FiO2, or fraction of inspired oxygen, is simply the percentage of oxygen in the air a person breathes in, expressed as a decimal. Room air has an FiO2 of 0.21, meaning 21% of each breath is oxygen. When supplemental oxygen is added, FiO2 rises, but how you calculate or estimate it depends entirely on the delivery device being used.
What FiO2 Means in Practice
FiO2 is written as a decimal between 0.21 and 1.0. Room air is 0.21, and pure oxygen is 1.0. If someone is breathing 40% oxygen, their FiO2 is 0.40. The number matters because it’s used in clinical formulas to assess how well the lungs are working. A patient needing very high FiO2 to maintain normal oxygen levels has a more serious lung problem than someone doing fine on room air.
The percentage of oxygen in the atmosphere stays at 21% regardless of altitude. What changes at higher elevations is barometric pressure, which lowers the actual amount of oxygen molecules available per breath. FiO2 itself remains 0.21 on top of a mountain, but the effective oxygen delivery to the lungs drops because the air is thinner.
Low-Flow Devices: Nasal Cannula
A standard nasal cannula delivers oxygen at 1 to 6 liters per minute and produces an estimated FiO2 range of 0.24 to 0.44. You may have seen a popular shortcut that adds roughly 4% for each liter of flow: 1 liter equals 24%, 2 liters equals 28%, 3 liters equals 32%, and so on up to 6 liters at about 44%.
This “rule of four” is widely taught, but it’s an approximation with no strong experimental basis. A given liter flow on a nasal cannula does not equal any specific FiO2. The actual oxygen concentration reaching the lungs depends on how fast and deeply the patient breathes. Someone breathing rapidly with their mouth open dilutes the supplemental oxygen with far more room air, lowering the effective FiO2. Someone breathing slowly through their nose gets a higher concentration from the same flow rate. The 4%-per-liter estimate is a rough clinical guideline, not a precise calculation.
Fixed-Performance Devices: Venturi Masks
When a precise FiO2 is needed, Venturi masks are the standard tool. These masks use color-coded valves that mix a specific ratio of oxygen and room air to deliver a consistent, predetermined oxygen concentration regardless of how the patient is breathing. Each valve requires a minimum flow rate to function correctly:
- Blue valve: 2 to 4 L/min, delivers 24% (FiO2 0.24)
- White valve: 4 to 6 L/min, delivers 28% (FiO2 0.28)
- Orange valve: 6 to 8 L/min, delivers 31% (FiO2 0.31)
- Yellow valve: 8 to 10 L/min, delivers 35% (FiO2 0.35)
- Red valve: 10 to 12 L/min, delivers 40% (FiO2 0.40)
- Green valve: 12 to 15 L/min, delivers 60% (FiO2 0.60)
With a Venturi mask, you don’t calculate FiO2. You select the valve that matches the target concentration. For patients breathing faster than 30 breaths per minute, the flow rate should be set 1.5 to 2 times higher than the standard maximum to ensure the device keeps up with their demand.
High-Flow Devices: Non-Rebreather Masks
A non-rebreather mask connects to an oxygen reservoir bag and runs at 10 to 15 liters per minute. Under ideal conditions, it can deliver up to 95% oxygen (FiO2 of 0.95). In real-world use, the actual FiO2 typically falls between 0.60 and 0.80, because small air leaks around the mask allow room air to mix in. Field studies have recorded effective delivery as low as FiO2 0.60, but even at that level, it remains the most effective constant-flow device available outside of a ventilator.
Because the delivered FiO2 varies with mask fit and breathing pattern, non-rebreather masks are considered variable-performance devices. You can estimate that the patient is receiving somewhere in the 60% to 80% range, but you can’t pin it to an exact number the way you can with a Venturi mask.
High-Flow Nasal Cannula Systems
High-flow nasal cannula (HFNC) systems are fundamentally different from standard nasal cannulas. They deliver heated, humidified oxygen at flow rates up to 60 liters per minute, which is enough to meet or exceed a patient’s total inspiratory demand. Because the device supplies the entire volume of gas the patient inhales, the FiO2 can be dialed in precisely on the machine, anywhere from 0.21 to 1.0. There’s no need to estimate or calculate. The clinician sets the desired FiO2 directly, and the device blends oxygen and air to match.
This is the key distinction between low-flow and high-flow systems. With low-flow devices, room air mixes in unpredictably, making FiO2 an estimate. With high-flow systems, the machine controls the entire gas mixture, making FiO2 a known, set value.
Mechanical Ventilators
On a mechanical ventilator, FiO2 is set directly as a percentage on the machine. There’s no estimation involved. After a patient is first placed on a ventilator, the FiO2 is typically reduced to 0.40 (40%) quickly to avoid delivering too much oxygen, which can cause its own damage. From there, clinicians adjust the FiO2 to the minimum level needed to keep oxygen saturation between 90% and 96% on a pulse oximeter.
For patients with acute respiratory distress syndrome (ARDS), the target oxygen saturation range is slightly lower, typically 88% to 95%. In lung conditions where the primary problem is airflow obstruction rather than oxygen exchange, FiO2 is generally kept at 0.40 because increasing it further won’t address the underlying issue.
The P/F Ratio: Using FiO2 to Assess Lung Function
One of the most important clinical uses of FiO2 is calculating the P/F ratio, which divides the partial pressure of oxygen in arterial blood (PaO2, measured from a blood draw) by the FiO2 the patient is receiving. The formula is straightforward:
P/F ratio = PaO2 ÷ FiO2
For example, if a patient has a PaO2 of 80 mmHg while breathing room air (FiO2 of 0.21), the P/F ratio is 80 ÷ 0.21 = 381. That’s a normal result. A healthy person breathing room air typically has a P/F ratio above 400.
Now consider a patient with a PaO2 of 80 mmHg who requires 60% oxygen (FiO2 of 0.60) to reach that level. Their P/F ratio is 80 ÷ 0.60 = 133. That same PaO2 now signals serious lung impairment, because the lungs needed three times the normal oxygen concentration to achieve it.
The P/F ratio is central to classifying ARDS severity under the Berlin definition: mild ARDS falls between 200 and 300, moderate between 100 and 200, and severe is 100 or below. Each step down in the P/F ratio reflects a significant worsening in the lungs’ ability to transfer oxygen into the bloodstream.
Quick Reference by Device Type
- Room air: FiO2 is always 0.21
- Nasal cannula (1 to 6 L/min): estimate 0.24 to 0.44, roughly adding 0.04 per liter
- Venturi mask: read the FiO2 from the selected color-coded valve (0.24 to 0.60)
- Non-rebreather mask (10 to 15 L/min): estimate 0.60 to 0.80 in practice
- High-flow nasal cannula: FiO2 is set directly on the device (0.21 to 1.0)
- Mechanical ventilator: FiO2 is set directly on the machine (0.21 to 1.0)