fio2/pao2 calculator

What is the FiO2/PaO2 relationship?

The FiO2/PaO2 relationship helps evaluate how well oxygen moves from the lungs into the blood. In clinical practice, the most commonly used value is the PaO2/FiO2 ratio (P/F ratio). A lower P/F ratio generally indicates worse oxygenation.

The inverse value, FiO2/PaO2, is used less often but can still be helpful for trend tracking in some workflows. This page calculates both values for convenience.

How to use this calculator

Step 1: Enter PaO2

Use the arterial blood gas PaO2 value in mmHg.

Step 2: Enter FiO2

You can enter FiO2 as either a percentage (like 40) or a fraction (like 0.40). Choose the matching unit.

Step 3: Review output

The calculator returns:

• P/F ratio (PaO2/FiO2)
• FiO2/PaO2 ratio
• A quick oxygenation interpretation band

Interpreting P/F ratio ranges (general guide)

• ≥ 400: Typically near-normal oxygenation in many settings
• 300-399: Mild reduction in oxygen transfer
• 200-299: Often considered mild ARDS range in the right clinical context
• 100-199: Often considered moderate ARDS range
• < 100: Often considered severe ARDS range

These cutoffs are screening-level references. Clinical diagnosis requires full context, including ventilator settings, PEEP, imaging, timeline, and other causes of hypoxemia.

Common FiO2 reference points

Approximate FiO2 values vary with device fit, flow, and patient factors, but common estimates include:

• Room air: ~21% (0.21)
• Nasal cannula: typically ~24-44% depending on flow and breathing pattern
• Simple face mask: often ~35-60%
• Non-rebreather and advanced support: may approach high FiO2 depending on setup
• Mechanical ventilation: FiO2 is set directly and measured more precisely

Why trend data matters

A single P/F ratio can be useful, but trends are usually more informative. A patient moving from a ratio of 120 to 190 may be improving, even if still critically ill. Likewise, a falling ratio can be an early warning sign and should trigger reassessment.

Limitations of any single oxygenation number

• P/F ratio can be influenced by PEEP and mean airway pressure
• Timing relative to interventions matters (positioning, suctioning, recruitment)
• Acid-base status, hemoglobin, and perfusion also affect oxygen delivery and outcomes
• Measurement errors in blood gas sampling can distort interpretation

Clinical note

This calculator is for educational and informational use only and is not a substitute for medical judgment. For diagnosis or treatment decisions, rely on qualified clinicians and current institutional protocols.