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Non-SI units mentioned in the SI

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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[1][2], because either the SI accepts their use as being multiples or submultiples of SI-units, or they have important contemporary application, or are otherwise commonly encountered.

Units officially accepted for use with the SI

Name Symbol Quantity Equivalent SI unit
Widely used units expected to be used indefinitely
minute min time (SI unit multiple) 1 min = 60 s
hour h time (SI unit multiple) 1 h = 60 min = 3600 s
day d time (SI unit multiple) 1 d = 24 h = 1440 min = 86400 s
degree of arc ° angle (dimensionless unit) 1° = (π/180) rad
minute of arc ′ angle (dimensionless unit) 1′ = (1/60)° = (π/10800) rad
second of arc ″ angle (dimensionless unit) 1″ = (1/60)′ = (1/3600)° = (π/648000) rad
hectare ha area (decimal unit multiple) 1 ha = 100 a = 10000 m2 = 1 hm2
litre l or L volume (decimal unit multiple) 1 l = 1 dm3 = 0.001 m3
tonne t mass (decimal unit multiple) 1 t = 103 kg = 1 Mg
Logarithmic units
neper Np dimensionless ratio of field quantities LF = ln(F/F0) Np
    dimensionless ratio of power quantities LP = 1⁄2ln(P/P0) Np
bel, decibel B, dB dimensionless ratio of field quantities LF = 2 log10(F/F0) B
    dimensionless ratio of power quantities LP = log10(P/P0) B
Units with experimentally determined values
electron-volt eV energy 1 eV = 1.60217653(14)×10−19 J
atomic mass unit
dalton 		u
Da 		mass 1 u = 1 Da = 1.66053886(28)×10−27 kg
astronomical unit ua[3][4][5] length 1 au = 1.49597870691(6)×1011 m
Natural units (n.u.)
n.u. of speed c0 speed 299792458 m/s (exact)[6]
n.u. of action ħ action 1.05457168(18)×10−34 J·s
n.u. of mass me mass of the electron 9.1093826(16)×10−31 kg
n.u. of time time 1.2880886677(86)×10−21 s
Atomic units (a.u.)
a.u. elementary charge e elementary charge 1.60217653(14)×10−19 C
a.u. of length (bohr) a0 Bohr radius 0.5291772108(18)×10−10 m
a.u. of energy (hartree) Eh Hartree energy 4.35974417(75)×10−18 J
a.u. of time ħ/Eh time 2.418884326505(16)×10−17 s

Common units not officially sanctioned

Name Symbol Quantity Equivalent SI unit
Ångström, angstrom Å length 1 Å = 0.1 nm = 10−10 m
nautical mile nm length 1 nautical mile = 1852 m
knot kt speed 1 knot = 1 nautical mile per hour = (1852/3600) m/s
are a area 1 a = 1 dam2 = 100 m2
barn b area 1 b = 10−28 m2
bar bar pressure 1 bar = 105 Pa
millibar mbar pressure 1 mbar = 1 hPa = 100 Pa (was used in atmospheric meteorology, the preferred unit is now the hectopascal)
atmosphere atm pressure 1 atm = 1013.25 mbar = 1013.25 hPa = 1.01325×105 [[Pa]] (commonly used in atmospheric meteorology, in oceanology and for pressures within liquids, or in the industry for pressures within containers of liquified gas)

See also

References

  1. ^ "Non-SI units accepted for use with the SI, and units based on fundamental constants" (PDF). SI brochure (8th edition). BIPM. Retrieved 2010-08-28.
  2. ^ Taylor, Barry N. (ed.) (2001). "The International System of Units (SI)" (PDF). Special Publication 330. Gaithersburg, Maryland: National Institute of Standards and Technology (NIST). {{cite web}}: |author= has generic name (help)
  3. ^ International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), p. 126, ISBN 92-822-2213-6, archived (PDF) from the original on 2021-06-04, retrieved 2021-12-16
  4. ^ ISO 80000
  5. ^ The IAU Style Manual (1989): The Preparation of Astronomical Papers and Reports (PDF), p. 23
  6. ^ In this case it is not the speed of light, but the length of the meter that is obtained by experiment
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