# P/F Ratio Calculator

URL: https://speedycalculator.com/calculators/health/pf-ratio/
Last updated: 2026-10-25

> The P/F ratio calculator divides arterial oxygen pressure (PaO₂) by the fraction of inspired oxygen (FiO₂). It is used to grade the severity of acute respiratory distress syndrome under the Berlin definition.

## Inputs

- PaO₂ (mmHg)
- FiO₂ (%)

## Results

- P/F ratio

## Formula

```
P/F = PaO₂ ÷ FiO₂ (as a decimal)
```

## Explanation

The ratio shows how well the lungs transfer oxygen relative to how much is being given.

### How to use the P/F Ratio Calculator

- **PaO₂**
- **FiO₂**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Divide PaO₂ in mmHg by FiO₂ expressed as a decimal, such as 0.4 for 40%.

### Inputs explained

FieldWhat to enterPaO₂In mmHgFiO₂In %

### Example values

How the main result changes when **PaO₂** changes and everything else stays at the example values above:

PaO₂ (mmHg)P/F ratio (mmHg)4511367.516990225112.5281135338180450
*Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.*

## Worked example

Inputs — PaO₂: 90 mmHg; FiO₂: 40 %.

Results — **P/F ratio = 225 mmHg**.

## FAQ

**What is normal?**
Around 400–500 mmHg breathing room air.

**Is a low P/F alone enough to diagnose ARDS?**
No, timing, imaging and cause also count.

**What formula does the P/F Ratio Calculator use?**
P/F = PaO₂ ÷ FiO₂ (as a decimal).


## Sources

- [ARDS Definition Task Force. Acute respiratory distress syndrome: the Berlin definition. JAMA, 2012](https://pubmed.ncbi.nlm.nih.gov/22797452/)