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Non-SI units mentioned in the SI

While the International System of Units (SI) is used throughout the world in all fields, many non-SI units continue to be used in the scientific, technical, and commercial literature. Some units are deeply embedded in history and culture, and their use has not been entirely replaced by their SI alternatives. The authority behind the SI system, the General Conference on Weights and Measures, recognised and acknowledged such traditions by compiling a list of non-SI units accepted for use with SI.[1]

While not an SI-unit, the litre may be used with SI units. It is equivalent to (10 cm)3 = (1 dm)3 = 10−3 m3.

Some units of time, angle, and legacy non-SI units have a long history of use. Most societies have used the solar day and its non-decimal subdivisions as a basis of time and, unlike the foot or the pound, these were the same regardless of where they were being measured. The radian, being 1/2π of a revolution, has mathematical advantages but is rarely used for navigation. Further, the units used in navigation around the world are similar. The tonne, litre, and hectare were adopted by the CGPM in 1879 and have been retained as units that may be used alongside SI units, having been given unique symbols. The catalogued units are given below.[2]

Most of these, in order to be converted to the corresponding SI unit, require conversion factors that are not powers of ten. Some common examples of such units are the customary units of time, namely the minute (conversion factor of 60 s/min, since 1 min = 60 s), the hour (3600 s), and the day (86400 s); the degree (for measuring plane angles, 1° = π⁄180 rad); and the electronvolt (a unit of energy, 1 eV = 1.602176634×10−19 J).[2]

This is a list of units that are not defined as part of the International System of Units (SI) but are otherwise mentioned in the SI Brochure,[2] listed as being accepted for use alongside SI-units, or for explanatory purposes.

Units officially accepted for use with the SI

NameSymbolQuantityValue in SI units
minutemintime1 min = 60 s
hourh1 h = 60 min = 3600 s
dayd1 d = 24 h = 1440 min = 86400 s
astronomical unitaulength1 au = 149597870700 m
degree°plane angle and phase angle1° = (π⁄180) rad
arcminute′1′ = (1⁄60)° = (π⁄10800) rad
arcsecond″1″ = (1⁄60)′ = (1⁄3600)° = (π⁄648000) rad
hectarehaarea1 ha = 1 hm2 = 10000 m2
litrel, Lvolume1 L = 1 dm3 = 1000 cm3 = 0.001 m3
tonnetmass1 t = 1 Mg = 1000 kg
daltonDa1 Da = 1.66053906660(50)×10−27 kg[3][note 1]
electronvolteVenergy1 eV = 1.602176634×10−19 J[4]
neperNplogarithmic ratio quantity1 Np = 1
bel, decibelB, dB—

The SI prefixes can be used with several of these units, but not, for example, with the non-SI units of time.

Other units defined but not officially sanctioned

The following table lists units that are effectively defined in sidenotes and footnotes in the 9th SI brochure. Units that are mentioned without a definition or that occur in historical material recorded in the appendices are not included.

NameSymbolQuantityEquivalent official unit
gal[note 2]Galacceleration1 Gal = 1 cm/s2 = 0.01 m/s2
unified atomic mass unit[note 3]umass1 u = 1 Da
volt-ampere reactivevarreactive power1 var = 1 VA

Changes to units mentioned in the SI

With the publication of each edition of the SI brochure, the list of non-SI units listed in tables changed compared to the preceding SI brochures.[5] The table below compares the status of each unit for which the status has changed between editions of the SI Brochure.

NameSymbol1st–3rd SI Brochures4–6th SI Brochures7th SI Brochure8th SI Brochure9th SI Brochure
astronomical unitau[note 4]acceptedomittedacceptedacceptedaccepted
parsecpcacceptedomittedomittedomittedomitted
neperNpomittedomittedacceptedlistedaccepted
belBomittedomittedacceptedlistedaccepted
decibeldBomittedomittedomittedlistedaccepted
unified atomic mass unituacceptedacceptedacceptedacceptedfootnote
daltonDaomittedomittedfootnoteacceptedaccepted
natural unit (n.u.) of speedc0omittedomittedomittedlistedomitted
n.u. of actionħomittedomittedomittedlistedomitted
n.u. of massmeomittedomittedomittedlistedomitted
n.u. of timeħ⁄(mec02)omittedomittedomittedlistedomitted
atomic unit (a.u.) of chargeeomittedomittedomittedlistedomitted
a.u. of massmeomittedomittedomittedlistedomitted
a.u. of actionħomittedomittedomittedlistedomitted
a.u. of lengtha0omittedomittedomittedlistedomitted
a.u. of energyEhomittedomittedomittedlistedomitted
a.u. of timeħ⁄Ehomittedomittedomittedlistedomitted
nautical mileMtemporarytemporarytemporarylistedomitted
knotkntemporarytemporarytemporarylistedomitted
ångströmÅtemporarytemporarytemporarylistedomitted
areatemporarytemporarytemporaryomittedomitted
hectarehatemporarytemporarytemporaryacceptedaccepted
barnbtemporarytemporarytemporarylistedomitted
barbartemporarytemporarytemporarylistedomitted
standard atmosphereatmtemporarylistedlistedomittedomitted
galGaltemporarytemporarylistedlistedfootnote
curieCitemporarytemporarylistedomittedomitted
roentgenRtemporarytemporarylistedomittedomitted
radradtemporarytemporarylistedomittedomitted
remremomittedtemporarylistedomittedomitted
ergerglistedlistedlistedlistedomitted
dynedynlistedlistedlistedlistedomitted
poisePlistedlistedlistedlistedomitted
stokesstlistedlistedlistedlistedomitted
maxwellMxlistedlistedlistedlistedomitted
gaussGlistedlistedlistedlistedomitted
ørstedOelistedlistedlistedlistedomitted
photphlistedlistedlistedlistedomitted
fermilistedlistedlistedomittedomitted
metric caratlistedlistedlistedomittedomitted
torrTorrlistedlistedlistedomittedomitted
kilogram-forcekgflistedlistedomittedomittedomitted
caloriecallistedlistedlistedomittedomitted
micronμlistedlistedlistedomittedomitted
x-unitxulistedlistedomittedomittedomitted
stilbsblistedlistedomittedomittedomitted
gamma (Mass)γlistedlistedlistedomittedomitted
γ (magnetic flux density)γlistedlistedomittedomittedomitted
lambdaλlistedlistedomittedomittedomitted
janskyJyomittedomittedlistedomittedomitted
millimetre of mercurymmHgomittedomittedomittedlistedomitted

In this table, the status descriptions have the following meanings:

  • "accepted": The unit is accepted for use with the SI.
  • "temporary": The unit is accepted for use with the SI, but is planned to be phased out.
  • "listed": The unit is defined in a table of units in the brochure but not accepted for use with the SI.
  • "footnote": The unit is defined in a footnote or side note, without any mention in the main text.
  • "omitted": There is no mention of the unit in the brochure, excluding historical appendices.

See also

Notes and references

Notes

References

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