acalculator

What is the PF ratio?

Enter the PaO2 from an arterial blood gas and the FiO2 to get the PF ratio and the Berlin Definition range it falls in. For information only. This is not medical advice or a diagnosis. Do not use it to make treatment decisions without a clinician. No licensed clinician has reviewed this page.

Your numbers

PaO2 units
PF ratio (mmHg)
200

The PF ratio is 200 mmHg (26.7 kPa): Moderate range (over 100, up to 200).

Moderate range (over 100, up to 200)

  1. Severe range (100 or less)
  2. Moderate range (over 100, up to 200)
  3. Mild range (over 200, up to 300)
  4. Over 300 (above the ARDS ranges)
PF ratio (kPa)
26.7
Berlin Definition range
Moderate range (over 100, up to 200)

PF ratio (mmHg): 200. The PF ratio is 200 mmHg (26.7 kPa): Moderate range (over 100, up to 200).

Where does the PF ratio sit?

This calculator gives general information only. It is not medical advice and does not replace a doctor or other health care provider. Talk with a professional about your results. Terms of use

How to calculate

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.

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).

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.

  • 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.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. PaO2 units mmHg, PaO2 (mmHg) 80, FiO2 40% gives PF ratio (mmHg) 200, PF ratio (kPa) 26.7, Berlin Definition range Moderate range (over 100, up to 200).Source: Berlin Definition 2012: 80 ÷ 0.40 = 200, moderate (100 < PF ≤ 200)
  2. PaO2 units mmHg, PaO2 (mmHg) 95, FiO2 21% gives PF ratio (mmHg) 452, PF ratio (kPa) 60.3, Berlin Definition range Over 300 (above the ARDS ranges).
  3. PaO2 units mmHg, PaO2 (mmHg) 90, FiO2 30% gives PF ratio (mmHg) 300, Berlin Definition range Mild range (over 200, up to 300).
  4. PaO2 units mmHg, PaO2 (mmHg) 60, FiO2 60% gives PF ratio (mmHg) 100, PF ratio (kPa) 13.3, Berlin Definition range Severe range (100 or less).
  5. PaO2 units kPa, PaO2 (kPa) 10.7, FiO2 50% gives PF ratio (mmHg) 161, PF ratio (kPa) 21.4, Berlin Definition range Moderate range (over 100, up to 200).

Medical disclaimer

This calculator is for information and education only. It is not medical advice, and its result is not a diagnosis. Do not use it to start, stop, or change a treatment without a clinician who knows your health history. No licensed clinician has reviewed this page. This site's editors checked the formulas and worked examples against published sources. If you think you have a medical emergency, call your local emergency number.

When this does not apply. The Berlin ranges are for adults on PEEP or CPAP of at least 5 cm H2O who meet the other ARDS criteria (timing, chest imaging, and a cause other than heart failure). The ratio changes with the ventilator settings and with altitude, so a value taken off pressure support or at high altitude is not comparable.

How it works

  1. FiO2 is typed as a percent from 21 to 100 and used as a fraction: FiO2 ÷ 100.
  2. PaO2 in kPa is turned into mmHg: mmHg = kPa ÷ 0.133322 (1 mmHg = 133.322 Pa).
  3. PF ratio (mmHg) = PaO2 (mmHg) ÷ (FiO2 ÷ 100), from the exact typed values, rounded once to a whole number, halves up.
  4. PF ratio (kPa) = the exact ratio × 0.133322, to 1 decimal. With PaO2 typed in kPa, this is PaO2 (kPa) ÷ (FiO2 ÷ 100).
  5. The range is read from the whole mmHg value shown, by the 2012 Berlin Definition: over 200 up to 300 mild; over 100 up to 200 moderate; 100 or less severe. Over 300 is above the ARDS ranges. Because the range follows the shown whole number, an exact ratio such as 300.4 shows as 300 and reads as mild.

Worked examples by hand

Page default. PaO2 80 mmHg, FiO2 40%. 80 ÷ 0.40 = 200, moderate. In kPa: 200 × 0.133322 = 26.66, 26.7.

Room air. PaO2 95, FiO2 21%. 95 ÷ 0.21 = 452.38, 452, over 300. In kPa: 452.38 × 0.133322 = 60.3.

Edge of mild. PaO2 90, FiO2 30%. 90 ÷ 0.30 = 300, mild (300 is in the mild range).

Edge of severe. PaO2 60, FiO2 60%. 60 ÷ 0.60 = 100, severe; 100 × 0.133322 = 13.3 kPa.

PaO2 in kPa. PaO2 10.7 kPa, FiO2 50%. 10.7 ÷ 0.133322 = 80.257 mmHg; ÷ 0.50 = 160.51, 161, moderate. In kPa: 10.7 ÷ 0.50 = 21.4.

Other questions people ask

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.