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PaO₂/FiO₂ (P/F) Ratio Calculator

Get the P/F oxygenation ratio in one step.

How it is calculated

The formula is P/F = PaO₂ ÷ FiO₂ with FiO₂ as a fraction. PaO₂ is the arterial oxygen partial pressure from a blood gas, in mmHg, and must be above 0 and no more than 800. FiO₂ can be typed either as a fraction from 0.21 to 1.0 or as a percentage from 21 to 100; the engine turns a percentage into a fraction by dividing by 100. Values between 1 and 21 (such as 5 or 1.5) are ambiguous and show no result, as do FiO₂ above 100 and any zero or negative input. Because 1 counts as the fraction 1.0, typing 1 means 100% oxygen. The answer is rounded to a whole number with no unit label. The tool only computes the ratio; it does not add a severity category or check PEEP, so the ARDS grading is read from the number by the clinician.

Worked example

Take an arterial blood gas with PaO₂ of 80 mmHg on an oxygen setting of 40%. Convert the percentage to a fraction: 40 ÷ 100 = 0.40. Then divide: 80 ÷ 0.40 = 200. The calculator returns 200. Typing 0.4 instead of 40 gives the same 200. For comparison, the same PaO₂ of 80 on room air (21%) would give 80 ÷ 0.21, about 381, so needing more oxygen for the same blood level lowers the ratio.

Try this example →

Common mistakes

  • Typing a number between 1 and 21 for FiO₂, such as 5 or 1.5. The tool cannot tell whether you meant a fraction or a percentage and shows no result.
  • Entering 1 meaning 1%. The tool reads 1 as the fraction 1.0, which is 100% oxygen, so the ratio equals the PaO₂ itself.
  • Entering the FiO₂ of a different moment than the blood gas. The ratio only means something if both values belong to the same time point.
  • Entering SpO₂ (pulse oximeter saturation in %) in the PaO₂ field. The tool needs the arterial partial pressure in mmHg.
  • Entering PaO₂ in kPa. A value such as 10.6 is accepted, but the formula and the usual cut-offs are in mmHg, so the ratio comes out roughly 7.5 times too low.

About this calculator

The PaO₂/FiO₂ ratio, often just called the P/F ratio, divides the partial pressure of oxygen in arterial blood (PaO₂, from an arterial blood gas, in mmHg) by the fraction of inspired oxygen a patient is receiving (FiO₂, expressed as a decimal — 0.21 for room air, 1.0 for 100% oxygen). It is the core measurement used to judge how well the lungs are transferring oxygen into the blood relative to how much oxygen is being delivered.

This ratio is the backbone of the Berlin Definition of ARDS (Acute Respiratory Distress Syndrome), which grades severity as mild (200-300 mmHg), moderate (100-200 mmHg), or severe (less than 100 mmHg), each requiring a minimum PEEP or CPAP of 5 cmH₂O. Intensivists, pulmonologists, and respiratory therapists in the ICU use it to classify ARDS severity, decide whether to escalate to prone positioning, higher PEEP, or ECMO referral, and track a patient's oxygenation trend over the course of mechanical ventilation.

This calculator takes the PaO₂ from a blood gas and the current FiO₂ setting and returns the P/F ratio instantly for bedside decision-making.

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