List of physical constants


The constants listed here are known values of physical constants expressed in SI units; that is, physical quantities that are generally believed to be universal in nature and thus are independent of the unit system in which they are measured. Many of these are redundant, in the sense that they obey a known relationship with other physical constants and can be determined from them.

Table of physical constants

SymbolQuantityValueRelativestandarduncertaintyRef
speed of light in vacuum
Planck constant
reduced Planck constant
Boltzmann constant
Newtonian constant of gravitation
cosmological constant
Stefan–Boltzmann constant
first radiation constant
first radiation constant for spectral radiance
second radiation constant
Wien wavelength displacement law constant
Wien frequency displacement law constant
Wien entropy displacement law constant
elementary charge
conductance quantum
inverse conductance quantum
von Klitzing constant
Josephson constant
magnetic flux quantum
fine-structure constant
inverse fine-structure constant
vacuum magnetic permeability
characteristic impedance of vacuum
vacuum electric permittivity
electron mass
muon mass
tau mass
proton mass
neutron mass
proton-to-electron mass ratio
W-to-Z mass ratio
sine-square weak mixing angle
electron g-factor
muon g-factor
proton g-factor
quantum of circulation
Bohr magneton
nuclear magneton
classical electron radius
Thomson cross section
Bohr radius
Rydberg constant
Rydberg constant#Rydberg [unit of energy|Rydberg unit of energy]
Hartree energy
Fermi coupling constant
Avogadro constant
molar gas constant
Faraday constant
molar Planck constant
molar mass of carbon-12
atomic mass constant
molar mass constant
molar volume of silicon
hyperfine transition frequency of 133Cs

Uncertainties

While the values of the physical constants are independent of the system of units in use, each uncertainty as stated reflects our lack of knowledge of the corresponding value as expressed in SI units, and is strongly dependent on how those units are defined. For example, the atomic mass constant is exactly known when expressed using the dalton, but the kilogram is not exactly known when using these units, the opposite of when expressing the same quantities using the kilogram.

Technical constants

Some of these constants are of a technical nature and do not give any true physical property, but they are included for convenience. Such a constant gives the correspondence ratio of a technical dimension with its corresponding underlying physical dimension. These include the Boltzmann constant, which gives the correspondence of the dimension temperature to the dimension of energy per degree of freedom, and the Avogadro constant, which gives the correspondence of the dimension of amount of substance with the dimension of count of entities. By implication, any product of powers of such constants is also such a constant, such as the molar gas constant.