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Weill Cornell Medicine — Pediatric Critical Care Medicine

Alveolar-arterial Gradient

All calculations should be independently verified prior to clinical use. These calculators are intended to supplement, not replace, clinical judgment.

Calculate the alveolar-arterial oxygen gradient given FiO2, PaCO2, and PaO2.

decimal
torr
torr
torr

High gradients result from impaired diffusion or, more commonly, ventilation-perfusion inequality of the "shunting" variety. A normal A-a gradient is less than 10 torr. The formula (age [years] + 10 / 4 provides another conservative estimate of a normal gradient. (1)

The calculation assumes 100% humidity and a respiratory quotient of 0.8, using the alveolar gas equation to determine PAO2:

PAO2 = ( FiO2 × (760 − 47) ) − ( PaCO2 / 0.8 )

A-a gradient = PAO2 − PaO2

See the Alveolar Gas Equation calculator, which allows manipulation of all variables including barometric pressure and respiratory quotient.

References

  1. Hantzidiamantis PJ, Amaro E. Physiology, Alveolar to Arterial Oxygen Gradient. [Updated 2023 Jun 5]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. 2026 Jan.