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Alveolar Gas Equation (PAO₂ Calculator)

· Last updated：2026-08-11

This tool is for informational purposes only and does not constitute medical, financial, or legal advice; consult a qualified professional for advice.

Calculate PAO₂ = FiO₂ × (P_atm − 47) − PaCO₂ / RQ. Defaults: P_atm 760 mmHg (sea level), RQ 0.8. Gives a directional hint vs expected ~100 mmHg. For full A-a gradient interpretation use a-a-gradient-calculator. Derived from StatPearls, LITFL, Cornell PICU, and West. Not medical advice.

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Tool usage guide

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Key facts

Category
Health
Input types
number
Output type
json
Sample coverage
4
API ready
Yes

Overview

The Alveolar Gas Equation Calculator estimates alveolar oxygen pressure (PAO₂) from PaCO₂, FiO₂, atmospheric pressure, and respiratory quotient using PAO₂ = FiO₂ × (P_atm − 47) − PaCO₂ / RQ. It uses sea-level pressure and an RQ of 0.8 by default and provides a directional comparison with expected PAO₂ values.

When to use

  • Estimate PAO₂ from arterial PaCO₂ and inspired oxygen values.
  • Adjust the calculation for altitude by entering a different atmospheric pressure.
  • Check how calculated PAO₂ compares with the expected value on room air at sea level.

How it works

  1. 1Enter PaCO₂ in mmHg and FiO₂ as a fraction, such as 0.21 for room air.
  2. 2Optionally replace the default atmospheric pressure of 760 mmHg and RQ of 0.8.
  3. 3The calculator applies PAO₂ = FiO₂ × (P_atm − 47) − PaCO₂ / RQ.
  4. 4The JSON result reports PAO₂, inputs, calculation method, and a directional category.

Use cases

  • Review arterial blood gas inputs by estimating the expected alveolar oxygen pressure.
  • Compare room-air PAO₂ at sea level with PAO₂ at reduced atmospheric pressure at altitude.
  • Explore how FiO₂, PaCO₂, atmospheric pressure, and RQ affect the calculated PAO₂.

Examples

1. Sea-level room-air PAO₂ estimate

Clinician reviewing an arterial blood gas

Background

The patient has a PaCO₂ of 40 mmHg while breathing room air at sea level.

Problem

Estimate the expected alveolar oxygen pressure and compare it with the textbook value near 100 mmHg.

How to use

Enter PaCO₂ as 40 and FiO₂ as 0.21. Leave atmospheric pressure and RQ at their defaults.

{"paco2":40,"fio2":0.21,"decimalPlaces":1}

Outcome

The calculator returns PAO₂ of approximately 99.7 mmHg and categorizes it as normal PAO₂ for sea-level room air.

2. PAO₂ estimate at Denver altitude

Respiratory physiology learner

Background

A person at higher altitude has a PaCO₂ of 35 mmHg while breathing room air. Atmospheric pressure is entered as 630 mmHg.

Problem

Show how reduced atmospheric pressure changes the estimated alveolar oxygen pressure.

How to use

Enter PaCO₂ as 35, FiO₂ as 0.21, and atmospheric pressure as 630 mmHg. Use the default RQ of 0.8.

FAQ

What does this calculator measure?

It estimates alveolar oxygen partial pressure, or PAO₂, in mmHg.

What FiO₂ should I enter for room air?

Enter 0.21 for room air.

What are the default atmospheric pressure and RQ?

The defaults are 760 mmHg for sea level and 0.8 for a mixed diet.

Can I calculate PAO₂ at altitude?

Yes. Enter the local atmospheric pressure in mmHg instead of using the sea-level default.

Does this replace a complete A-a gradient interpretation?

No. This tool focuses on PAO₂ and does not provide a full A-a gradient interpretation. It is not medical advice.

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{"paco2":35,"fio2":0.21,"patm":630,"decimalPlaces":1}

Outcome

The calculator returns PAO₂ of approximately 78.7 mmHg and identifies the result as reduced PAO₂ due to altitude.