How to use this calculator
- Enter or choose absolute pressure.
- Enter or choose pressure unit.
- Enter or choose temperature.
- Enter or choose temperature unit.
- Enter or choose molar mass of the gas.
- The gas density updates as you type. Use Copy result or Share to keep it.
Method
ρ = p × M ÷ (R × T); Vm = R × T ÷ p; v = 1 ÷ ρ
- p
- absolute pressure in Pa
- M
- molar mass in kg/mol
- R
- molar gas constant, 8.314462618 J/(mol·K)
- T
- absolute temperature in K
Worked example
- T = 15 + 273.15 = 288.15 K; p = 101 325 Pa; M = 0.0289647 kg/mol.
- ρ = 101 325 × 0.0289647 ÷ (8.314462618 × 288.15) = 2934.85 ÷ 2395.82 = 1.22499 kg/m³.
- Molar volume = 8.314462618 × 288.15 ÷ 101 325 = 0.0236448 m³ = 23.6448 L/mol.
Dry air at sea level and 15 °C has a density of about 1.225 kg/m³, the International Standard Atmosphere value.
What the result means
Density falls as air warms or as you climb, which lowers engine power, lift and fan output. For gas mixtures, use the mole-fraction-weighted average molar mass.
Assumptions and limits
- Ideal gas behaviour (compressibility factor Z = 1).
- Pressure is absolute.
- Real gases deviate at high pressure or near condensation; steam and refrigerants need property tables.
- Humid air is lighter than dry air; this uses the molar mass you enter.
Questions
Why does humid air have a lower density?
Water vapour (18.015 g/mol) is lighter than the nitrogen and oxygen it displaces (about 29 g/mol), so moist air at the same temperature and pressure is slightly less dense.
Is kg/m³ the same as g/L?
Yes. 1 kg/m³ = 1 g/L, so air at 1.225 kg/m³ weighs 1.225 g per litre.