PAFI Calculator (PaO2/FiO2)
Use this tool to quickly calculate the oxygenation index often called PAFI. Enter arterial oxygen pressure (PaO2) and inspired oxygen (FiO2), then click calculate.
What is “PAFI” and why is it useful?
PAFI is a quick clinical ratio: PaO2/FiO2. It helps estimate how well oxygen is moving from the lungs into the blood. In many Spanish-speaking settings, clinicians refer to this as “cálculo PAFI.”
The concept is simple: if a patient needs more oxygen (higher FiO2) but still has a low PaO2, the ratio drops, suggesting worse oxygenation. Because the ratio is fast to calculate, it is widely used in emergency medicine, critical care, and respiratory monitoring.
The formula
The equation is:
PAFI = PaO2 (mmHg) / FiO2 (fraction)
- PaO2 comes from arterial blood gas (ABG).
- FiO2 must be expressed as a fraction, not a percent.
- Example conversion: 40% oxygen = 0.40 FiO2.
Quick example
If PaO2 is 80 mmHg and FiO2 is 40%:
FiO2 fraction = 0.40
PAFI = 80 / 0.40 = 200
How to use the calculator above
- Enter PaO2 in mmHg.
- Enter FiO2 either as a percent (e.g., 50) or fraction (e.g., 0.50).
- Select the correct FiO2 format.
- Click Calculate PAFI to see the result and interpretation.
| PAFI Range | General Interpretation |
|---|---|
| > 300 | Relatively preserved oxygenation |
| 201–300 | Mild oxygenation impairment |
| 101–200 | Moderate oxygenation impairment |
| ≤ 100 | Severe oxygenation impairment |
Clinical interpretation: important context
PAFI is valuable, but it should never be interpreted in isolation. A complete respiratory assessment includes:
- Current respiratory support (nasal cannula, mask, HFNC, NIV, mechanical ventilation)
- PEEP/CPAP levels in ventilated patients
- Work of breathing and respiratory rate
- Chest imaging findings
- Trend over time, not just one snapshot
Common mistakes in “pafi calculo”
1) Forgetting to convert FiO2 percent to fraction
This is the most frequent error. If FiO2 is 60%, use 0.60, not 60.
2) Using uncertain FiO2 estimates
With low-flow oxygen devices, real FiO2 can vary by breathing pattern and mask fit. Interpret PAFI cautiously when FiO2 is estimated rather than measured.
3) Ignoring trend direction
A PAFI rising from 110 to 170 can indicate meaningful improvement even if still abnormal. Repeated measurements often provide better insight than a single value.
Why this ratio remains practical
Despite limitations, PAFI is still one of the fastest ways to communicate oxygenation status across teams. It supports triage discussions, helps monitor progression, and improves clarity during handoffs.
In everyday use, clinicians often combine PAFI with pulse oximetry, hemodynamics, and imaging to make safer treatment decisions.
Frequently asked questions
Is a higher PAFI better?
Yes. Higher values generally indicate better oxygen transfer from lungs to blood.
Can I use this calculator for education?
Absolutely. It is designed for rapid educational and bedside support calculations.
Does this replace medical judgment?
No. The calculator provides a numerical aid, not a diagnosis. Clinical context is essential.
Educational content only. This page is not a substitute for professional medical evaluation, diagnosis, or treatment.