Flue Gas Calculators

Chimney Draft Calculator

Calculate residential or small commercial chimney draft in mm water column from chimney height and temperature difference.

Dry Flue Gas Calculator

Convert wet flue gas volume to dry basis by removing the water vapor fraction.

Excess Air from O2 Calculator

Calculate excess combustion air percentage directly from a measured flue gas oxygen reading, without needing full fuel and air flow data.

Flue Gas CO2 Calculator

Estimate flue gas CO2 percentage (dry basis) from a fuel's theoretical CO2 max and the excess air percentage.

Flue Gas Density Calculator

Calculate flue gas density from temperature, pressure, and average molar mass using the ideal gas law.

Flue Gas Dew Point Calculator

Calculate the water vapor dew point temperature of flue gas from its water vapor partial pressure using the Magnus formula.

Flue Gas Flow Calculator

Calculate flue gas flow rate in Nm3/hr from fuel flow rate and gas volume per kg of fuel, for stack and fan sizing.

Flue Gas Mass Flow Calculator

Calculate flue gas mass flow rate from volumetric flow rate and gas density for use in mass and energy balance calculations.

Flue Gas Molecular Weight Calculator

Calculate the effective molecular weight of a flue gas mixture from the mole fractions of CO2, O2, N2, and H2O.

Flue Gas Nitrogen Calculator

Calculate flue gas nitrogen percentage (dry basis) by difference from measured O2 and CO2 percentages.

Flue Gas Oxygen Calculator

Estimate flue gas oxygen percentage (dry basis) from the excess air percentage supplied to combustion.

Flue Gas SO2 Calculator

Calculate sulfur dioxide (SO2) mass produced from a fuel's sulfur content using direct combustion stoichiometry.

Flue Gas Specific Volume Calculator

Calculate flue gas specific volume, the volume occupied by one kilogram of gas, from its density.

Flue Gas Temperature Calculator

Calculate flue gas exit temperature after heat is absorbed by a heat exchanger, economizer, or boiler.

Flue Gas Velocity Pressure Calculator

Calculate flue gas velocity pressure (dynamic pressure) from gas density and velocity, as used in EPA Method 2 stack testing.

Flue Gas Volume Calculator

Calculate total flue gas volume produced from fuel mass burned and gas volume per kg of fuel.

Flue Gas Water Vapor Calculator

Calculate the mass of water vapor produced in flue gas from a fuel's hydrogen content and mass burned.

Stack Draft Calculator

Calculate natural stack draft in Pascals from stack height, ambient temperature, and flue gas temperature.

Stack Gas Velocity Calculator

Calculate flue gas velocity in a stack or exhaust duct from volumetric flow rate and cross-sectional area.

Wet Flue Gas Calculator

Calculate total wet flue gas volume by adding water vapor volume to dry flue gas volume.

Reading the Exhaust to Understand the Fire

Flue gas composition is a window into how completely combustion actually happened — its oxygen, CO2, and moisture content reveal excess air levels and efficiency without needing to inspect the flame itself. These thirteen calculators cover flue gas analysis math.

Reading Flue Gas Oxygen and CO2 Together, Step by Step

Measure flue gas oxygen percentage. Higher O2 in the flue gas means more excess air passed through the combustion process than was chemically needed.
Cross-check against flue gas CO2 percentage. These two move in opposite directions — as excess air (and O2) rises, CO2 concentration falls, since the same combustion products are diluted by more excess air.
Compare both to the fuel's expected values. Each fuel type has a known theoretical maximum CO2 percentage at zero excess air; the gap between that maximum and the measured reading estimates the excess air level.
Check flue gas temperature alongside these readings. High O2 combined with high stack temperature usually points to both excess air and heat loss happening together, a double efficiency hit worth correcting.

Run these readings through the Flue Gas Oxygen Calculator and Flue Gas CO2 Calculator to estimate excess air directly from a combustion analyzer reading.

Popular Flue Gas Calculators

Chimney Draft Calculator — estimates the natural draft a chimney generates from height and temperature differential.
Flue Gas Dew Point-adjacent tools: the Flue Gas Water Vapor Calculator estimates moisture content relevant to condensation risk.
Stack Gas Velocity Calculator — calculates exhaust velocity from flow rate and stack cross-sectional area.