The gas constant, also known as the universal, usually denoted by symbol R is a physical constant which is featured in a large number of fundamental equations in the physical sciences, such as the ideal gas law and the Nernst equation. It is equivalent to the Boltzmann constant, and is expressed in units of energy per Kelvin per mole.

Its value is:

R  = 8.314472(15) J ·K ^-1  · mol ^-1

Where the number in parentheses represents the uncertainty in the last two digits.

The gas constant occurs in the simplest equation of state, the ideal gas law.

The gas constant is the constant of proportionality (R) in the equation

                        PV = nRT

Where P is the absolute Pressure, V is the Volume,  n is the number of moles, T is the absolute Temperature.

The above equation relates the pressure and volume of a quantity of gas to the absolute temperature. The specific gas constant  of a gas or a mixture of gases ( R ) isgiven by the universal gas constant, divided by the molar mass ( M) of the gas/mixture.
The gas constant can also be defined as the pressure of the gas multiplied by its volume, divided by the number of moles of the gas multiplied by its temperature in Kelvin. Ideal gases are hypothetical i.e., they strictly obey all gas laws and have a molar volume of 22.4141 liters at standard temperature and pressure (STP), which is 273 Kelvin,     1 atmosphere. At STP, however, most gases behave like ideal gases, so the value of R is generally 0.0821 L atm / mol K or 8.3145 J / mol K. At a low temperature or under high pressure conditions the gas molecules are moving closer together and more slowly, so intermolecular forces, called Vander Waals, cause the measured pressure to be lower than expected. When the molecules are closer together, the volume of the actual molecules also becomes a higher fraction of the total volume of the gas.