Are SpO2 and FiO2 the Same?


Oxygen is a free element that occurs naturally; life cannot exist without it. However, supplemental oxygen is used when your body does not get enough oxygen when you breathe. Low oxygen levels can lead to serious health risks such as heart and brain damage. You may have heard of oxygen saturation (SpO2) and a fraction of inspired oxygen (FiO2), but what exactly are they? Are they the same?

No. SpO2 and FiO2 are not the same, but they are related. SpO2 determines the amount of oxygen in your blood, while FiO2 is the percentage of oxygen in the air you breathe.

While most people may know about SpO2, FiO2 may be a relatively new term to many. This article looks closely at SpO2 and FiO2 and how they are related.

What Is FiO2?

FiO2 is the fraction, or concentration, of oxygen in the gas mixture that a person inhales when they breathe in. Typically, natural room air has a FiO2 of 21 percent, regardless of the person’s altitude.

The air we breathe is composed of three components:

  • 21% oxygen
  • 68% nitrogen
  • 1% other gasses such as neon, hydrogen, and carbon dioxide.

The 21% FiO2 is enough to keep your cells healthy when you're in good health. However, one may need more than 21% (0.21) of FiO2 to maintain enough oxygen saturation when sick. In such cases, supplemental oxygen, also known as oxygen therapy, is given.  

One may require varying amounts of FiO2 supplementation depending on how sick they are. Therefore, an ill patient needs higher levels. Generally, FiO2 is kept below 0.5 to avoid oxygen toxicity, which the high partial pressure of inspired oxygen can cause. FiO2 can be increased to reach 100% (1.00).

The partial pressure of oxygen (PaO2) changes when the barometric temperature changes. It remains at 21%. A change in altitude does not affect the fraction of inspired oxygen.

PaO2 refers to how much oxygen is dissolved in your blood and how well oxygen is delivered to the cells. A normal PaO2 is between 75 and 100 mmHg, or 10.5 and 13.5 kilopascals (kPa). The lower the partial pressure, the less oxygen is delivered to cells that need it, leading to hypoxia.

Why Is FiO2 Important?

FiO2 ensures our bodies are getting the oxygen they need. The fraction of inspired oxygen is significant because it can help doctors and nurses determine how well a patient is doing and if they need more oxygen. Furthermore, monitoring FiO2 can help prevent low and high oxygen levels that can lead to adverse effects. Knowing the actual levels is crucial for adequately treating several conditions, such as hypoxemia.

Measuring FiO2

We have to be careful when measuring FiO2 by looking at the concentration of oxygen in the gas mixture. The standard amount of FiO2 in a gas mixture at room temperature should be at an average of 21%. However, this differs depending on levels of gas exchanges and a number of other factors, such as:

Oxygen Flow Rate and the FiO2

When delivering supplemental oxygen, there are two crucial elements to consider: oxygen flow rate and FiO2. As we stated above, FiO2 is the percentage of oxygen in the air you breathe.

On the other hand, the oxygen flow rate measures how much oxygen is flowing to your body at a given time. It is typically measured in liters per minute (L/min).

Oxygen delivery devices predict the flow rate and the FiO2 depending on their algorithm. Typically, the FiO2 increases by 4% for every liter of oxygen delivered. For instance, a nasal cannula with a 1 L/min flow rate provides 28% FiO2.

Generally, FiO2 is never precise, but some guidelines can help achieve an accurate estimation. The actual O2 concentration will depend on breathing rate and tidal volume. However, you can use this simple formula to convert oxygen flow in liters per minute to FiO2.

FiO2 = 20% + (4 x oxygen liter flow)

When you use this formula, you will end up with values similar to the ones in the table below:

Oxygen Flow (LPM)

Approximate FiO2





















Oxygen devices that deliver oxygen include:

  • High-flow nasal cannula.
  • Venturi mask.
  • Non-rebreather mask
  • Partial rebreather Mask
  • Face tent
  • Simple mask

Each device can deliver varying FiO2. Below is a table that can guide you in understanding the amount of oxygen a device can deliver in L/min and the oxygen concentration it can provide in percentage. 


Flow Rates and Oxygen Percentage

Nasal Cannula

Flow rate: 1-6 L/min

FiO2: 24% to 44%

Venturi mask

10 to 12 L/min

FiO2: 24% to 60% 

High-Flow Nasal Cannula

Flow rate: up to 60 L/min

FiO2: Up to 100%

Face tent (low-flow system)

Flow rate: minimum of 15 L/min

FiO2: 28% to 100% 

Simple Mask

Flow rate:  6-10 L/min

FiO2:  28% to 50%

Non-Rebreather Mask

Flow rate: 10 to 15 L/min

FiO2: 60-80%

Partial re-breather mask 

Flow rate: 10 to 12 L/min

FiO2: 80% to 90%

What Happens if FiO2 Is High?

If FiO2 remains high for too long, typically because of ventilation with high oxygen concentration levels, the lungs can be harmed by oxygen toxicity. When FiO2 is too high, you may experience headaches, dizziness, nausea, coughing, and difficulty breathing. In some critical cases, it can lead to death.

What Happens if FiO2 Is Low?

If your FiO2 is low, it means that the oxygen level in your blood is lower than it should be. When FiO2 is lower than it should be, it can lead to several health problems, including difficulty breathing, fatigue, dizziness, and even heart problems. Low FiO2 can also lead to hypoxia.

What Causes Decreased FiO2?

Several facets can lead to low levels of FiO2, including hypoventilation, pulmonary shunting, and impaired alveolar diffusion. In addition, respiratory drive, respiration rate, airway resistance, and lung compliance can also determine FiO2 levels.

What Is the P/F Ratio?

The P/F ratio, also known as the PaO2/FiO2 ratio, is calculated by dividing the arterial partial pressure of oxygen (PaO2) by the inspired oxygen concentration (FiO2). The PaO2/FiO2 ratio helps determine the severity of any issue related to how the lungs transfer oxygen to the blood. This includes acute respiratory distress syndrome (ARDS) and acute lung injury.

ICUs frequently employ the PaO2/FiO2 ratio to gather information on the oxygenation status of critically ill patients. Patients in the intensive care unit (ICU) have a higher risk of mortality and more extended hospital stays when their PaO2/FiO2 values are low. Pathological conditions, such as ARDS, pneumonia, and acute pulmonary edema, can lead to a low P/F ratio. 

What is SpO2?

SpO2, also known as oxygen saturation, is a measurement of oxygen in the blood at any given time. Measuring SpO2 through pulse oximetry has become a standard procedure in hospitals as the levels can tell your doctor a lot about your health. Low SpO2 can signify a serious health condition like heart or lung disease. 

Some people may confuse SpO2 with SaO2. The main difference is that SpO2 is a measurement of hemoglobin present in peripheral blood saturated with oxygen. On the other hand, SaO2 is the percentage of hemoglobin in your arterial blood that is saturated with oxygen. 

Measuring SpO2

Measuring SpO2 is often done through pulse oximetry, with a small device you clip to your finger. The pulse oximeter shines a light into your finger and measures how much light is absorbed, which corresponds to the amount of oxygen in your blood. 

The regular SpO2 reading through pulse oximetry is 95–100%. This means 95–100% of your blood is saturated with oxygen. Anything less than 95% in the pulse oximetry device is considered low and may indicate that you have hypoxia. 

Pulse oximetry utilization skyrocketed during the COVID-19 pandemic to help monitor oxygen levels. 

What Is the SpO2/FiO2 Ratio?

The oxygen saturation to fraction of inspired oxygen (SpO2/FiO2) or S/F ratio is a non-invasive measure that can detect ARDS in high-risk patients. The SpO2/FiO2 ratio, also known as the S/F ratio, can be used as a substitute for invasive PF during oxygen supplementation. Studies show a high correlation between the SpO2/FiO2 and PaO2/FiO2 ratios.


So, are oxygen saturation and fraction of inspired oxygen the same? No, but they are closely related. Despite not being the same, they are both crucial as they can help estimate the degree of hypoxemia and other acute respiratory problems.