**Molar gas constant** (also known as *universal gas constant*, usually denoted by symbol *R*) is the constant occurring in the universal gas equation, i.e. the equation of state of an ideal gas:

*pV*=*nRT*

It can be shown that *R* is an universal constant, equal for all gases. Real gases obey this equation only in an approximation of very diluted gases.

R also appears in other popular equation, e.g. Nernst's.

Its value is:

*R*= 8.314570(70) J K^{-1}mol^{-1}

The Boltzmann constant *k _{B}* is defined as a ratio of molar gas constant and the Avogadro's number: