# Stefan-Boltzmann constant

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

blackbody 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.