Gas Mixture Molecular Weight and Density Calculator
Enter the composition of a gas mixture in mole (volume) percent. The calculator returns the mixture molecular weight, specific gravity relative to air and the ideal gas density at your pressure and temperature.
- MW = Σ(yi × MWi) ÷ Σyi = 17.824 g/mol
- SG = MW ÷ 28.9647
- ρ = P × MW ÷ (R × T) = 0.75384 kg/m³
Mixture Properties
SG = MWmix ÷ 28.9647
ρ = P × MW ÷ (R × T)
yi is the mole fraction of each component (equal to volume fraction for ideal gases), MWi its molecular weight, 28.9647 the molecular weight of dry air, P the absolute pressure, T in kelvin and R = 8.314 J/mol·K. Density assumes ideal gas (Z = 1); at high pressure divide by Z.
Worked example
A natural gas of 90 % methane, 5 % ethane, 2 % propane, 1 % CO₂ and 2 % N₂ has MW ≈ 17.82 g/mol, SG ≈ 0.615 and a density of about 0.754 kg/m³ at 15 °C and 1.01325 bar a.
Frequently Asked Questions
Is mole percent the same as volume percent?
For ideal gases, yes. Most gas analyses are reported in mol % or vol %.
Why is the result normalised?
Analyses rarely add to exactly 100 %. Normalising scales each component so the total is 100 %.
When is ideal gas density not accurate?
At high pressure or near condensation. Use a compressibility factor from AGA 8 or GERG for natural gas at pipeline pressure.
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