FiO2 Calculator (LPM to %)
Estimate Fraction of Inspired Oxygen for Nasal Cannula
How to Calculate FiO2 from Flow Rate
In clinical respiratory care, FiO2 (Fraction of Inspired Oxygen) refers to the concentration of oxygen a patient inhales. Room air naturally contains approximately 21% oxygen. When supplemental oxygen is delivered via a low-flow system like a nasal cannula, the FiO2 increases based on the flow rate measured in Liters Per Minute (LPM).
The Rule of Four Formula
The standard "Rule of Thumb" used by healthcare professionals to estimate FiO2 for a nasal cannula is the Rule of Four. This formula assumes that for every 1 liter per minute of oxygen added, the FiO2 increases by approximately 4%.
FiO2 Conversion Chart
| Flow Rate (LPM) | Estimated FiO2 (%) |
|---|---|
| Room Air (0 LPM) | 21% |
| 1 LPM | 25% |
| 2 LPM | 29% |
| 3 LPM | 33% |
| 4 LPM | 37% |
| 5 LPM | 41% |
| 6 LPM | 45% |
Clinical Example
If a patient is currently on a nasal cannula at 3 LPM, you can calculate their FiO2 as follows:
- Start with the baseline oxygen concentration of room air: 21%
- Multiply the flow rate by 4: (3 LPM × 4 = 12)
- Add the two numbers: 21 + 12 = 33% FiO2
Important Limitations
It is crucial to remember that this calculation is an estimation. Several factors can influence the actual FiO2 delivered to the patient's lungs, including:
- Respiratory Rate: Faster breathing dilutes the oxygen with more room air.
- Tidal Volume: The depth of each breath affects the ratio of pure oxygen to room air.
- Mouth vs. Nose Breathing: Nasal cannulas are less effective for chronic mouth breathers.
- Flow Limit: Nasal cannulas are generally not used above 6 LPM as they cause nasal mucosal drying and do not significantly increase FiO2 further.