PaO₂/FiO₂ Ratio Calculator — Horowitz Index
The PaO₂/FiO₂ ratio (also called the Horowitz index or P/F ratio) quantifies how efficiently the lungs transfer oxygen. Enter the arterial PaO₂ from a blood gas and the FiO₂ to get the ratio and its Berlin 2012 ARDS severity classification.
mmHg
(0–1)
Normal (> 400)
- 1
FiO₂ (fraction of inspired oxygen)
0.6Room air = 0.21; 100% supplemental O₂ = 1.00 - 2
P/F ratio
300 ÷ 0.6 = 500
How does this calculator work?
PaO₂/FiO₂ ratio = PaO₂ ÷ FiO₂. Values above 400 mmHg are normal. The Berlin 2012 ARDS definition classifies mild ARDS as 200–300, moderate as 101–200, and severe as ≤ 100 mmHg. Enter the ABG PaO₂ and the delivered FiO₂ to get the ratio and its severity category instantly.
Formula
How this is calculated
The PaO₂/FiO₂ ratio divides the arterial partial pressure of oxygen (measured by ABG in mmHg) by the fraction of inspired oxygen (a dimensionless decimal from 0.21 for room air to 1.00 for 100% oxygen). A healthy adult breathing room air with a normal PaO₂ of 95 mmHg has a ratio of about 95 / 0.21 ≈ 452 mmHg. As lung function deteriorates, PaO₂ falls for a given FiO₂, so the ratio drops.
The Berlin Definition of ARDS (2012) uses the P/F ratio — measured on a minimum of 5 cmH₂O PEEP — to grade severity: Mild 200–300 mmHg, Moderate 101–200 mmHg, Severe ≤ 100 mmHg. A ratio between 300 and 400 is often labelled near-normal or indicative of hypoxaemic respiratory failure without meeting ARDS criteria. Values above 400 are generally considered within the normal physiological range.
This tool is for educational and clinical reference only. ARDS diagnosis requires bilateral opacities on chest imaging, respiratory failure not fully explained by cardiac failure or fluid overload, and acute onset — the P/F ratio alone is not sufficient. Always integrate ABG interpretation with the full clinical picture and your institution's protocols.
Frequently asked questions
A value above 400 mmHg is considered normal. On room air (FiO₂ 0.21) this corresponds to a PaO₂ above roughly 84 mmHg. Values between 300 and 400 represent mild hypoxaemia; below 300 meets criteria for at least mild ARDS under the Berlin 2012 definition.
If a patient's PaO₂ stays the same but you increase the FiO₂ (deliver more supplemental oxygen), the ratio falls because the denominator increases — the lungs are working harder just to maintain that PaO₂. A falling P/F ratio despite rising FiO₂ is a sign of worsening respiratory failure.
No. The oxygenation index (OI) also incorporates mean airway pressure: OI = (FiO₂ × MAP × 100) / PaO₂. It is a more sensitive marker in mechanically ventilated patients. The P/F ratio is simpler and does not require airway pressure data.
Also known as
TG we-Calculate Editorial Team. (2026). PaO₂/FiO₂ Ratio Calculator — Horowitz Index [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/pao2-fio2-ratio-calculator
TG we-Calculate Editorial Team. "PaO₂/FiO₂ Ratio Calculator — Horowitz Index." TG we-Calculate. 2026. https://we-calculate.com/calculator/pao2-fio2-ratio-calculator.
TG we-Calculate Editorial Team, "PaO₂/FiO₂ Ratio Calculator — Horowitz Index," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/pao2-fio2-ratio-calculator
@misc{wecalculate_pao2_fio2_ratio_calculator, title = {PaO₂/FiO₂ Ratio Calculator — Horowitz Index}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/pao2-fio2-ratio-calculator}}, year = {2026}, note = {TG we-Calculate} }
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