The equation that characterizes any amount of a gas is

where p is the pressure, V the volume, n the number of the gas moles and T the absolute temperature of the gas; R the universal gas constant (approximately 8.314472 ).

The gas law is accurately valid for an ideal gas, but a good approximation for real gases.

The law contains the following gas laws:

• Boyle–Mariotte law (pV = constant when n and T are constants)
• Gay–Lussac law ( = constant when n and p are constants)
• Avogadro’s law (in equal conditions, equal volumes of different gases contain the same number of molecules)

0.1 Alternative definition

The ideal gas law can also be defined using Boltzmann’s constant:

The symbols are those defined above, with the difference that N is the number of gas molecules, and k is the Boltzmann constant, k ≈ 1.38 × 10⁻²³JK⁻¹. This form is often preferred by physicists who consider the number of molecules to be more fundamental than the number of moles.