Blood Oxygen (SpO2)
How accurate your wearable's blood oxygen reading actually is, especially the overnight number most devices report, and a real, documented accuracy gap by skin tone the research doesn't let us ignore.
Blood oxygen, or SpO2, shows up on nearly every modern watch and ring as a clean percentage, usually somewhere in the high 90s. It feels like a lab reading. It isn't one. The sensor shines red and infrared light through your skin and infers oxygen saturation from how much of each color bounces back, a method with real, well-documented limits, some of them specific to the device sitting on your wrist or finger, not just to blood oxygen itself. Here's what the research actually says about how well that estimate holds up, and where it holds up less evenly than the single number on your screen suggests.
| Device | What they call it | What it actually measures |
|---|---|---|
| Apple Watch | "Blood Oxygen app" | An on-demand, roughly 15-second spot-check reading, available on Series 6 and later plus Ultra models. Not continuous or automatic overnight in the US market as of this writing. |
| Garmin | "Pulse Ox" | Can run as an on-demand spot check or as continuous all-day/overnight tracking, depending on device settings, unlike Apple's on-demand-only model. |
| Oura Ring | "Blood Oxygen Sensing (SpO2)" | Overnight-only by design. Measures during sleep periods longer than three hours and reports Average Blood Oxygen plus a separate Breathing Regularity score. No daytime spot-check exists on Oura. |
| Fitbit | "Estimated Oxygen Variation" (EOV) | Deliberately not a raw SpO2 percentage. It's a relative overnight graph showing how much blood oxygen fluctuates, with a red/yellow/green pattern flagging possible breathing disturbances, a different approach from every other device listed here. |
| Whoop | "Blood Oxygen" | Added with the 4.0 hardware generation. Overnight-only, paired with a skin temperature sensor that feeds into Recovery; no daytime spot-check, the same overnight-only design as Oura's ring. |
| RingConn | "Blood Oxygen" | Continuous PPG tracking, reported as a percentage rather than a relative graph, closer to Oura's or Garmin's approach than Fitbit's. |
| Samsung Galaxy Watch | "Blood Oxygen" | Available both on-demand and as part of overnight Sleep Coaching tracking. |
| Ultrahuman Ring | "SpO2" | One of Ultrahuman's core individual markers, tracked alongside HRV, resting heart rate, VO2 max, and temperature. |
| Amazfit | "Blood Oxygen" | Available both on-demand and as continuous overnight monitoring via the BioTracker sensor. |
| Withings ScanWatch | "SpO2" (used for sleep apnea signal detection) | Tracks blood oxygen, heart rate, movement, and respiratory rate overnight. SpO2 itself is FDA-cleared for AFib detection, but sleep apnea detection specifically has not received separate FDA clearance. |
Two different, real findings, not one: general accuracy against a reference oximeter holds up reasonably well across the studies reviewed here. But a separate, well-documented body of research finds pulse oximetry accuracy, including in consumer wearables, is reduced by skin pigmentation, and the direction of that bias is more nuanced than a simple light-skin/dark-skin split, one study found devices missed a dangerously low reading in 21% of darkly pigmented subjects under low-perfusion conditions, versus about 1% in lightly pigmented subjects.
General accuracy: Uncertain · on the Research Desk Accuracy by skin tone: Uncertain · on the Research DeskSources: Apple's own support documentation (support.apple.com) for the Blood Oxygen app. Garmin's own Pulse Ox support pages (garmin.com) for on-demand vs continuous modes. Oura's own Blood Oxygen Sensing blog post and support article (ouraring.com) for overnight-only measurement and the two reported metrics. Fitbit's own help documentation for Estimated Oxygen Variation's relative-graph design. Whoop's own product documentation for the 4.0-generation blood oxygen sensor. Full study-by-study evidence lives on the Research Desk linked above.