
Become familiar with the seven defining constants of the SI.
The following seven SI base unit definitions are based on the BIPM SI Brochure (9th Edition).
Definitions of the SI base units

Unit of length 
meter 
The meter, 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
Δν_{Cs}.


Unit of mass 
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 × 10^{34} when expressed in the unit J s, which is equal to kg m^{2} s^{1}, where the meter and the second are defined in terms of
c and Δν_{Cs}.


Unit of time 
second 
The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the cesium frequency Δν_{Cs}, the unperturbed groundstate hyperfine transition frequency of the cesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s^{1}.


Unit of
electric current 
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 Δν_{Cs}.


Unit of
thermodynamic
temperature 
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, meter and second are defined in terms of h, c and Δν_{Cs}.


Unit of
amount of
substance 
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.


Unit of
luminous
intensity 
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, meter and second are defined in terms of h, c and Δν_{Cs}.


Go to SI Units Background or SI base units
Online: March 1998  Last update: June 2019
