Is my number normal?

What does SpO2 mean?

By GyroscopeUpdated November 14, 20267 sources

SpO2 is your blood oxygen as a sensor measures it. It’s the share of the oxygen seats in your red blood cells that are filled, worked out from light shone into your skin. Awake and resting, 95 to 100 is normal.

What do the letters stand for?

The S is saturation, which means how full. O2 is oxygen. The small p tells you the number came from a pulse oximeter, the clip on your finger or the sensor on the back of a watch or ring. Doctors write SaO2, with an a, for the same thing measured in a blood sample from an artery. When researchers want to know how far a sensor was off, they put the two side by side, which is how a 2020 hospital study caught pulse oximeters showing too high in some patients.

What is it measuring?

Your red blood cells carry oxygen from your lungs to the rest of you, and each one only has so many seats for it. SpO2 is the share of those seats that are taken. At 97, about 97 of every 100 are full.

Think of a bus line where every bus leaves the depot nearly full. A bus at 96% and one at 99% carry almost the same number of people, so healthy blood at 96 and 99 delivers about the same oxygen. The number tells you how full the buses are. It tells you nothing about how many buses are running.

How does a sensor see it through your skin?

Blood carrying oxygen soaks up red and infrared light differently from blood without it. The sensor shines both into you, measures what comes back out, and only counts the part of the light that changes with each heartbeat.

That trick was found by accident. In 1972 Takuo Aoyagi, an engineer at Nihon Kohden in Tokyo, was using an ear oximeter to measure how much blood the heart pumps, and the pulse kept getting in the way. He tried to cancel it out by balancing the red and infrared signals, and found that changes in oxygen broke his cancelling. The noise was the measurement. Minolta sold an instrument built on his idea around 1978, and by the mid 1980s other companies were selling pulse oximeters around the world.

A finger clip shines its light through your fingertip to a detector on the other side. A watch, or a ring like Oura, shines it into your skin and catches what bounces back, which is harder. In 9 young adults breathing controlled low oxygen, an Apple Watch Series 7 missed the blood test by more than a medical finger oximeter did (more in how devices measure).

What’s a normal SpO2?

The tool below takes any number, the moment it came from and the device that showed it, and tells you where it sits. Every whole number from 85 to 100 also has its own page.