The constant that relates the pressure and volume of an ideal gas to its temperature and amount is the gas constant. It is represented by R and appears in the ideal gas law, PV = nRT, where P is pressure, V is volume, n is the amount of gas in moles, and T is absolute temperature.
The gas constant is called the universal or molar gas constant because it connects macroscopic measurements made for a mole of particles. Its exact SI value is 8.31446261815324 joules per kelvin per mole. In many classroom calculations, it is rounded to 8.314 J/(mol·K).
The gas constant combines two other important constants: the Avogadro constant and the Boltzmann constant, according to R = Nₐkᵦ. This explains a common mix-up: the Boltzmann constant is used when counting individual particles, while the gas constant is used when measuring amounts in moles. The gas constant also appears in equations for chemical equilibria, thermodynamics, and reaction rates.