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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
- <math>\sigma = \frac{2\pi^5k^4}{15h^3c^2} = \frac{\pi^2k^4}{60\hbar^3c^2},<math>
where h is Planck's constant, ħ is h-bar, and c is the speed of light.
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