The Stefan–Boltzmann constant (also Stefan's constant), a physical constant denoted by the Greek letter σ, is the constant of proportionality in the Stefan–Boltzmann law: the total energy radiated per unit surface area of a black body in unit time is proportional to the fourth power of the thermodynamic temperature.
The value of the Stefan–Boltzmann constant is given in SI units by
In cgs units the Stefan-Boltzmann constant is:
In US customary units the Stefan-Boltzmann constant is [2]:
The value of the Stefan–Boltzmann constant is derivable as well as experimentally determinable; see Stefan–Boltzmann law for details. It can be defined in terms of the Boltzmann constant as:
where:
The CODATA recommended value is calculated from the measured value of the gas constant:
where:
A related constant is the radiation constant (or radiation density constant) a which is given by:[3][4]