P/F Ratio Calculator
PaO₂/FiO₂ ratio for oxygenation assessment
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Need a fast answer on PaO₂/FiO₂ ratio for oxygenation assessment? P/F Ratio Calculator answers with the steps beside it. Just enter PaO₂ (mmHg) and FiO₂ (%); P/F Ratio updates the result as you type. Nothing in P/F Ratio is a black box. P/F Ratio replaces guesswork. You can disconnect once P/F Ratio has loaded; the arithmetic runs locally. P/F Ratio is part of the Health collection on CalcProMaster, alongside pf ratio calculator pao2 fio2 oxygenation ards berlin criteria, free online p/f ratio calculator and more. The labels in P/F Ratio match the terms in its result. Start with P/F Ratio; the first number takes seconds.
What the defaults give
PaO₂ of 100 mmHg on an FiO₂ of 50% gives a P/F ratio of 200.
The ratio compares the oxygen in arterial blood with the fraction being delivered, so it falls either when oxygenation worsens or when the oxygen supply is reduced — which is exactly what makes it useful for tracking a patient over time.
How the P/F ratio is calculated
The P/F ratio compares arterial oxygen to the fraction of oxygen you are breathing:
P/F ratio = PaO₂ ÷ FiO₂ (as a decimal)
A PaO₂ of 100 mmHg on 50% oxygen: 100 ÷ 0.50 = 200.
| PaO₂ (mmHg) | FiO₂ | P/F ratio | Band |
|---|---|---|---|
| 100 | 21% | 476 | Normal |
| 100 | 40% | 250 | Mild |
| 100 | 50% | 200 | Mild / moderate |
| 80 | 60% | 133 | Moderate |
| 60 | 80% | 75 | Severe |
Lower ratios mean worse oxygenation. This tool is for reference only — clinical decisions belong with the treating team. Related: anion gap and health calculators.
The formula
P/F = PaO₂ ÷ FiO₂ (as a fraction).
100 ÷ 0.50 = 200. The FiO₂ field takes a percentage, so it is divided by 100 before use — 50% becomes 0.50, not 50. Entering it as a fraction directly gives answers off by a factor of 100.
Worked example
PaO₂ 100 mmHg
FiO₂ 50% = 0.50
P/F = 100 ÷ 0.50 = 200
On room air (0.21), a normal PaO₂ of 90 gives a ratio of 428. The same PaO₂ on 100% oxygen gives 90 — a dramatic fall with no change in the blood gas measurement itself.
The bands
- Above 400 — normal oxygenation.
- 300–400 — mild impairment.
- 200–300 — mild ARDS (Berlin definition uses 200–300 with PEEP ≥ 5).
- 100–200 — moderate ARDS.
- Below 100 — severe ARDS.
A ratio of exactly 200 sits on the boundary between mild and moderate, which is why the trend matters more than a single reading on the line.
Limits worth knowing
The ratio assumes steady-state arterial sampling. It is unreliable in the first minutes after an FiO₂ change, it is affected by altitude and by the patient’s position, and a venous sample produces a much lower figure that should never be read as a P/F ratio. It also says nothing about ventilation — CO₂ clearance is a separate question.
What it is used for
Tracking oxygenation over time, grading ARDS severity, and titrating oxygen and PEEP. Trends across hours carry more information than any single value.
Frequently Asked Questions
Why does 100 on 50% give 200?
Because FiO₂ is converted to a fraction: 50% = 0.50, and 100 ÷ 0.50 = 200. The ratio is PaO₂ relative to the oxygen fraction being delivered.
What P/F ratio indicates ARDS?
Below 300 with PEEP of at least 5 cmH₂O and compatible imaging. 200–300 is mild, 100–200 moderate, and below 100 severe.
Is a higher P/F ratio better?
Yes. A higher ratio means more oxygen in the blood for the fraction being delivered. A falling ratio at a constant FiO₂ is a deterioration.
Does FiO₂ need to be a percentage or a fraction?
This calculator takes a percentage — enter 50 for 50%. Internally it is divided by 100, so entering 0.5 would give a ratio a hundred times too high.
What is a normal P/F ratio?
Around 400–500 on room air. A healthy PaO₂ of 90 on 21% oxygen gives about 428; anything above 400 is generally read as normal oxygenation.
What P/F ratio needs intubation?
No ratio alone decides it. Severe ARDS is defined below 100 with PEEP of at least 5, and that band usually means mechanical ventilation — but the decision is clinical, based on work of breathing, gas exchange trend and the patient’s overall state.
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