Enter any two values (target FiO₂, air flow, or O₂ flow) to calculate the missing variable. This calculator can also show the air:oxygen ratio needed to achieve a specific oxygen percentage when blending room air (≈21% O₂) with 100% oxygen.
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Air to Oxygen Ratio Formula
The following formula is used to calculate the air to oxygen ratio (Air Flow ÷ O₂ Flow) when blending room air (≈21% O₂) with 100% oxygen to achieve a target FiO₂:
AOR = (100 - FiO_2) / (FiO_2 - 21)
Variables:
- AOR is the air to oxygen ratio (Air Flow ÷ O₂ Flow), a unitless ratio
- FiO₂ (%) is the target oxygen percentage in the blended gas mixture
- 21 is the approximate oxygen percentage in room air (more precisely ≈20.9% in dry air)
To calculate the air to oxygen ratio, subtract the target FiO₂ from 100 and then divide by (FiO₂ − 21). This relationship is valid for FiO₂ values greater than 21% up to 100%.
What is Air to Oxygen Ratio?
The air to oxygen ratio (AOR) describes how much room air flow is required per unit of pure oxygen flow to achieve a desired oxygen concentration (FiO₂) in a blended gas mixture. It is commonly used for oxygen blending in medical respiratory care (e.g., blenders and ventilator gas supplies) and other processes that require controlled oxygen enrichment.
How to Calculate Air to Oxygen Ratio?
The following steps outline how to calculate the Air to Oxygen Ratio:
- First, determine the target oxygen percentage (FiO₂ %) in the mixture.
- Next, use the formula AOR = (100 − FiO₂) / (FiO₂ − 21).
- Finally, calculate the Air to Oxygen Ratio (AOR).
- After inserting the variable and calculating the result, check your answer with the calculator above.
Example Problem:
Use the following variable as an example problem to test your knowledge.
Oxygen percentage (FiO₂ %) = 60%