The fuel factor, f<sub>o</sub>, is the ratio of created CO<sub>2</sub> to depleted oxygen in a combustion reaction, used to check the accuracy of an emission measurement system. It can be calculated using the equation
Where %O<sub>2</sub> is the percent O<sub>2</sub> by volume, dry basis, %CO<sub>2</sub> is the percent CO<sub>2</sub> by volume, dry basis, and 20.9 is the percent O<sub>2</sub> by volume in ambient air. The Fuel factor can be corrected for the amount of CO, by adding the percent CO on a dry basis to the CO<sub>2</sub>, and subtracting half of the percent CO from the O<sub>2</sub>.