Specific Gas Constant Calculator
Find the specific gas constant from molar mass.
Results update as you type.
How it works
The specific gas constant, also called the individual gas constant, is the universal gas constant divided by a gas's molar mass, and this calculator finds it. You enter the molar mass of the gas, and it returns the specific gas constant in joules per kilogram-kelvin. This constant is used in the mass-based form of the ideal gas law, PV = mRT, where m is the mass of gas and R is the specific gas constant, in contrast to the molar form that uses the universal gas constant and moles. Because it is tailored to a particular gas through its molar mass, the specific gas constant lets engineers work directly with the mass of gas rather than converting to moles, which is convenient in thermodynamics, aerodynamics, and HVAC calculations. Lighter gases have larger specific gas constants — hydrogen's is very high, while heavy gases like carbon dioxide have small ones — because dividing the fixed universal constant by a smaller molar mass gives a larger result. For dry air, with a molar mass of about 28.97 grams per mole, the specific gas constant is approximately 287 joules per kilogram-kelvin, a value used constantly in aviation and meteorology. Enter the molar mass to find the specific gas constant.
R_specific = R ÷ M (universal gas constant divided by molar mass)
The specific (individual) gas constant equals the universal gas constant, 8.314 joules per mole-kelvin, divided by the gas's molar mass. Using molar mass in kilograms per mole gives the constant in joules per kilogram-kelvin. Lighter gases have larger specific gas constants.
Source:Thermodynamics (ideal gas law)
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Worked example
How to use this calculator
- 1
Enter your values
Enter molar mass.
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Fine-tune any options to match your situation.
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Frequently asked questions
What is the specific gas constant?
The specific gas constant is the universal gas constant divided by a gas's molar mass, giving a value particular to that gas. It appears in the mass-based ideal gas law, PV = mRT, letting you use the mass of gas directly. It has units of joules per kilogram-kelvin and differs for every gas depending on its molar mass.
How is it different from the universal gas constant?
The universal gas constant, about 8.314 joules per mole-kelvin, is the same for all gases and is used with moles in PV = nRT. The specific gas constant is the universal constant divided by a particular gas's molar mass, so it differs per gas and is used with mass in PV = mRT. One counts moles, the other uses mass.
Why do lighter gases have larger specific gas constants?
Because the specific gas constant is the fixed universal constant divided by molar mass, a smaller molar mass gives a larger result. Light gases like hydrogen and helium therefore have large specific gas constants, while heavy gases like carbon dioxide have small ones. This reflects that a given mass of a light gas contains more molecules and exerts more pressure.
What is the specific gas constant of air?
For dry air, with an average molar mass of about 28.97 grams per mole, the specific gas constant is approximately 287 joules per kilogram-kelvin. This widely used value appears throughout aerodynamics, meteorology, and thermodynamics whenever calculations involve the mass of air, such as computing air density, lift, or atmospheric behavior.
Where is the specific gas constant used?
It is used in engineering and physics wherever the mass-based ideal gas law is convenient, including thermodynamics, aerodynamics, meteorology, and HVAC design. Working with mass directly avoids converting to moles, which is practical when dealing with flowing gases, air density, engine cycles, and atmospheric calculations where mass is the natural quantity to measure.