Editing Mass (section)

== Units of mass ==

{{further|Orders of magnitude (mass)}}

[[File:SI base unit.svg|thumb|The kilogram is one of the seven [[SI base units]].]]

The [[International System of Units]] (SI) unit of mass is the [[kilogram]] (kg). The kilogram is 1000&nbsp;grams (g), and was first defined in 1795 as the mass of one cubic decimetre of water at the [[melting point]] of ice. However, because precise measurement of a cubic decimetre of water at the specified temperature and pressure was difficult, in 1889 the kilogram was redefined as the mass of a metal object, and thus became independent of the metre and the properties of water, this being a copper prototype of the [[Grave (unit)|grave]] in 1793, the platinum [[Kilogramme des Archives]] in 1799, and the platinum–iridium [[International Prototype of the Kilogram]] (IPK) in 1889.

However, the mass of the IPK and its national copies have been found to drift over time. The [[2019 revision of the SI|re-definition of the kilogram and several other units]] came into effect on 20 May 2019, following a final vote by the [[CGPM]] in November 2018.<ref>{{cite journal
|url = https://www.nature.com/nphys/journal/v13/n2/pdf/nphys4029.pdf
|title = Metrology in 2019 |first=Klaus|last=von Klitzing|journal=Nature Physics|volume=13 |issue=2 |date=February 2017 |page=198|arxiv=1707.06785|bibcode=2017SSPMA..47l9503L|doi=10.1360/SSPMA2017-00044|s2cid = 133817316 }}</ref> The new definition uses only invariant quantities of nature: the [[speed of light]], the [[Caesium standard|caesium hyperfine frequency]], the [[Planck constant]] and the [[elementary charge]].<ref name=Brochure9_2016>
{{cite web
 |title=Draft of the ninth SI Brochure
 |publisher=BIPM
 |url=http://www.bipm.org/utils/common/pdf/si-brochure-draft-2016b.pdf
 |pages=2–9
 |date=10 November 2016
 |access-date=2017-09-10
}}</ref>

Non-SI units accepted for use with SI units include:
* the [[tonne]] (t) (or "metric ton"), equal to 1000&nbsp;kg
* the [[Electronvolt (mass)|electronvolt]] (eV), a unit of [[energy]], used to express mass in units of eV/''c''<sup>2</sup> through [[mass–energy equivalence]]
* the [[Dalton (unit)|dalton]] (Da), equal to 1/12 of the mass of a free [[carbon-12]] atom, approximately {{val|1.66|e=-27|u=kg}}.<ref group="note">The dalton is convenient for expressing the masses of atoms and molecules.</ref>

Outside the SI system, other units of mass include:
* the [[slug (unit)|slug]] (sl), an [[Imperial units|Imperial unit]] of mass (about 14.6&nbsp;kg)
* the [[Pound (mass)|pound]] (lb), a unit of mass (about 0.45&nbsp;kg), which is used alongside the similarly named [[pound (force)]] (about 4.5&nbsp;N), a unit of force<ref group="note">These are used mainly in the United States except in scientific contexts where SI units are usually used instead.</ref>
* the [[Planck mass]] (about {{val|2.18|e=-8|u=kg}}), a quantity derived from fundamental constants
* the [[solar mass]] ({{Solar mass}}), defined as the mass of the [[Sun]], primarily used in astronomy to compare large masses such as stars or galaxies (≈&nbsp;{{val|1.99|e=30|u=kg}})
* the mass of a particle, as identified with its inverse [[Compton wavelength]] ({{nowrap|1&nbsp;cm<sup>−1</sup> ≘ {{val|3.52|e=-41|u=kg}}}})
* the mass of a star or [[black hole]], as identified with its [[Schwarzschild radius]] ({{nowrap|1&nbsp;cm ≘ {{val|6.73|e=24|u=kg}}}}).

=== History of units of mass ===
{{excerpt|History of measurement#Units of mass}}