Flue Gas Molecular Weight Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: July 27, 2026One number representing a mixture of gases
Flue gas is a mixture of several different gases, each with its own molecular weight - a mole-fraction-weighted average gives the mixture's effective molecular weight, a required input for density and specific volume calculations using the ideal gas law.
Worked example
For a typical flue gas composition of 13% CO2, 4% O2, 72% N2, and 11% H2O (by mole fraction):
MW = (0.13 x 44.01) + (0.04 x 32.00) + (0.72 x 28.01) + (0.11 x 18.02) = 29.151 g/mol
This result closely matches typical published flue gas molecular weight figures (generally 28-30 g/mol), slightly above dry air's 28.97 g/mol due to the heavier CO2 content offsetting the lighter water vapor contribution.
Make sure the four mole fractions entered sum to approximately 1.0 (100%) for an accurate result - this simplified four-component model covers the overwhelming majority of typical combustion flue gas composition.