Specific Gas Constant Calculator — Rₛ = R / M
The specific gas constant Rₛ = R / M links the universal gas constant R to a particular gas via its molar mass M. Enter any gas and conditions to get Rₛ, density and specific volume from the ideal-gas law.
Gas
K
Pa
R / M — universal constant divided by molar mass
- 1
Molar mass M
28.964 g/mol = 0.028964 kg/mol - 2
Specific gas constant Rₛ = R ÷ M
8.31446 ÷ 0.028964 = 287.06R = 8.31446 J/(mol·K) is the universal gas constant.
How does this calculator work?
The specific gas constant is Rₛ = R/M where R = 8.31446 J/(mol·K) and M is the molar mass in kg/mol. For dry air (M = 28.97 g/mol), Rₛ ≈ 287 J/(kg·K). Gas density at any conditions follows from ρ = P/(Rₛ·T) using the ideal-gas law.
Formula
How this is calculated
The ideal-gas law is usually written PV = nRT, where n is the amount of gas in moles. Engineers often prefer a mass-specific form: PV = mRₛT, where m is mass in kilograms and Rₛ = R/M is the specific (per-unit-mass) gas constant in J/(kg·K). This form is convenient because engineering problems usually deal with mass flow rates and densities rather than molar quantities.
Given Rₛ, the gas density at any temperature T (in Kelvin) and pressure P (in Pascals) follows directly from rearranging the ideal-gas law: ρ = P/(RₛT). The specific volume v = 1/ρ is the inverse. These are exact results under the ideal-gas assumption, which holds well for common gases (air, nitrogen, argon, helium) at moderate pressures (< ≈ 10 MPa) and temperatures well above the gas's boiling point. The assumption breaks down near the critical point and at very high pressures where intermolecular forces matter.
The indicative speed of sound shown assumes an isentropic ratio γ = 1.4 (diatomic gas) via c = √(γRₛT). This is accurate for dry air at ordinary conditions but will differ for monoatomic gases (γ = 5/3) or polyatomic gases (γ < 1.4); use it as an order-of-magnitude guide for non-diatomic gases.
Frequently asked questions
The universal gas constant R = 8.31446 J/(mol·K) is a physical constant that applies to all ideal gases. The specific gas constant Rₛ = R/M is the per-unit-mass version for a particular gas of molar mass M. For air (M ≈ 28.97 g/mol), Rₛ ≈ 287 J/(kg·K).
The ideal-gas law requires absolute temperature. Convert: K = °C + 273.15. A temperature of 20 °C = 293.15 K. Standard atmosphere (ISA) reference conditions are 288.15 K (15 °C) and 101 325 Pa.
For air, nitrogen, oxygen and noble gases at pressures below about 1 MPa and temperatures above 200 K, the ideal-gas law is accurate to within 0.1–0.5%. At high pressures (gas pipelines, compressed cylinders) or near the boiling point, real-gas equations of state (van der Waals, Peng-Robinson) are needed.
Also known as
TG we-Calculate Editorial Team. (2026). Specific Gas Constant Calculator — Rₛ = R / M [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/specific-gas-constant-calculator
TG we-Calculate Editorial Team. "Specific Gas Constant Calculator — Rₛ = R / M." TG we-Calculate. 2026. https://we-calculate.com/calculator/specific-gas-constant-calculator.
TG we-Calculate Editorial Team, "Specific Gas Constant Calculator — Rₛ = R / M," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/specific-gas-constant-calculator
@misc{wecalculate_specific_gas_constant_calculator, title = {Specific Gas Constant Calculator — Rₛ = R / M}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/specific-gas-constant-calculator}}, year = {2026}, note = {TG we-Calculate} }
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