`PAO_2 = ( "(FIO_2)" ( "BP" - "WVP" )-( "(PaCO_2)" / "R" ))`

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The alveolar gas equation is usually used to calculate the partial pressure of oxygen in the alveoli:

PAO₂ = FIO₂ (BP – WVP) – (PaCO₂/R)

PAO₂ = alveolar partial pressure of O₂
 FIO₂ = [O₂] in inspired air
 PaCO₂ = arterial partial pressure of CO₂
    R = respiratory quotient
    BP = barometric pressure (normally 760 mmHg)
   WVP = water vapor pressure (normally 47 mmHg)

The FiO₂ is the fraction of inspired oxygen (usually as a fraction, but entered here as a percentage for ease of use). BP is the barometric pressure, which is 760 mmHg at sea level. WVP is water vapor pressure at 37C and is equal to 47 mmHg. The respiratory quotient (R) is the ratio of CO₂ eliminated divided by the O₂ consumed, and its value is typically 0.8 but can range from 0.7 to 1.0.

The alveolar gas equation is most commonly used in the calculation of the Alveolar - arterial gradient:

A-a gradient = PAO₂ - PaO₂