An ideal gas satisfies these conditions:

- It consists of a large number of identical molecules.

- The volume occupied by the molecules themselves is negligible compared to the volume of the container they're in.

- The molecules obey Newton's laws of motion, and they move in random motion.

- The molecules experience forces only during collisions; any collisions are completely elastic, and take a negligible amount of time.

The ideal gas law states that:

PV = nRT

where P is pressure, V is volume, n is the number of moles, T is the absolute temperature, and R = 8.31 J/(mol K) is the universal gas constant.

This can be written in terms of N, the number of molecules, instead.

N = nN_{A}, so:

PV = nN_{A} (R/N_{A}) T = NkT

where k is the Boltzmann constant, 1.38 x 10^{-23} J/K. This is the universal gas constant divided by Avogadro's number.