How does the Apple Watch Series 6 measure blood oxygen levels

The Apple Watch Series 6 has a high-tech new feature that, it says, can accurately measure a wearer’s blood oxygen levels using red and green LEDs and infrared light.

How does the Apple Watch Series 6 measure blood oxygen levels

The technique used by the Apple Watch is similar to non-evasive hospital-grade tests using Pulse Oximetry (the clip-like device that is commonly placed onto a wearer’s finger).

A Pulse Oximetry device sends beams of light through the wearer’s finger and calculates the amount of oxygen by measuring the changes of light absorption in oxygenated or deoxygenated blood.

And the Apple Watch does largely the same. The difference is that the Apple Watch uses green and green LEDs, infrared and the four photodiodes on the Apple Watch’s back crystal and measures the light reflected back from your blood (not passed through).

See also: What is Apple Fitness+? Is it available in NZ?

Here’s Apple’s explanation of how its blood oxygen measurements work (verbatim):

“In Apple Watch Series 6, the optical heart sensor has been redesigned to add blood oxygen measurement capabilities. During a blood oxygen measurement, the back crystal shines red and green LEDs and infrared light onto your wrist. Photodiodes then measure the amount of light reflected back.

Advanced algorithms use this data to calculate the color of your blood. The color determines your blood oxygen level — bright red blood has more oxygen, while dark red blood has less.”

How to get the best results

  1. Rest your arms on a table or in your lap while you take a measurement with the Apple Watch facing up. Keep your wrist flat, and hold as still as you can.
  2. Make sure that your Apple Watch isn’t loose on your wrist. The band should be snug but comfortable, and the back of your Apple Watch needs to be touching your wrist. 
  3. Make sure that the back of your Apple Watch is flush with the top of your wrist. If your wrist bones interfere with this, move your watch up your arm away from your wrist bone.
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Read next: Which iPad should I buy in NZ? (Updated September 2020)

David Court
David is a professional journalist. He's been reviewing lifestyle and technology products since 2007. His CV boasts a series of high-profile websites that he's previously edited and managed. These sites include,,, and is his new project.


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