The Combined gas law or General Gas Equation is obtained by combining Boyles Law, Charless law, and Gay-Lussac's Law. It shows the relationship between the pressure, volume, and temperature for a fixed mass of gas:
This can also be written as:
With the addition of Avogadro's law, the combined gas law develops into the ideal gas law:
:where P  is the pressure, V is volume, n is the number of moles, R is the universal gas constant and T is the absolute temperature.
:The proportionality constant, now named R, is the universal gas constant with a value of 8.3144598 (kPa∙L)/(mol∙K).
An equivalent formulation of this law is:
:where P is the pressure, V is the volume, N is the number of gas molecules, k is the Boltzmann constant (1.381×10J·K in SI units) and T is the absolute temperature.
These equations are exact only for an ideal gas, which neglects various intermolecular effects (see real gas). However, the ideal gas law is a good approximation for most gases under moderate pressure and temperature.
This law has the following important consequences:
# If temperature and pressure are kept constant, then the volume of the gas is directly proportional to the number of molecules of gas.
# If the temperature and volume remain constant, then the pressure of the gas changes is directly proportional to the number of molecules of gas present.
# If the number of gas molecules and the temperature remain constant, then the pressure is inversely proportional to the volume.
# If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change in direct proportion to the temperature.