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.

1. Stefan–Boltzmann constant
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Not to be confused with Boltzmann constant.
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
σ = 5.670373(21)×10−8 W m−2 K−4.[1]
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:
 kB is the Boltzmann constant;
 h is the Planck constant;
 ħ is the reduced Planck constant;
 c is the speed of light in vacuum.
The CODATA recommended value is calculated from the measured value of the gas
constant:
where:
 R is the Universal gas constant;
 NA is the Avogadro constant;
 R∞ is the Rydberg constant;

2.  Ar(e) is the "relative atomic mass" of the electron;
 Mu is the molar mass constant (1 g/mol by definition);
 α is the fine structure constant.
A related constant is the radiation constant (or radiation density constant) a which is
given by:[3]
A simple rule to remember the Stefan–Boltzmann constant is to think "5-6-7-8;" and try not
to forget the negative sign before the final eight.