Free · Respiratory measurements

Arterial Oxygen to Inspired Oxygen Ratio

Calculate the PaO₂/FiO₂ ratio using arterial pressure, inspired oxygen fraction and selectable pressure units.

  • Formula & worked example
  • Private in your browser
  • No signup
Go to tool

Calculator inputs

Loading calculator…

How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Use the result with context

Health results are educational estimates, not a diagnosis or treatment recommendation. A qualified clinician can interpret measurements in the context of your health.

Formula and method

PaO₂ 80 mmHg and FiO₂ 40% give a P/F ratio of 200 mmHg.

P/F ratio = arterial oxygen partial pressure / inspired oxygen fraction.

Worked example

Enter these known values and leave the other values blank.

PaO₂
80 mmHg
FiO₂
40 %
PaO₂/FiO₂ ratio
200 mmHg

Assumptions and limitations

  • Use arterial PaO₂ and contemporaneous FiO₂, expressed as a fraction in the equation and a percent in the control. It is not an oxygen saturation ratio. The captured pressure-unit factors retain their numerical precision.
  • The historical Berlin adult categories use 200–300 mmHg, 100–200 mmHg and at most 100 mmHg along with other required findings and PEEP. Newer definitions can use different respiratory-support settings; this tool computes the measurement relationship, not a treatment or diagnosis.

Common questions

Is FiO₂ entered as percent?

Yes. Enter 40 for 40%; the equation divides by 0.40.

Does a low ratio prove ARDS?

No. The 2012 Berlin definition also requires timing, imaging and assessment of edema origin and respiratory support.

References

The calculation equations, inverse formulas, units, and input rules were imported from this source. Bookify provides the interface and equation solver.

Explore respiratory measurements or browse all health calculators.