Respiratory

Alveolar Gas Equation (PAO₂) Calculator

Expected alveolar oxygen pressure from FiO2, barometric pressure and PaCO2; the first step toward the A-a gradient.

Reviewed by Alex G, BSN, RN, CEN, CCRN, CMC, NREMT-P · From the Reference tab of the app

Turn on JavaScript to use this calculator, or open it in the web app.

Formula

(FiO2/100) * (Patm - 47) - (PaCO2 / 0.8)

Reference: About 100 mmHg on room air at sea level (PaCO₂ 40)

How it is used

• PAO₂ = FiO₂ × (PB − 47) − PaCO₂ ÷ 0.8: about 100 mm Hg on room air at sea level with a PaCO₂ of 40, about 663 on FiO₂ 1.0.
• Subtract the measured PaO₂ to get the A–a gradient, normally about age ÷ 4 + 4 on room air.
• Barometric pressure is 760 mm Hg at sea level and lower at altitude, which is why PAO₂ falls on a mountain or in flight.

Same formula as the app. Open it in the web app.

Related one-pagers

• ABG Interpretation in 6 StepsPaO₂ · 80–100 mm Hg · Room air; some labs 75–100
• Ventilator Modes and SettingsPaO₂ or SpO₂ · FiO₂ or PEEP
• Neuro: ICP and StrokePaO₂ · Little effect until below 50–60, then CBF rises steeply
• ARDSOxygenation · SpO₂ 88–95% or PaO₂ 55–80 mm Hg · ARDSNet

Related calculators

• A-a Gradient
• Arterial Oxygen Content (CaO₂)
• Minute Ventilation
• P/F Ratio (PaO₂/FiO₂)
• Predicted Body Weight (PBW)
• SpO₂/FiO₂ Ratio
• Ventilator Rate for a Target PaCO₂

For education and exam preparation only. Follow your institution’s protocols. For education and exam preparation only. Verify results independently and follow your local protocols and medical direction. Not a substitute for clinical judgment.