Calculate FiO₂, air flow, or O₂ flow from FiO₂ and the air:O₂ ratio, with L/min or mL/min inputs to determine the missing value for oxygen blending.
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Air to Oxygen Ratio Formula
The air to oxygen ratio calculator assumes you are blending room air, treated as 21% oxygen, with pure oxygen, treated as 100% oxygen. The air:O₂ ratio is air flow divided by oxygen flow.
AOR = (100 - FiO2)/(FiO2 - 21)
FiO2 = (100 + 21*AOR)/(AOR + 1)
AOR = Air Flow/O2 Flow
Air Flow = AOR*O2 Flow
O2 Flow = Air Flow/AOR
- AOR = air to oxygen ratio, written as air flow divided by O₂ flow
- FiO2 = fraction of inspired oxygen, entered as a percentage
- Air Flow = room air flow rate
- O2 Flow = oxygen flow rate
If you enter FiO₂ only, the calculator finds the air:O₂ ratio needed to produce that oxygen percentage. If you enter air flow and O₂ flow, it finds FiO₂. If you enter FiO₂ with one flow value, it calculates the missing air or oxygen flow. Flow units can be entered in L/min or mL/min, and the calculation converts them internally to L/min before applying the formulas.
Common FiO2 to Air:O2 Ratios
The values below show approximate air:O₂ ratios for common FiO₂ targets using the same 21% room air and 100% oxygen assumption.
| FiO₂ Target | Air:O₂ Ratio | Meaning |
|---|---|---|
| 24% | 25.33:1 | Much more air than oxygen |
| 28% | 10.29:1 | Low oxygen enrichment |
| 40% | 3.16:1 | Moderate oxygen enrichment |
| 60% | 1.03:1 | Nearly equal air and oxygen flows |
| 100% | 0:1 | No room air is mixed in |
Flow Unit Conversions
| Unit | Conversion | Example |
|---|---|---|
| L/min | Base unit | 2 L/min = 2 L/min |
| mL/min | Divide by 1000 to get L/min | 500 mL/min = 0.5 L/min |
Examples
Example 1: Calculate FiO2 from air and oxygen flow
You have an air flow of 6 L/min and an O₂ flow of 2 L/min.
First find the air:O₂ ratio:
AOR = 6/2 = 3
Then calculate FiO₂:
FiO2 = (100 + 21*3)/(3 + 1) = 40.75%
Example 2: Calculate air flow from FiO2 and oxygen flow
You want 40% FiO₂ and have an O₂ flow of 2 L/min.
Find the required air:O₂ ratio:
AOR = (100 - 40)/(40 - 21) = 3.1579
Then calculate air flow:
Air Flow = 3.1579*2 = 6.3158 L/min
FAQ
What does an air:O2 ratio of 3 mean?
An air:O₂ ratio of 3 means the air flow is 3 times the oxygen flow. For example, 6 L/min of air and 2 L/min of oxygen have a ratio of 3:1.
Why does the formula use 21 and 100?
The formula uses 21 because room air is treated as about 21% oxygen. It uses 100 because the oxygen source is treated as 100% oxygen. If your actual gas sources differ from those assumptions, the calculated ratio will not match the real blended oxygen concentration exactly.
Can FiO2 be 21% or 100%?
FiO₂ of 21% represents room air only, so the air:O₂ ratio would approach infinity because no added oxygen is needed. FiO₂ of 100% represents pure oxygen only, so the air flow is 0 and the air:O₂ ratio is 0.
