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Non-SI units mentioned in the SI

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Title: Non-SI units mentioned in the SI  
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Subject: Tonne, Degree (angle), Litre, Atomic mass unit, Astronomical unit
Collection: Non-Si Metric Units, Si Units
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Non-SI units mentioned in the SI

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 General Conference on Weights and Measures (CGPM) accepts their use as being multiples or submultiples of SI-units, they have important contemporary application worldwide, or are otherwise commonly encountered worldwide.


  • Units officially accepted for use with the SI 1
  • Common units not officially sanctioned 2
  • See also 3
  • References 4

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 ° plane angle (dimensionless unit) 1° = (π / 180) rad
minute plane angle (dimensionless unit) 1′ = (1 / 60)° = (π / 10800) rad
second plane 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 = 1000 cc = 1000 cm3
tonne t mass (decimal unit multiple) 1 t = 103 kg = 1 Mg
astronomical unit au or AU[1][3] length 1 AU = 1.495978707×1011 m (exact)
Logarithmic units
neper Np level of a field quantity 1 Np = 1[4]
    level of a power quantity 1 Np = 1[5]
bel B level of a field quantity 1 B = ln(10)/2 Np = ln(10)/2[6]
    level of a power quantity
decibel dB level of a field quantity 1 dB = 0.1 B [7]
    level of a power quantity
Units with experimentally determined values
electronvolt eV energy 1 eV = 1.60217653(14)×10−19 J
unified atomic mass unit
mass 1 u = 1 Da = 1.66053886(28)×10−27 kg
Natural units (n.u.)
n.u. of speed (speed of light in vacuum) c0 speed 299792458 m/s (exact)[8]
n.u. of action (reduced Planck constant) ħ action 1.05457168(18)×10−34 J·s
n.u. of mass (electron mass) me mass 9.1093826(16)×10−31 kg
n.u. of time ħ / mec02 time 1.2880886677(86)×10−21 s
Atomic units (a.u.)
a.u. of charge (elementary charge) e electric charge 1.60217653(14)×10−19 C
a.u. of mass (electron mass) me mass 9.1093826(16)×10−31 kg
a.u. of action (reduced Planck constant) ħ action 1.05457168(18)×10−34 J·s
a.u. of length, bohr (Bohr radius) a0 length 0.5291772108(18)×10−10 m
a.u. of energy, hartree (Hartree energy) Eh 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 = 100 pm
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
atmosphere atm pressure 1 atm = 1013.25 mbar = 1013.25 hPa = 101325 Pa exactly (commonly used in atmospheric meteorology, in oceanography and for pressures within liquids, or in the industry for pressures within containers of liquefied gas)
dyne per square centimetre (barye) Ba pressure 1 Ba = 0.1 Pa = 0.1 N/m2 = 100 g·cm−1·s−2
millimetre of mercury mmHg pressure 1 mmHg = 133.322387415 Pa at 0 °C = 133.322387415 N/m2 at 0 °C = 133.322387415 kg·cm−1·s−2 at 0 °C
torr Torr pressure 1 Torr = 1/760 atm = 133.322368421... Pa = 133.322368421... N/m2 = 133.322368421... kg·cm−1·s−2

See also


  1. ^ a b "Units outside the SI". SI brochure (8th edition). BIPM. 2014 [2008]. Retrieved 2015-06-16. 
  2. ^ Taylor, Barry N. (ed.) (2001). "The International System of Units (SI)" (PDF). Special Publication 330. Gaithersburg, Maryland:  
  3. ^ "Manuscript Preparation: AJ & ApJ Author Instructions". American Astronomical Society. Retrieved 2015-06-15. 
  4. ^ LF = ln(F / F0) Np
  5. ^ LP = 12 ln(P / P0) Np
  6. ^ LF = 2 log10(F / F0) B
  7. ^ LP = log10(P / P0) B
  8. ^ In this case it is not the speed of light, but the length of the metre that is obtained by experiment
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