"The SI is an evolving system that reflects the best current practices in measurement 

Extract from the SI brochure, CIPM's Consultative Committee for Units, 8th edition, 2006

The SI's base units

The International System of Units (SI, abbreviated from the French "Système international (des unités)") is the set of units of measurement consisting of the 7 base units associated with the 7 base quantities. It is completed by derived units and prefixes.

The seven basic quantities are: length, mass, time, electric current, thermodynamic temperature, amount of substance and luminous intensity. The base units are the base on which all the units used to quantitatively express the measured quantities are built. Standards and measurement references are materializations of the measured quantity.

To date, their definitions, are as follows:


The metre

SI - metre

Symbol : m

Quantity : length

Definition : The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.



Metre derived units

Quantity Unit Expressed in SI based units
area square meter m2
volume cubic meter m3
plane angle radian (rad) m.m-1
solid angle steradian (sr)  m2.m-2
absorbed dose gray (Gy) m2.s-2


The kilogram

SI - kilogramme

Symbol : kg

Quantity : mass

Definition : The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.



Kilogram derived units

Quantity Unit Expressed in terms of SI based units
force newton (N) m.kg.s-2
pressure pascal (Pa) m-1.kg.s-2
electric potential difference volt (V) m2.kg.s-3.A-1
energy joule (J) m2.kg.s-2
power, radiant flux watt (W) m2.kg.s-3 


The second

SI - seconde

Symbol : s

Quantity : time

DefinitionThe second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.



Second derived units

Quantity Unit Expressed in terms of SI based units
frequency hertz (Hz) s-1
activity refered to a radionucleide becquerel (Bq) s-1
dose equivalent sievert (Sv) m2.s-2 
absorbed dose gray (Gy) m2.s-2


The ampere

SI - ampere

Symbol : A

Quantity : electric current

Definition : The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length.


Ampere derived units

Quantity Unit Expressed in terms of SI based units
electric charge coulomb (C) s.A
electric potential difference volt (V) m2.kg.s−3.A−1
electric resistance ohm () m2.kg.s−3.A−2
capacitance farad (F) m−2.kg−1.s4.A2
inductance henry (H) m2.kg.s−2.A−2
magnetic flux density tesla (T) kg.s−2.A−1


The kelvin

SI - kelvin

Symbol : K

Quantity : thermodynamic temperature

Definition : The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.



Kelvin derived units

Quantity Unit Expressed in terms of SI based units

Celsius temperature

degré Celsius (°C)


T/K – 273,15
thermal conductivity watt per metre kelvin m.kg.s-3.K-1
surface thermal resistance square-metre kelvin per watt kg-1.s3.K
heat capacity joule per kelvin  kg.m2.s-2.K-1


The mole

SI - mole

Symbol : mol

Quantity : amount of substance

Definition1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is "mol".
2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

Mole derived units

Quantity Unit Expressed in terms of SI based units
molar concentration mole per cubic metre


catalytic activity katal (kat)  mol.s-1



The candela

SI - candela

Symbol : cd

Quantity : luminous intensity

DefinitionThe candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.


Candela derived units

Quantity    Unit Expressed in terms of SI based units
luminous flux lumen (lm) cd.sr = m2.m-2.cd
illuminance lux (lx) lm.m-2 = m-2.cd 


Multiples and sub-multiples

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