# What is the PF ratio?

Divides the arterial oxygen pressure (PaO2) by the inspired oxygen fraction (FiO2) to give the PF ratio, and shows the Berlin Definition range it falls in.

- Page: https://www.acalculator.org/health/pf-ratio-calculator
- JSON spec: https://www.acalculator.org/health/pf-ratio-calculator.json
- Version: 9af1336777ab

## Default answer

Example with the default inputs (PaO2 units mmHg, PaO2 (mmHg) 80, FiO2 40%): The PF ratio is 200 mmHg (26.7 kPa): Moderate range (over 100, up to 200).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| units | PaO2 units | The unit of the blood gas PaO2: mmHg or kPa. |
| pao2 | PaO2 (mmHg) | Arterial partial pressure of oxygen from a blood gas, 20 to 700 mmHg. |
| pao2Kpa | PaO2 (kPa) | Arterial partial pressure of oxygen from a blood gas, 2.7 to 93.3 kPa. |
| fio2 | FiO2 | Fraction of inspired oxygen, from 21% (room air) to 100%. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| pf | PF ratio (mmHg) | PaO2 in mmHg ÷ FiO2 as a fraction, to a whole number. |
| pfKpa | PF ratio (kPa) | The same ratio with PaO2 in kPa, to 1 decimal. |
| range | Berlin Definition range | The oxygenation range of the 2012 Berlin Definition of ARDS that holds the ratio. |

## Method

PF ratio = PaO2 (mmHg) ÷ (FiO2 % ÷ 100), to a whole mmHg. PaO2 in kPa ÷ 0.133322 = mmHg. Berlin Definition: over 200 to 300 mild, over 100 to 200 moderate, 100 or less severe.

## Assumptions

- PaO2 and FiO2 are taken at the same time.
- The ratio is worked from the exact typed values and rounded once, halves up; the range is read from the whole mmHg value shown.
- The Berlin ranges apply only with PEEP or CPAP of 5 cm H2O or more and the other ARDS criteria; the page does not check those.

## Worked examples

1. units = mmhg, pao2 = 80, fio2 = 40% gives pf = 200, pfKpa = 26.7, range = Moderate range (over 100, up to 200). Source: Berlin Definition 2012: 80 ÷ 0.40 = 200, moderate (100 < PF ≤ 200).
2. units = mmhg, pao2 = 95, fio2 = 21% gives pf = 452, pfKpa = 60.3, range = Over 300 (above the ARDS ranges).
3. units = mmhg, pao2 = 90, fio2 = 30% gives pf = 300, range = Mild range (over 200, up to 300).
4. units = mmhg, pao2 = 60, fio2 = 60% gives pf = 100, pfKpa = 13.3, range = Severe range (100 or less).
5. units = kpa, pao2Kpa = 10.7, fio2 = 50% gives pf = 161, pfKpa = 21.4, range = Moderate range (over 100, up to 200).

## FAQ

### How is the PF ratio calculated?

Divide the arterial oxygen pressure (PaO2, in mmHg) by the fraction of inspired oxygen (FiO2, as a decimal). A PaO2 of 80 mmHg on 40% oxygen gives 80 ÷ 0.40 = 200.

### What is a normal PF ratio?

A healthy person breathing room air (FiO2 0.21) with a PaO2 of 80 to 100 mmHg has a ratio of about 380 to 480. The Berlin Definition of ARDS uses ratios of 300 or less.

### What do the Berlin Definition ranges mean?

In the 2012 Berlin Definition of acute respiratory distress syndrome (ARDS), a ratio over 200 up to 300 is mild, over 100 up to 200 is moderate, and 100 or less is severe. The task force found higher death rates with each step (27%, 32%, and 45%).

### Does a low PF ratio mean ARDS?

Not by itself. The Berlin Definition also needs a start within 1 week, shadows on both lungs on imaging, a cause other than heart failure or fluid overload, and PEEP or CPAP of at least 5 cm H2O. The page checks only the ratio.

### How do I use a PaO2 in kPa?

Pick kPa. The page divides by 0.133322 to get mmHg, then works the ratio. It shows the ratio in both units; in kPa, 300 mmHg is about 40 kPa.

### What FiO2 does room air have?

About 21%. Enter 21 for a person breathing room air.

## Sources

- ARDS Definition Task Force; Ranieri VM, Rubenfeld GD, Thompson BT, et al. Acute respiratory distress syndrome: the Berlin Definition. JAMA. 2012;307(23):2526-2533. https://doi.org/10.1001/jama.2012.5669 (retrieved 2026-10-05)
- Taylor BN, Thompson A. NIST Special Publication 811: Guide for the Use of the International System of Units, appendix B.8 (1 mmHg = 133.322 Pa). https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-05)