access=free }}</ref>

As part of this project, a variety of very [[Alternative approaches to redefining the kilogram|different technologies and approaches]] were considered and explored over many years. Some of these approaches were based on equipment and procedures that would enable the reproducible production of new, kilogram-mass prototypes on demand (albeit with extraordinary effort) using measurement techniques and material properties that are ultimately based on, or traceable to, physical constants. Others were based on devices that measured either the acceleration or weight of hand-tuned kilogram test masses and that expressed their magnitudes in electrical terms via special components that permit traceability to physical constants. All approaches depend on converting a weight measurement to a mass and therefore require precise measurement of the strength of gravity in laboratories ([[gravimetry]]). All approaches would have precisely fixed one or more constants of nature at a defined value.{{Citation needed|date=July 2023}}

== SI multiples ==
{{main|Orders of magnitude (mass)}}<!--NOTE TO EDITORS: This section is linked from [[Caffeine]] as well as many other Wikipedia articles. Please do not delete or rename without fixing them.-->
{{Redirect|Milligram|the American band|Milligram (band)|the Serbian band|Miligram (band)|the horse|Milligram (horse)}}
Because an SI unit may not have multiple prefixes (see [[SI prefix]]), prefixes are added to ''[[gram]]'', rather than the base unit ''kilogram'', which already has a prefix as part of its name.<ref>BIPM: SI Brochure: Section 3.2, ''[http://www.bipm.org/en/publications/si-brochure/section3-2.html The kilogram]'' {{Webarchive|url=https://web.archive.org/web/20160329162832/http://www.bipm.org/en/publications/si-brochure/section3-2.html |date=March 29, 2016 }}</ref> For instance, one-millionth of a kilogram is 1{{nbsp}}mg (one milligram), not 1{{nbsp}}μkg (one microkilogram).
<div style="float:center; margin-left: 1em;">
{{SI multiples
| unit = gram
| symbol = g
| p= | n= | mc= | m= | k= | M= | G= | T=
| note = Common prefixed units are in bold face.<ref group="Note">Criterion: A combined total of at least five occurrences on the [[British National Corpus]] and the [[Corpus of Contemporary American English]], including both the singular and the plural for both the -''gram'' and the -''gramme'' spelling.</ref>
}}
</div>

== Usage and practical issues with SI mass units ==
<!--EDITORS NOTE REGARDING BULLET-POINT TEXT SIZE: The below bullet notes have alternately been tried in both normal-size text, then small text, then normal, etc. While it may be tempting to reduce the below section (as they are near-parenthetical, footnote-type notes), small text has proven too difficult to read on many computer systems. Accordingly, please leave the below text in normal size. -->
* Serious medication errors have been made by confusing milligrams and micrograms when micrograms has been abbreviated.<ref name="SPCG">{{cite web |url=http://www.palliativecareguidelines.scot.nhs.uk/guidelines/about-the-guidelines/pharmacological-considerations/prescribing-information-for-liquid-medicines.aspx |title=Prescribing Information for Liquid Medicines |website=Scottish Palliative Care Guidelines |access-date=June 15, 2015 |archive-url=https://web.archive.org/web/20180710152658/http://www.palliativecareguidelines.scot.nhs.uk/guidelines/about-the-guidelines/pharmacological-considerations/prescribing-information-for-liquid-medicines.aspx |archive-date=July 10, 2018 |url-status=dead }}</ref> The abbreviation "mcg" rather than the SI symbol "μg" is formally mandated for medical practitioners in the US by the [[Joint Commission on Accreditation of Healthcare Organizations]] (JCAHO).<ref>{{cite web |title=New Joint Commission "Do Not Use" List: Abbreviations, Acronyms, and Symbols |url=http://www.aapmr.org/practice/guidelines/Pages/New-Joint-Commission-symbols.aspx |publisher=American Academy of Physical Medicine and Rehabilitation |access-date=19 February 2024 |archive-url=https://web.archive.org/web/20150915012112/http://www.aapmr.org/practice/guidelines/Pages/New-Joint-Commission-symbols.aspx |archive-date=15 September 2015}}</ref> In the United Kingdom, the [[National Institute for Health and Care Excellence]] and Scottish Palliative Care Guidelines state that "micrograms" and "nanograms" must both be written in full, and never abbreviated as "mcg", "μg" or "ng" respectively.<ref name="SPCG" /><ref>{{cite web |title=Prescription writing |url=https://bnf.nice.org.uk/medicines-guidance/prescription-writing/ |publisher=[[National Institute for Health and Care Excellence]] |access-date=19 February 2024}}</ref>
* The hectogram (100&nbsp;g) (Italian: ''ettogrammo'' or ''etto'') is a very commonly used unit in the retail food trade in Italy.<ref>Tom Stobart, ''The Cook's Encyclopedia'', 1981, p. 525</ref><ref>J.J. Kinder, V.M. Savini, ''Using Italian: A Guide to Contemporary Usage'', 2004, {{isbn|0521485568}}, p. 231</ref><ref>Giacomo Devoto, Gian Carlo Oli, ''Nuovo vocabolario illustrato della lingua italiana'', 1987, ''s.v.'' 'ètto': "frequentissima nell'uso comune: ''un e. di caffè, un e. di mortadella; formaggio a 2000 lire l'etto''"</ref>
* The former standard spelling and abbreviation "deka-" and "dk" produced abbreviations such as "dkm" (dekametre) and "dkg" (dekagram).<ref>U.S. National Bureau of Standards, ''The International Metric System of Weights and Measures'', "Official Abbreviations of International Metric Units", 1932, [https://books.google.com/books?id=ujwM_fwA9lEC&pg=PA13 p. 13]</ref> {{As of|2020|post=,}} the abbreviation "dkg" (10&nbsp;g) is still used in parts of central Europe in retail for some foods such as cheese and meat.<ref name="Czeck dkg ham">{{cite web |title=Jestřebická hovězí šunka 10 dkg {{!}} Rancherské speciality |url=https://eshop.rancherskespeciality.cz/Jestrebicka-hovezi-sunka-10-dkg-d189.htm |website=eshop.rancherskespeciality.cz |language=cs|access-date=June 16, 2020|archive-url=https://web.archive.org/web/20200616032253/https://eshop.rancherskespeciality.cz/Jestrebicka-hovezi-sunka-10-dkg-d189.htm|archive-date=June 16, 2020}}</ref><ref name="Slovak dkg ham">{{cite web |title=Sedliacka šunka 1 dkg {{!}} Gazdovský dvor – Farma Busov Gaboltov |url=http://farmabusoveshop.sk/sedliacka-sunka-1-dkg |website=Sedliacka šunka 1 dkg |language=sk|access-date=June 16, 2020|archive-url=https://web.archive.org/web/20200616033111/http://farmabusoveshop.sk/sedliacka-sunka-1-dkg|archive-date=June 16, 2020}}</ref><ref name="Czech dkg cheese">{{cite web |title=sýr bazalkový – Farmářské Trhy |url=http://www.e-farmarsketrhy.cz/syry-z-kravskeho-mleka/syr-bazalkovy |website=www.e-farmarsketrhy.cz |language=cs|access-date=June 16, 2020|archive-url=https://web.archive.org/web/20200616033522/http://www.e-farmarsketrhy.cz/syry-z-kravskeho-mleka/syr-bazalkovy|archive-date=June 16, 2020}}</ref><ref name="Hungarian menu">{{cite web |title=English Menu – Cafe Mediterran |url=http://www.mediterrangrill.hu/english-menu/ |language=en|access-date=June 16, 2020|archive-url=https://web.archive.org/web/20200616034445/http://www.mediterrangrill.hu/english-menu/|archive-date=June 16, 2020|quote= Beef steak 20 dkg; Beef steak 40 dkg;Thick crust 35 dkg}}</ref><ref name="Hungarian dkg cheese">{{cite web |title=Termékek – Csíz Sajtműhely |url=https://csizsajtmuhely.hu/sajtrendeles/ |language=hu|access-date=June 16, 2020|archive-url=https://web.archive.org/web/20200616035724/https://csizsajtmuhely.hu/sajtrendeles/|archive-date=June 16, 2020}}</ref>
* The unit name ''megagram'' is rarely used, and even then typically only in technical fields in contexts where especially rigorous consistency with the SI standard is desired. For most purposes, the name ''[[tonne]]'' is instead used. The tonne and its symbol, "t", were adopted by the CIPM in 1879. It is a non-SI unit accepted by the BIPM for use with the SI. According to the BIPM, "This unit is sometimes referred to as 'metric ton' in some English-speaking countries."<ref>''Non-SI units that are accepted for use with the SI'', [https://www.bipm.org/en/publications/si-brochure/ SI Brochure: Section 4 (Table 8)], BIPM</ref>

== See also ==
{{Portal|Physics}}
* {{Annotated link|Inertia}}
* {{Annotated link|Kibble balance}}
* {{Annotated link|Kilogram-force}}
* {{Annotated link|Mass versus weight}}
* {{Annotated link|Metric system}}
* {{Annotated link|National Institute of Standards and Technology}} (NIST)
* {{Annotated link|Newton (unit)|Newton}}
* {{Annotated link|Specific quantity}}
* {{Annotated link|Standard gravity}}
* {{Annotated link|Weight}}

== Notes ==
{{reflist|group="Note"}}

== References ==
{{reflist|30em}}

== External links ==
{{Commons category}}
{{External media
| float = right
| width = 40%
| image2 = NIST: [http://patapsco.nist.gov/imagegallery/retrieve.cfm?imageid=49&dpi=72&fileformat=jpg K20, the US National Prototype Kilogram] resting on an egg crate fluorescent light panel
| image3 = BIPM: ''[https://web.archive.org/web/20070926215612/http://www.bipm.org/en/scientific/mass/pictures_mass/cleaning.html Steam cleaning a 1&nbsp;kg prototype before a mass comparison]''
| image4 = BIPM: [https://web.archive.org/web/20070926221138/http://www.bipm.org/en/scientific/mass/pictures_mass/vault.html The IPK and its six sister copies in their vault]
| image5 = The Age: [http://www.theage.com.au/ffximage/2007/06/14/rgN1506_csiro_wideweb__470x343,0.jpg Silicon sphere for the Avogadro Project]
| image6 = NPL: [http://www.npl.co.uk/content/conMediaFile/1083 The NPL's Watt Balance project]
| image7 = NIST: This particular [http://museum.nist.gov/object.asp?ObjID=51 Rueprecht Balance], an Austrian-made precision balance, was used by the NIST from 1945 until 1960
| image8 = BIPM: [https://web.archive.org/web/20070926215949/http://www.bipm.org/en/scientific/mass/research_mass/flexure-strip.html The FB{{nbhyph}}2 flexure-strip balance], the BIPM's modern precision balance featuring a standard deviation of one ten-billionth of a kilogram (0.1{{nbsp}}μg)
| image9 = BIPM: [https://web.archive.org/web/20070926220910/http://www.bipm.org/en/scientific/mass/pictures_mass/mettler.html Mettler HK1000 balance], featuring 1{{nbsp}}μg resolution and a 4{{nbsp}}kg maximum mass. Also used by NIST and Sandia National Laboratories' Primary Standards Laboratory
| image10 = Micro-g LaCoste: [http://www.microglacoste.com/images/FG5.small.jpg FG{{nbhyph}}5 absolute gravimeter], ([http://www.microglacoste.com/images/fg5schem.jpg diagram]), used in national laboratories to measure gravity to 2{{nbsp}}[[Gal (acceleration)|μGal]]<!--NOTE TO EDITORS: The Micro-g LaCoste Web site does not display properly on some computer systems. The accuracy is NOT 2&nbsp;mGal and is indeed 2&nbsp;μGal. Please see discussion topic at [[Talk:Kilogram#Micro-g_LaCoste_FG-5_accuracy]]--> accuracy
}}
* [https://www.nist.gov/public_affairs/releases/electrokilogram.cfm NIST Improves Accuracy of 'Watt Balance' Method for Defining the Kilogram]
* The UK's National Physical Laboratory (NPL): [<!-- http://www.npl.co.uk/server.php?show=ConWebDoc.2087 -->https://web.archive.org/web/20160323212037/http://www.npl.co.uk/reference/faqs/are-any-problems-caused-by-having-the-kilogram-defined-in-terms-of-a-physical-artefact-(faq-mass-and-density) <!-- An overview of the problems with an artefact-based kilogram -->Are any problems caused by having the kilogram defined in terms of a physical artefact? (FAQ – Mass & Density)]
* NPL: ''[http://www.npl.co.uk/science-technology/mass-and-force/research/npl-kibble-balance NPL Kibble balance]''
* Metrology in France: ''[http://www.french-metrology.com/en/feature/watt-balance.asp Watt balance] {{Webarchive|url=https://web.archive.org/web/20140319084923/http://www.french-metrology.com/en/feature/watt-balance.asp |date=March 19, 2014 }}''
* Australian National Measurement Institute: ''[http://www.measurement.gov.au/SCIENCETECHNOLOGY/Pages/MassandRelatedQuantities.aspx Redefining the kilogram through the Avogadro constant]''
* International Bureau of Weights and Measures (BIPM): [http://www.bipm.org/en/home/ Home page]
* NZZ Folio: ''[http://www.nzzfolio.ch/www/d80bd71b-b264-4db4-afd0-277884b93470/showarticle/fb0ba22e-46b7-43a5-8320-ef16483b7e91.aspx What a kilogram really weighs]''
* NPL: ''[<!-- http://www.npl.co.uk/server.php?show=ConWebDoc.1380 -->http://www.npl.co.uk/reference/faqs/what-are-the-differences-between-mass,-weight,-force-and-load-(faq-mass-and-density) What are the differences between mass, weight, force and load?]''
* BBC: ''[https://news.bbc.co.uk/2/hi/science/nature/7084099.stm Getting the measure of a kilogram]''
* NPR: ''[https://www.npr.org/templates/story/story.php?storyId=112003322 This Kilogram Has A Weight-Loss Problem]'', an interview with [[National Institute of Standards and Technology]] physicist Richard Steiner
* [https://web.archive.org/web/20110717124427/http://www.inrim.it/Nah/Web_Nah/home.htm Avogadro and molar Planck constants for the redefinition of the kilogram]
* [http://arquivo.pt/wayback/20160513163237/http://www.inrim.it/know/ Realization of the awaited definition of the kilogram]
* {{cite news |url=https://www.theguardian.com/science/2018/nov/09/in-the-balance-scientists-vote-on-first-change-to-kilogram-in-century |title=In the balance: scientists vote on first change to kilogram in a century |newspaper=[[The Guardian]] |first=Ian |last=Sample |date=November 9, 2018 |access-date=November 9, 2018}}

=== Videos ===
* [https://www.youtube.com/thebipm The BIPM] – [[YouTube]] channel
* [https://www.youtube.com/watch?v=yhgb23tAFFs "The role of the Planck constant in physics" – presentation at 26th CGPM meeting at Versailles, France, November 2018 when voting on superseding the IPK took place] on [[YouTube]]

{{SI units}}
{{Authority control}}
{{Good article}}

[[Category:SI base units]]
[[Category:Units of mass]]
[[Category:1000 (number)]]