About the Specific Gas Constant
The specific gas constant (also called the individual gas constant), R_specific, links a gas's pressure, density, and absolute temperature through the ideal gas law. Unlike the universal gas constant R, which is the same 8.314462618 J/(mol·K) for every ideal gas, R_specific is scaled by the gas's own molar mass: R_specific = R / M, where M is the molar mass in kg/mol. Because M is different for every gas, R_specific is different for every gas — a light gas such as hydrogen has a large specific gas constant, while a heavy gas such as carbon dioxide has a small one.
Deriving R_specific from molar mass
Start from the ideal gas law written in molar form, PV = nRT, where n is the number of moles. Since the number of moles equals mass divided by molar mass (n = m/M), substituting gives PV = (m/M)RT, which rearranges to PV = m(R/M)T. Defining R_specific = R/M turns this into the mass-based ideal gas law, PV = m·R_specific·T — the form used throughout engineering thermodynamics. Dividing both sides by volume V and using density ρ = m/V gives the equivalent form P = ρ·R_specific·T, which this calculator uses to estimate gas density from pressure and temperature.
Units and unit conversions
R_specific is most often reported in J/(kg·K) or kJ/(kg·K) in SI work, and in ft·lbf/(lbm·°R) in US customary engineering — for example, air's specific gas constant is about 287.05 J/(kg·K), equivalent to roughly 53.35 ft·lbf/(lbm·°R). Enter molar mass in g/mol or kg/mol (not lb/lbmol), and always convert temperature to an absolute scale — Kelvin (K = °C + 273.15) or Rankine — before using it in P = ρ·R_specific·T. Plugging a Celsius or Fahrenheit reading directly into that formula gives an incorrect density because those scales do not start at absolute zero.