The Stefan-Boltzmann constant (also Stefan's constant), denoted with a Greek letter σ, is a derivable physical constant, the constant of proportionality between the total energy radiated per unit surface area of a black body in unit time and the fourth power of the thermodynamic temperature, as per the Stefan-Boltzmann law.
Its value is 5.670 400(40) × 10−8 J s−1 m−2 K−4.
The Stefan-Boltzmann constant can be defined in terms of the Boltzmann constant k by
where h is Planck's constant, ħ is h-bar, and c is the speed of light.