Calculate an ideal air:oxygen ratio, mixture oxygen percentage, air flow or oxygen flow, assuming 21% oxygen air blended with pure oxygen.

Ideal blending of 21% oxygen room air with 100% oxygen, at the same flow reference conditions. This is mixture arithmetic, not a prescription or a prediction of oxygen actually inhaled through a device.

Enter percent, such as 40, not a fraction such as 0.40.

Related Calculators

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

Choose the value to solve for, enter the displayed inputs, then select Calculate. Ratio mode needs only the oxygen percentage; mixture percentage mode needs both flows. Air flow and oxygen flow modes each need the percentage and the other flow. Flow inputs convert between L/min and mL/min; flow results show both units. This ideal blending model does not determine a patient’s actual inspired oxygen through a delivery device or prescribe oxygen treatment.

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.