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]

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
Name Symbol Quantity Value in SI units
minute min time 1 min = 60 s
hour h 1 h = 60 min = 3600 s
day d 1 d = 24 h = 1440 min = 86400 s
astronomical unit au length 1 au = 149597870700 m
degree ° plane angle and phase angle 1° = (π⁄180) rad
arcminute ′ 1′ = (1⁄60)° = (π⁄10800) rad
arcsecond ″ 1″ = (1⁄60)′ = (1⁄3600)° = (π⁄648000) rad
hectare ha area 1 ha = 1 hm2 = 10000 m2
litre l, L volume 1 L = 1 dm3 = 1000 cm3 = 0.001 m3
tonne t mass 1 t = 1 Mg = 1000 kg
dalton Da 1 Da = 1.66053906892(52)×10−27 kg[3][note 1]
electronvolt eV energy 1 eV = 1.602176634×10−19 J[4]
neper Np logarithmic ratio quantity 1 Np = 1
bel, decibel B, 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.

Name Symbol Quantity Equivalent official unit
gal[note 2] Gal acceleration 1 Gal = 1 cm/s2 = 0.01 m/s2
unified atomic mass unit[note 3] u mass 1 u = 1 Da
volt-ampere reactive var reactive power 1 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.

Name Symbol 1st–3rd SI Brochures 4–6th SI Brochures 7th SI Brochure 8th SI Brochure 9th SI Brochure
astronomical unitau[note 4] acceptedomittedacceptedacceptedaccepted
parsecpc acceptedomittedomittedomittedomitted
neperNp omittedomittedacceptedlistedaccepted
belB omittedomittedacceptedlistedaccepted
decibeldB omittedomittedomittedlistedaccepted
unified atomic mass unitu acceptedacceptedacceptedacceptedfootnote
daltonDa omittedomittedfootnoteacceptedaccepted
natural unit (n.u.) of speedc0 omittedomittedomittedlistedomitted
n.u. of actionħ omittedomittedomittedlistedomitted
n.u. of massme omittedomittedomittedlistedomitted
n.u. of timeħ⁄(mec02) omittedomittedomittedlistedomitted
atomic unit (a.u.) of chargee omittedomittedomittedlistedomitted
a.u. of massme omittedomittedomittedlistedomitted
a.u. of actionħ omittedomittedomittedlistedomitted
a.u. of lengtha0 omittedomittedomittedlistedomitted
a.u. of energyEh omittedomittedomittedlistedomitted
a.u. of timeħ⁄Eh omittedomittedomittedlistedomitted
nautical mileM temporarytemporarytemporarylistedomitted
knotkn temporarytemporarytemporarylistedomitted
ångströmÅ temporary temporary temporary listed omitted
area temporary temporary temporary omitted omitted
hectareha temporary temporary temporary accepted accepted
barnb temporary temporary temporary listed omitted
barbar temporary temporary temporary listed omitted
standard atmosphereatm temporary listed listed omitted omitted
galGal temporary temporary listed listed footnote
curieCi temporary temporary listed omitted omitted
roentgenR temporary temporary listed omitted omitted
radrad temporary temporary listed omitted omitted
remrem omitted temporary listed omitted omitted
ergerg listedlistedlistedlistedomitted
dynedyn listedlistedlistedlistedomitted
poiseP listedlistedlistedlistedomitted
stokesst listedlistedlistedlistedomitted
maxwellMx listedlistedlistedlistedomitted
gaussG listedlistedlistedlistedomitted
ørstedOe listedlistedlistedlistedomitted
photph listedlistedlistedlistedomitted
fermi listedlistedlistedomittedomitted
metric carat listedlistedlistedomittedomitted
torrTorr listedlistedlistedomittedomitted
kilogram-forcekgf listedlistedomittedomittedomitted
caloriecal listedlistedlistedomittedomitted
micronμ listedlistedlistedomittedomitted
x-unitxu listedlistedomittedomittedomitted
stilbsb listedlistedomittedomittedomitted
gamma (Mass)γ listedlistedlistedomittedomitted
γ (magnetic flux density)γ listedlistedomittedomittedomitted
lambdaλ listedlistedomittedomittedomitted
janskyJy omittedomittedlistedomittedomitted
millimetre of mercurymmHg omittedomittedomittedlistedomitted

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
  1. A footnote in the 9th SI Brochure gives an exact definition of the dalton.
  2. This is a unit employed in geodesy and geophysics to express acceleration due to gravity.
  3. The unified mass unit (u) is a synonym of the dalton (Da). In edition 9 (2019) of the SI Brochure, the unified mass unit is no longer listed as being accepted for use with SI units, though it notes its equivalence with the dalton in a footnote. In edition 8 (2006), both names were mentioned in parallel. In edition 7 (1998), position was the reverse of that in edition 9. Earlier editions mentioned only u.
  4. The symbol given for the astronomical unit changed from ua in the 8th to au in the 9th SI Brochure.

References
  1. International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), ISBN 92-822-2213-6, archived (PDF) from the original on 2021-06-04, retrieved 2021-12-16
  2. Bureau international des poids et mesures, "Non-SI units that are accepted for use with the SI", in: Le Système international d'unités (SI) / The International System of Units (SI), 9th ed. (Sèvres: 2019), ISBN 9789282222720, c. 4, pp. 145–146.
  3. "2022 CODATA Value: atomic mass constant". The NIST Reference on Constants, Units, and Uncertainty. NIST. May 2024. Retrieved 2024-05-18.
  4. "2022 CODATA Value: electron volt". The NIST Reference on Constants, Units, and Uncertainty. NIST. May 2024. Retrieved 2024-05-18.
  5. Bureau international des poids et mesures, Le Système international d'unités (SI) / The International System of Units (SI), 8th ed. (Sèvres: Organisation Intergouvernementale de la Convention du Mètre, 2006‑05), ISBN 9282222136.
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