Specific Gas Constant Calculator

The specific gas constant (R_specific) is the ratio of the universal gas constant (R = 8.31446 J/mol·K) to the molar mass of the gas. It is measured in J/kg·K and is unique to each gas or gas mixture. It appears in the ideal gas law when mass-based (rather than mole-based) quantities are used.

There are two main methods. First, divide the universal gas constant (8.31446 J/mol·K) by the molar mass of the gas in kg/mol: R_specific = R / M. Second, subtract the specific heat at constant volume (Cv) from the specific heat at constant pressure (Cp): R_specific = Cp − Cv. Both methods yield the same result for ideal gases.

Look up the molar mass of the gas (in g/mol), convert it to kg/mol by dividing by 1000, then divide 8.31446 by that value. For example, for air (M = 0.02897 kg/mol): R_specific = 8.31446 / 0.02897 ≈ 287.05 J/kg·K.

For an ideal gas, the specific gas constant equals the difference between the specific heat at constant pressure (Cp) and specific heat at constant volume (Cv): R_specific = Cp − Cv. This relationship is known as Mayer's relation and links thermodynamic properties of a gas.

Oxygen (O₂) has a molar mass of approximately 31.999 g/mol. Its specific gas constant is R_specific = 8314.46 / 31.999 ≈ 259.84 J/kg·K.

Steam (water vapor, H₂O) has a molar mass of 18.015 g/mol. Its specific gas constant is R_specific = 8314.46 / 18.015 ≈ 461.52 J/kg·K.

Dry air has a mean molar mass of about 28.97 g/mol, giving a specific gas constant of approximately 287.05 J/kg·K. This value is widely used in aerodynamics and atmospheric science.

The specific gas constant is used in compressible flow calculations, gas turbine and compressor design, pipeline engineering, and aerodynamics. It also appears in formulas for the speed of sound, gas density, and thermodynamic cycles involving specific gas mixtures.