Because a nasal cannula doesn't deliver a precise, fixed oxygen percentage, this tool estimates the approximate FiO₂ a patient is receiving based on their oxygen flow rate.
| Flow | Approx. FiO₂ |
|---|---|
| 1 L/min | ~24% |
| 2 L/min | ~27% |
| 4 L/min | ~33% |
| 6 L/min | ~39% |
Given: nasal cannula at 3 L/min.
21 + 9 = ~30%.Delivered oxygen is diluted by entrained room air that varies with each breath.
Room air mixes with the delivered oxygen in a way that depends on the patient's own breathing — tidal volume, rate, and whether they mouth-breathe all change the final fraction, so no fixed value is guaranteed.
As a rough teaching rule, each liter per minute adds about 3% above the 21% of room air — so 2 L ≈ 27%, 4 L ≈ 33%, 6 L ≈ 39%.
Beyond about 6 L/min the added oxygen plateaus and the cannula also becomes uncomfortable and drying, so higher-flow or different devices are used instead.
No — it's a conceptual estimate for teaching. When a precise, known FiO₂ is needed, a fixed-performance device like a Venturi mask is used instead.