Cardiopulmonary Function Assessment Scores (CPET)

FiO2 Estimation & Oxygen Correction Calculator

Clinical calculation support · Verify results before clinical use

Arterial Oxygenation Inputs

Enter the measured arterial blood gas values and the delivered FiO₂ to calculate estimated alveolar oxygen pressure, the A–a gradient, and the PaO₂/FiO₂ ratio.

%FiO₂

Inspired oxygen fraction (FiO₂, %) Enter percentage, e.g. 21 for room air or 40 for 40% oxygen. For low-flow nasal cannula, actual FiO₂ is variable and may not be reliably known.

Arterial PaCO₂ (mmHg) Use arterial PaCO₂ from the same blood gas sample as PaO₂.

Arterial PaO₂ (mmHg) Measured PaO₂ from arterial blood gas analysis.

Barometric pressure (mmHg) Use local barometric pressure when known. 760 mmHg is the conventional sea-level approximation.

Respiratory quotient (RQ) Default 0.8 is commonly used for a resting adult; the actual value varies with metabolic state.

Clinical background

This page calculates standard oxygenation parameters from FiO₂ and arterial blood gas measurements. It does not estimate a patient’s required oxygen flow or prescribe an oxygen device.

Equations used

Parameter	Equation	Meaning
Alveolar PO₂ (PAO₂)	(PB − 47) × FiO₂ − PaCO₂/RQ	Estimated alveolar oxygen partial pressure; PB is barometric pressure and 47 mmHg is water-vapour pressure at 37°C.
A–a oxygen gradient	PAO₂ − PaO₂	Difference between estimated alveolar and measured arterial oxygen partial pressure.
PaO₂/FiO₂ ratio	PaO₂ ÷ FiO₂ fraction	Common oxygenation index. FiO₂ must be entered as a fraction for the calculation.

Important considerations

Use PaCO₂ and PaO₂ from the same arterial blood gas sample whenever possible.
The simplified equation assumes water-vapour pressure of 47 mmHg and uses the entered barometric pressure and respiratory quotient.
FiO₂ from low-flow nasal cannulae and other variable-performance devices is only an estimate; breathing pattern and patient factors affect the delivered concentration.
Do not use this calculator alone to make oxygen-treatment decisions or diagnose acute respiratory distress syndrome.

References

Hendrix JM, Burns B. Alveolar Gas Equation.

StatPearls. Updated September 11, 2024.

NCBI Bookshelf

British Thoracic Society Guideline for oxygen use in adults in healthcare and emergency settings.

Thorax. 2017;72(Suppl 1):ii1–ii90. doi:10.1136/thoraxjnl-2016-209729.

Official DOI

Merck Manual Professional Edition. Equations for Calculating Alveolar Oxygen Pressure and Alveolar to Arterial Oxygen Gradient.

Reference table for standard alveolar gas and A–a gradient equations.

Official reference

Clinical disclaimer: This calculator is an educational aid and does not replace clinical assessment, arterial blood gas interpretation, local protocols, or specialist advice. The original template's HCO₃⁻-based “H plus value” and “AH” outputs, its derived FiO₂ estimate, and its oxygen-flow correction mapping were not standard validated equations and have therefore not been carried forward. Confirm the delivered FiO₂ and clinical context before interpreting oxygenation indices.