What is the approximate value of the Boltzmann constant in joules per kelvin?

The approximate value of the Boltzmann constant is 1.38 × 10⁻²³ joules per kelvin. Its symbol is kᵦ, or sometimes simply k, and its exact SI value is 1.380649 × 10⁻²³ J/K.

The constant provides a bridge between the microscopic and macroscopic views of temperature. It relates the average thermal energy of individual particles to the temperature of the system. For example, in the particle-count form of the ideal gas law, PV = NkᵦT, N represents the number of molecules rather than the amount in moles.

A frequent source of confusion is the gas constant, R, which is about 8.314 J/(mol·K). The two are connected by R = Nₐkᵦ, where Nₐ is the Avogadro constant. The Boltzmann constant also appears in statistical mechanics, entropy, black-body radiation, and calculations of thermal noise. Since the 2019 SI revision, its numerical value has been defined exactly, and that definition helps establish the kelvin.