P/F Ratio Calculator
PaO₂ ÷ FiO₂ — the oxygenation index behind the Berlin ARDS definition.
Enter PaO₂ and FiO₂ (21–100%) to calculate. Room air is 21%.
Reading the number
The P/F ratio answers “how hard is oxygenation working for the support it’s getting?” A PaO₂ of 90 looks fine — but on 100% FiO₂ it’s a P/F of 90, severe ARDS territory. That context is why the ratio beats PaO₂ alone.
Berlin ARDS classification also requires acute onset, bilateral opacities, ≥ 5 cm H₂O PEEP, and a non-cardiac cause — the ratio is the oxygenation piece, not the whole diagnosis. The SpO₂/FiO₂ (S/F) ratio is a noninvasive surrogate used for trending between ABGs.
Educational use only. Ventilator changes and ARDS management are provider-directed; use the ratio for trending and escalation awareness. This material supports nursing education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional policy, or medical direction. Always follow facility protocols and current provider orders.