PV = nRT — The ideal gas law is the equation of state of a hypothetical ideal gas, first stated by Benoît Paul Émile Clapeyron in 1834. The state of an amount of gas is determined by its pressure, volume, and temperature according to the equation.

P is the absolute pressure,
V is the volume of the vessel,
n is the amount of substance of gas,
R is the ideal gas constant,
T is the absolute temperature.

References: Wikipedia, the free encyclopedia – http://en.wikipedia.org/wiki/Ideal_gas_law