What is a normal PaO2/FiO2 ratio?

A normal P/F ratio in a healthy adult breathing room air is approximately 400–500 mmHg (PaO2 ~80–100 mmHg, FiO2 0.21). Values below 300 mmHg are considered abnormal and suggest impaired gas exchange.

What are the Berlin Definition ARDS thresholds?

The 2012 Berlin Definition classifies ARDS as mild (P/F 200–300 mmHg), moderate (P/F 100–200 mmHg), or severe (P/F below 100 mmHg), all requiring at least 5 cmH2O of PEEP or CPAP.

Should I enter FiO2 as a decimal or percentage?

This calculator accepts both. You can enter 40 (meaning 40%) or 0.40 — it detects values greater than 1.0 and treats them as a percentage automatically.

Does altitude or barometric pressure affect the P/F ratio?

PaO2 is directly affected by barometric pressure at altitude, which can lower the P/F ratio without true lung disease. Some centers apply altitude corrections, but the standard Berlin criteria do not mandate this.

What is the difference between P/F ratio and oxygenation index (OI)?

The oxygenation index (OI) factors in mean airway pressure: OI = (FiO2 × MAP × 100) / PaO2. The P/F ratio is simpler and more widely used, while OI is preferred in pediatric ARDS assessments.

Is P/F ratio alone sufficient to diagnose ARDS?

No. The Berlin Definition requires acute onset within 1 week, bilateral chest opacities not fully explained by effusions or atelectasis, respiratory failure not fully explained by cardiac failure, and P/F ratio below 300 on PEEP/CPAP ≥5 cmH2O.

PaO2/FiO2 Ratio Calculator

Enter arterial oxygen partial pressure (PaO2) and fraction of inspired oxygen (FiO2) to calculate the P/F ratio and classify ARDS severity per Berlin criteria.

Input Values

PaO2 — Arterial Oxygen Tension (mmHg)

Normal range: 75–100 mmHg (breathing room air)

FiO2 — Fraction of Inspired Oxygen (% or decimal)

Room air = 21 or 0.21 | 100% oxygen = 100 or 1.0

Berlin Definition Thresholds

Mild ARDS 200 < P/F ≤ 300

Moderate ARDS 100 < P/F ≤ 200

Severe ARDS P/F ≤ 100

Normal / No ARDS P/F > 300

All ARDS categories require PEEP or CPAP ≥ 5 cmH2O on mechanical ventilation.

Enter PaO2 and FiO2 values and press Calculate to see results.

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Summary

The PaO2/FiO2 (P/F) ratio is a key clinical measure of respiratory function, calculated by dividing arterial oxygen tension (PaO2, in mmHg) by the fraction of inspired oxygen (FiO2, expressed as a decimal from 0.21 to 1.0). A lower P/F ratio indicates worse oxygenation. According to the Berlin Definition, ARDS is classified as mild (P/F 200–300), moderate (P/F 100–200), or severe (P/F below 100), always assessed on at least 5 cmH2O of PEEP or CPAP.

How it works

Enter the arterial oxygen tension (PaO2) in mmHg from an arterial blood gas (ABG) result.
Enter the fraction of inspired oxygen (FiO2) as a percentage (e.g., 40 for 40%) or as a decimal (e.g., 0.40).
The calculator divides PaO2 by FiO2 (after converting FiO2 to a decimal) to produce the P/F ratio.
The result is interpreted against the Berlin Definition thresholds to classify ARDS severity.
Review the severity classification and associated clinical context displayed below the result.

Use cases

Classify ARDS severity at ICU admission or triage.
Monitor treatment response in ventilated patients over time.
Guide decisions on prone positioning, ECMO referral, or lung-protective ventilation strategies.
Communicate oxygenation status clearly in handoffs and clinical notes.
Educate medical students and residents on Berlin Definition criteria.
Screen patients for enrollment in ARDS clinical trials.

Frequently Asked Questions

Last updated: 2026-05-29 · Reviewed by Nham Vu