The **universal gas equation**, or **ideal gas law**, is an equation of state of an ideal gas:

*pV*=*nRT*

*p*is the pressure of gas,

*V*the volume it occupies,

*n*the number of moles of gas,

*R*the molar gas constant, and

*T*the temperature of the gas. In SI units, the pressure is expressed in pascals, the volume in litres, and the temperature in Kelvin. The ideal gas law generalizes the three classical gas laws.

The equation is valid only for an ideal gas. Real gases obey this equation only approximately, but its validity increases as the density of the gas tends to zero.

The picture shows some isotherms of an ideal gas (i.e. the relation between pressure p and volume V at fixed temperature T; plotted for a set of temperatures, with increasing T from lower to upper curve):

See also: