kT is the product of the Boltzmann constant, k, and the temperature, T. This product is used in physics as a scaling factor for energy values in molecular-scale systems (sometimes it is used as a pseudo-unit of energy), as the rates and frequencies of many processes and phenomena depend not on their energy alone, but on the ratio of that energy and kT, that is, on E / kT (see Arrhenius equation). More fundamentally, kT is the amount of heat required to increase the thermodynamic entropy of a system, in natural units, by one nat.

In macroscopic scale systems, with large numbers of molecules, RT value is commonly used; its SI units are joules per mole (J/mol): RT = kT × (N_A). At room temperature 25 °C (77 °F, 298 K) 1kT is equivalent to 4.11x10⁻²¹ J , 9.83x10⁻²² cal, or 0.0256 eV; correspondingly, RT =2.479 kJ·mol⁻¹=0.593 kcal·mol⁻¹ .