Watches and oximeters

How accurate is Apple Watch blood oxygen?

By GyroscopeUpdated November 14, 20265 sources

Good enough to follow your own nights, and least trustworthy at the low numbers you’d worry about. Below a true 88, an Apple Watch Series 7 was off by about 6 points, more often too high, in 9 young adults breathing controlled low oxygen. Apple says its Blood Oxygen measurements aren’t intended for medical use.

How far off was it in tests?

One careful test took 9 healthy adults, median age 25, into a lab at Duke University, lowered the oxygen they breathed, and checked the devices against blood drawn from an artery. An Apple Watch Series 7 and a medical-grade finger oximeter (a Masimo MightySat Rx) measured at the same time.

The typical miss here is a kind of average error (the root mean square), so single values can miss by more. Both devices leaned high. The FDA’s limit for a cleared sensor of the watch kind is 3.5 points and for a finger clip 3, as the study’s authors state them, and under low oxygen both devices went over their limit. The authors also cite a test of 50 healthy adults where an Apple Watch Series 6 missed by about 2.2 points, with the same lean toward too high.

It’s like a car’s fuel gauge. It’s good at telling you the tank is going down, and least exact when you’re nearly empty, which is when you want it most.

Why is a watch harder than a finger clip?

A finger clip shines light through your fingertip to a detector on the other side. A watch shines light into your wrist and catches what bounces back, which is harder. Apple lists what gets in the way: how much blood flows through your skin, some tattoos, moving around, and a loose band. Cold hands and bright light throw off oximeters too.

How does it measure while you sleep?

Only with Track Sleep on, and only now and then. The watch takes background measurements when you’re still, and in the big Apple Heart and Movement Study those were tried about every 30 minutes. That study gathered about 72 million values from Series 6 watches, and the average came out at 96.2 by day and 95.4 at night, right where you’d expect healthy blood to be. Across a crowd and across many nights, the watch tells a sensible story. On one night at one moment, it can be several points off.

The year the watch’s oxygen moved to the iPhone

On August 14, 2025, Apple brought Blood Oxygen back for some Series 9, Series 10 and Ultra 2 owners in the US who didn’t have it, after a US Customs ruling. On those watches, the sensor data still comes from the watch, but it’s measured and calculated on the paired iPhone, and the result shows up in the Health app under Respiratory. Apple’s support page says this applies to US models with part numbers ending in LW/A or LS/A bought on or after January 18, 2024.

How to get better numbers from it

• Wear it snug but comfortable, with the back of the watch touching your wrist.
• For a spot check, rest your arm on a table or in your lap, keep your wrist flat with the watch facing up, and hold still for the full 15 seconds.
• Compare tonight with your own past nights, never with someone else’s watch.

Apple’s Blood Oxygen app isn’t available to anyone under 18. The heart rate variability from the same watch has its own quirks (Apple Watch on hrv.guide).