FiO2 stands for a fraction of inspired oxygen. It is the volumetric or molar fraction of oxygen in inhaled gas.

When patients experience difficulty in breathing, they are given oxygen-enriched air. This air has higher FiO2 than natural air. There is 21% oxygen in natural air, and it means a FiO2 of 0.21.

The oxygen-enriched air has higher FiO2 up to 1.00 (100% oxygen). But usually, it’s kept below 0.5, to avoid the toxicity of oxygen, even with mechanical ventilation.

But it’s routine to use 100% in some applications.

The calculation of the percentage of oxygen received by a given flow cannot be precise, but a relatively accurate estimate can be obtained by relying on some guidelines.
Comparison of FiO2, a fraction of inspired oxygen in the air to a transportable oxygen device, is indicated in percentage.

As a general rule of thumb, under normal conditions, effective FiO2 can be gauged by:

  • At 1 LPM (liters per minute), the estimated FiO2 is 24%.
  • At 2 LPM, it is 28%.
  • At 3 LPM, it is 32%.
  • At 4 LPM, it is 36%.
  • At 5 LMP, it is 40%.
  • At 6 LPM, it is 44%.
  • At 7 LPM, it is 48%.
  • At 8 LPM, it is 52%.
  • At 9 LPM, it is 56%.
  • At 10 LPM, it is 60%.

Therefore, there is a 4% increase in FiO2 for every LPM use of oxygen from portable oxygen concentrators.

The formula for calculating FiO2 percentage

FiO2= 20% + (4 X oxygen liter flow) But this calculation is valid under normal conditions. Various factors affect the FiO2. These factors include:

  • Breathe size, how small or large it is.
  • Breathe rate, how slow or fast the person is breathing.
  • Inspiratory flow rate, affected by both breath rate and breathe size.
  • Different types of oxygen tubes affect the flow.

Health care providers are trained in monitoring and considering the patient’s condition when deciding about the oxygen flow.

When oxygen is provided using a nasal cannula, the liter flow will be set between 1 and 6 liters.

While using a simple face mask, the meter will be set between 5 and 12 liters. With this type of mask, the meter should never be less than 5 liters.

The oxygen delivered by a partial rebreather mask will be 35 to 60 percent. The liter flow dial must be set between 8 to 15 liters. The bag must always be kept half-inflated. If the bag is not inflated enough, then you have to increase the oxygen flow.

A rebreather mask and a non-breather mask looks similar, but a non-breather mask delivers a high concentration of oxygen. The liter flow is set between 8 to 15 liters but the percentage of oxygen delivered is 60 to 90 percent. The bag must be kept inflated, and the liter flow must never be decreased to less than 8 liters.

If the patient is using a venti mask for oxygen, the flow meter is always set at 15 liters. The mask has different color-changeable plastic pieces. Each color signifies a different oxygen delivery percentage. The mask can deliver between 24 to 50 percent of oxygen.

The oxygen flow can only be changed by changing the plastic adaptor.

Not following the protocols can cause serious O2 toxicity. You must never decrease or increase the oxygen flow without your doctor’s permission.


Dr. Ayesha Saeed is PhD in Biochemistry. She has been serving as a lecturer in different colleges for last 7 years. Currently, she is vice-principal in a privately owned medical college. She recently joined biomadam as a volunteer writer to share her knowledge through our platform.

Write A Comment

I accept the Privacy Policy

Pin It