
The SI base units:

Base quantity 

Base unit 



Name 
Typical symbol 

Name 
Symbol 

time 
t 

second 
s 
length 
l, x, r, etc. 

metre 
m 
mass 
m 

kilogram 
kg 
electric current 
I, i 

ampere 
A 
thermodynamic temperature 
T 

kelvin 
K 
amount of substance 
n 

mole 
mol 
luminous intensity 
I_{v} 

candela 
cd 

Definitions
Starting from the definition of the SI in terms of fixed numerical values of the defining constants, definitions of each of the seven base units are deduced by using, as appropriate, one or more of these defining constants to give the following set of definitions:
The second
The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency _{Cs}, the unperturbed groundstate hyperfine transition frequency of the caesium133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s^{–1}.
The metre
The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s^{–1}, where the second is defined in terms of the caesium frequency _{Cs}.
The kilogram
The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 x 10^{–34} when expressed in the unit J s, which is equal to kg m^{2} s^{–1}, where the metre and the second are defined in terms of c and _{Cs}.
The ampere
The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 x 10^{–19} when expressed in the unit C, which is equal to A s, where the second is defined in terms of _{Cs}.
The kelvin
The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 x 10^{–23} when expressed in the unit J K^{–1}, which is equal to kg m^{2} s^{–2} K^{–1}, where the kilogram, metre and second are defined in terms of h, c and _{Cs}.
The mole
The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 x 10^{23} elementary entities. This number is the fixed numerical value of the Avogadro constant, N_{A}, when expressed in the unit mol^{–1} and is called the Avogadro number.
The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.


This definition implies the exact relation N_{A} = 6.022 140 76 x 10^{23} mol^{–1}. Inverting this relation gives an exact expression for the mole in terms of the defining constant N_{A}:
The effect of this definition is that the mole is the amount of substance of a system that contains 6.022 140 76 x 10^{23} specified elementary entities.

The candela
The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 x 10^{12} Hz, K_{cd}, to be 683 when expressed in the unit lm W^{–1}, which is equal to cd sr W^{–1}, or
cd sr kg^{–1} m^{–2} s^{3}, where the kilogram, metre and second are defined in terms of h, c and _{Cs}.
All other SI units can be derived from these, by multiplying together different powers of the base units.

