Boltzmann Constant Definition and Units

Boltzmann Constant
The Boltzmann constant is a physical constant that relates the average kinetic energy of a gas particle to its absolute temperature.

The Boltzmann constant (k or kB) is a proportionality factor that relates the average kinetic energy of an ideal gas particle to its absolute temperature. It is one of the seven SI base units, with a defined value of exactly 1.380694 x10-23 J/K, which is the same as 1.380694 x10-23 m2⋅kg/(s2⋅K). The Boltzmann constant is the ratio of the ideal gas constant (R) to Avogadro’s number (NA). Several chemistry and physics formulas use the Boltzmann constant, including variations of the ideal gas law, the definitions of the gas constant and kelvin unit, Boltzmann’s entropy formula, and Planck’s law of black-body radiation. The unit gets its name from Austrian physicist Ludwig Boltzmann.

The Boltzmann Constant in the Ideal Gas Law

The ideal gas law relates the pressure and volume of an ideal gas to its number of moles and absolute temperature:

PV = nRT

Here, P is pressure, V is volume, n is number of moles, R is the ideal gas constant, and T is absolute temperature.

But, the ideal gas constant depends on the Boltzmann constant and Avogadro’s number:

R = kNA

So, another way of writing the ideal gas law includes the Boltzmann constant:

PV = NkT

Here, N is the number of molecules of an ideal gas.

All of the ideal gas laws relate to the Boltzmann constant:

Gas LawRelationship to k
Ideal gas law PV = NkT
Combined gas lawPV/T = k
Boyle’s lawPV = k
Charles’s lawV/T = k
Gay-Lussac’s lawP/T = k
Avogadro’s lawV/N = k

Boltzmann Constant Values in Different Units

This table gives values of the Boltzmann constant in different units.

Value of kUnit
5.657302466×10−24ft lb/°R
3.1668 ×10−6Eh/K (Eh = Hartree)

The Other Boltzmann Constant and Planck’s Constant

There are actually two different Boltzmann constants. One is the Boltzmann constant (k) and the other is the Stefan-Boltzmann constant or Stefan constant (σ). The Stefan-Boltzmann constant is a proportionality constant in the Stefan-Boltzmann law, which states that total radiated intensity of a black body increases as temperature increases.

There is a relationship between the Stefan-Boltzmann constant, Boltzmann constant, and Planck’s constant (h). The Stefan-Boltzmann constant’s definition relies on values of the other two constants, pi, and the speed of light:

σ = 2πk4/15h3c3


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