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International System of Units (SI)

Last updated:26 June 2020

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SI Base Unit

Quantity Base unit Definition
Name Symbol
Time second s The second [s] is a unit of time. It is defined by taking the fixed numerical value of the caesium frequency ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit s-1(equivalent to Hz).
Length metre m The metre [m] is a unit of length. It is defined by taking the fixed numerical value of the speed of light in a vacuum c to be 299,792,458 when expressed in the unit m· s-1.
Mass kilogram kg The kilogram [kg] is a unit of mass. It is defined by taking the value of the Planck constant h to be 6.62607015 × 10-34 when expressed in the unit s-1·m2·kg (equivalent to J·s).
Current ampere A The ampere [A] is a unit of electric current. It is defined by taking the value of the elementary charge e to be 1.602176634 × 10-19.Its unit is C, and is equivalent to A·s.
Thermodynamic temperature kelvin K The kelvin [K] is a unit of thermodynamic temperature. It is defined by taking the value of Boltzmann’s constant k to be 1.380649 × 10-23 when expressed in the unit s-2·m2·kg K-1 (equivalent to J·K-1).
Amount of substance mole mol The mole [mol] is a unit of the amount of a substance. 1 mole contains exactly 6.02214076 × 1023 elementary particles. Its value is the fixed numerical value the Avogadro constant NA when expressed in the unit mol-1, and is referred to as Avogadro’s number.
Luminous intensity candela cd The candela [cd] is a unit of luminous intensity, and is defined by taking the value of the luminous efficiency of monochromatic radiation of frequency 540 × 1012 Hz to be 683 when expressed in the unit s3·m-2·kg-1·cd·sr or cd·sr·W-1 (equivalent to lm·W-1).

Consistent Derived SI Units with Special Names and Symbols

Quantity Name Symbol Other SI unit equivalent SI base unit equivalent
Plane angle radian rad 1 m/m
Solid angle steradian sr 1 m2/m2
Frequency hertz Hz s1
Force newton N m・kg・s-2
Pressure, stress pascal Pa N/m2 m-1・kg・s-2
Energy, work, heat joule J N・m m2・kg・s-2
Power, radiant flux watt W J/s m2・kg・s-3
Electric charge, quantity of electricity coulomb C s・A
Electrical potential difference (voltage), electromotive force volt V W/A m 2・kg・s-3・A-1
Capacitance farad F C/V m -2・kg-1・s4・A2
Electrical resistance ohm Ω V/A m 2・kg・s-3・A-2
Conductance siemens S A/V m -2・kg-1・s3・A2
Magnetic flux weber Wb V・s m 2・kg・s-2・A-1
Magnetic flux density tesla T Wb/m2 kg・s-2・A-1
Inductance henry H Wb/A m2・kg・s-2・A-2
Celsius temperature degree Celsius °C K  
Luminous flux lumen lm cd・sr cd・m2/m2=cd
Illuminance lux lx lm/m2 m-2・cd
Radioactivity of radionuclides becquerel Bq s-1
Absorbed dose, specific energy distribution, kerma gray Gy J/kg m2・s-2
Dose equivalent, ambient dose equivalent, directional dose equivalent, individual dose equivalent sievert Sv J/kg m2・s-2
Enzymatic activity katal kat s-1・mol

SI Prefixes

Multiplier Name Symbol Decimal notation Decimal notation Name Symbol Decimal notation
101 deca da 10 10-1 deci d 0.1
102 hecto h 100 10-2 centi C 0.01
103 kilo k 1000 10-3 milli m 0.001
106 mega M 1000 000 10-6 micro μ 0.000 001
109 giga G 1000 000 000 10-9 nano n 0.000 000 001
1012 tera T 1000 000 000 000 10-12 pico p 0.000 000 000 001
1015 peta P 1000 000 000 000 000 10-15 femto f 0.000 000 000 000 001
1018 exa E 1000 000 000 000 000 000 10-18 atto a 0.000 000 000 000 000 001
1021 zetta Z 1000 000 000 000 000 000 000 10-21 zepto z 0.000 000 000 000 000 000 001
1024 yotta Y 1000 000 000 000 000 000 000 000 10-24 yocto y 0.000 000 000 000 000 000 000 001

Main SI Unit Conversion Factors

Force

N dyn kgf
1 1×105 1.019 72×10-1
1×10-5 1 1.019 72×10-6
9.806 65 9.806 65×105 1

Stress

Pa or N/m2 MPa or N/mm2 kgf/mm2 kgf/cm2
1 1×10-6 1.019 72×10-7 1.019 72×10-5
1×106 1 1.019 72×10-1 1.019 72×10
9.806 65×106 9.806 65 1 1×102
9.806 65×104 9.806 65×10-2 1×10-2 1
※1Pa=1N/m2, 1MPa=1N/mm2

Pressure

Pa kPa MPa bar kgf/cm2 atm mmH2O mmHg or Torr
1 1×10-3 1×10-6 1×10-5 1.019 72×10-5 9.869 23×10-6 1.019 72×10-1 7.500 62×10-3
1×10-3 1 1×10-3 1×10-2 1.019 72×10-2 9.869 23×10-3 1.019 72×102 7.500 62
1×106 1×103 1 1×10 1.019 72×10 9.869 23 1.019 72×105 7.500 62×103
1×105 1×102 1×10-1 1 1.019 72 9.869 23×10-1 1.019 72×104 7.500 62×102
9.806 65×104 9.806 65×10 9.806 65×10-2 9.806 65×10-1 1 9.678 41×10-1 1×104 7.355 59×102
1.013 25×105 1.013 25×102 1.013 25×10-1 1.013 25 1.033 23 1 1.033 23×104 7.600 00×102
9.806 65 9.806 65×10-3 9.806 65×10-6 9.806 65×10-5 1×10-4 9.678 41×10-5 1 7.355 59×10-2
1.333 22×102 1.333 22×10-1 1.333 22×10-4 1.333 22×10-3 1.359 51×10-3 1.315 79×10-3 1.359 51×10 1
※1Pa=1N/m2

Energy, Work, Heat

J kW・h kgf・m kcal
1 2.777 78×10-7 1.019 72×10-1 2.388 89×10-4
3.600×106 1 3.670 98×105 8.600 0×102
9.806 65 2.724 07×10-6 1 2.342 70×10-3
4.186 05×103 1.162 79×10-3 4.268 58×102 1
※1J=1W・s, 1J=1N・m

Power, Heat Flow

W kgf・m/s PS kcal/h
1 1.019 72×10-1 1.359 62×10-3 8.600 0×10-1
9.806 65 1 1.333 33×10-2 8.433 71
7.355 ×102 7.5 ×10 1 6.325 29×102
1.162 79 1.185 72×10-1 1.580 95×10-3 1
※1W=1J/s, PS:French horse power

Viscosity

Pa・s cP P
1 1×103 1×10
1×103 1 1×10-2
1×10-1 1×102 1
※1P=1dyn・s/cm2 =1g/cm・s, 1Pa・s=1N・s/m2, 1cP=1mPa・s

Kinematic Viscosity

m2/s cSt St
1 1×106 1×104
1×10-6 1 1×10-2
1×10-4 1×102 1
※1St=1cm2/s, 1cSt=1mm2/s

Thermal Conductivity

W/(m・k) kcal/(h・m・°C)
1 8.600 0×10-1
1.162 79 1

Heat Transfer Coefficient

W(m2・k) kcal/(h・m2・°C)
1 8.600 0×10-1
1.162 79 1

Specific Heat

J/(kg・K) kcal/(kg・°C)
cal/(g・°C)
1 2.388 89×10-4
4.186 05×103 1

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