Excess Air = 100 x  (20.9%) / (20.9% -O₂m%)  - 100%   

Where O₂ m% = The measured value of oxygen in the exhaust.

Examples:

When O₂m% = 0%    Then excess air = 0 

When O₂m% = 5%    Then:  excess air = 100 x  (20.9%) / (20.9%-5%) - 100%

                                                   excess air = 100 x  (20.9%) / (15.9%)  - 100%

                                                   excess air = 100 x  (1.31) - 100%

                                                   excess air = 31%