Oxygenation & ARDS
P/F Ratio Explained: PaO₂/FiO₂ Ratio Made Simple
Learn how to calculate the P/F ratio step by step, interpret normal versus abnormal oxygenation, classify ARDS severity, and use the PaO₂/FiO₂ ratio alongside ABGs and the A–a gradient at the bedside.
What Is the P/F Ratio?
The P/F ratio is the ratio of arterial oxygen tension (PaO₂) to the fraction of inspired oxygen (FiO₂). It is one of the fastest ways to estimate oxygenation efficiency.
In practice, it helps answer a simple but important question: how much oxygen support is the patient receiving, and how much oxygen is actually making it into arterial blood?
For a broader oxygenation framework, pair this page with our A–a Gradient guide. For the full blood gas framework, see the ABG Step-by-Step Guide.
P/F Ratio Formula
FiO₂ should be expressed as a decimal. For example, 50% oxygen is written as 0.50.
Example conversions: room air = 0.21, 40% oxygen = 0.40, 100% oxygen = 1.00.
How to Calculate the P/F Ratio Step by Step
1. Measure PaO₂ from the ABG
Use the arterial oxygen tension from the blood gas.
2. Convert FiO₂ to a decimal
Convert the oxygen percentage to decimal form before dividing.
3. Divide PaO₂ by FiO₂
The result is the P/F ratio, which can then be used to describe oxygenation severity.
ARDS Interpretation
| P/F Ratio | Interpretation |
|---|---|
| >300 | Normal or near normal oxygenation |
| 200–300 | Mild ARDS range |
| 100–200 | Moderate ARDS range |
| <100 | Severe ARDS range |
The P/F ratio is heavily used in ARDS classification, but it should always be interpreted in clinical context along with imaging, ventilator settings, and the rest of the Berlin criteria.
Worked P/F Ratio Examples
Example 1: Mild Oxygenation Impairment
PaO₂ 90 mmHg / FiO₂ 0.30
90 ÷ 0.30 = 300
A P/F ratio of 300 is right at the threshold where oxygenation begins to look abnormal. This is not severe hypoxemia, but it is not fully reassuring either in the right clinical context.
Example 2: Moderate ARDS Range
PaO₂ 80 mmHg / FiO₂ 0.40
80 ÷ 0.40 = 200
A P/F ratio of 200 falls into the moderate ARDS range and suggests significant impairment in oxygen transfer.
Example 3: Severe Oxygenation Failure
PaO₂ 55 mmHg / FiO₂ 0.80
55 ÷ 0.80 = 68.75
A P/F ratio below 100 suggests severe oxygenation failure and is consistent with severe ARDS when the rest of the Berlin criteria are met.
Clinical Use
- Assess the severity of hypoxemia
- Help classify ARDS
- Track response to oxygen and ventilator changes
- Support escalation decisions from HFNC to NIV to intubation
If you are also working through acid-base disorders, review Compensation Formulas Cheat Sheet.
Limitations of the P/F Ratio
- It is affected by ventilator settings such as PEEP.
- It depends on an accurate and stable FiO₂ estimate.
- It does not directly explain why oxygenation is low.
- It should not replace full ABG interpretation.
That is why the P/F ratio works best when paired with the A–a gradient, which helps explain the mechanism of hypoxemia.
Use the P/F Ratio Calculator
Use the PulmTools oxygenation calculator to compute the P/F ratio quickly, then pair it with the ABG analyzer and A–a gradient page for a more complete oxygenation assessment.