A smartwatch measures blood oxygen by shining red and infrared light into your wrist and analyzing how much light bounces back. The ratio of absorbed light reveals how much oxygen your hemoglobin is carrying, displayed as an SpO2 percentage. It uses the same core principle as the fingertip clip at a doctor’s office, but adapted for the wrist, which introduces some important differences in accuracy.
Reflectance vs. Transmissive Pulse Oximetry
A standard medical pulse oximeter clips onto your fingertip and shines light through one side while a detector on the other side captures what passes through. This is called transmissive pulse oximetry, and it works well because fingers and earlobes are thin enough for light to travel all the way through.
Your wrist is too thick for that approach. So smartwatches use reflectance pulse oximetry instead: the LEDs and the photodetector sit on the same side of the sensor, pressed against the underside of your wrist. Rather than measuring light that passes through tissue, the watch measures light that bounces back from blood vessels beneath the skin. This reflected signal is weaker and noisier, which is why wrist readings are less reliable than fingertip ones.
How Red and Infrared Light Reveal Oxygen Levels
The sensor on the back of your watch contains at least two types of LEDs. One emits red light at around 660 nanometers, and the other emits infrared light at around 940 nanometers. These two wavelengths interact with blood in opposite ways depending on how much oxygen it carries.
Hemoglobin that’s loaded with oxygen absorbs more infrared light and lets more red light reflect back. Hemoglobin that’s low on oxygen does the reverse: it absorbs more red light and reflects more infrared. By comparing how much of each wavelength bounces back to the photodetector, the watch can distinguish oxygen-rich blood from oxygen-poor blood.
The watch’s processor separates each light signal into two components: a pulsing part that changes with every heartbeat (as fresh arterial blood flows in) and a steady background part from tissue, bone, and venous blood. It then calculates a “ratio of ratios,” comparing the pulsing-to-steady ratio of red light against the same ratio for infrared light. This single number maps directly to an SpO2 value using a calibration curve built into the software. A healthy reading typically falls between 95% and 100%.
What Affects Accuracy
Consumer smartwatches are not as precise as clinical devices, and several factors widen the gap.
- Motion and wrist position. Even small movements disrupt the light signal. Apple’s guidance is to rest your arm on a table or in your lap, keep your wrist flat with the watch facing up, and hold still for about 15 seconds. Hanging your arms at your sides or clenching your fist can cause a failed reading.
- Fit and placement. The sensor needs consistent contact with your skin. A loose band lets ambient light leak in and weakens the reflected signal. Tattoos, thick hair, or very dry skin can also interfere.
- Skin pigmentation. Higher melanin levels absorb more light before it reaches blood vessels, which can reduce signal strength. The FDA has acknowledged accuracy differences between lighter and darker skin tones and has proposed updated testing requirements to address the gap.
- Ambient light. Bright sunlight or overhead lighting can flood the photodetector with extra photons, adding noise to the measurement.
- Altitude. Accuracy tends to degrade when oxygen levels are genuinely low. One study found that the difference between a Garmin watch and a clinical blood gas test grew from about 4.7% at sea level to 13.1% at 5,500 meters of altitude.
How Accurate Are Current Smartwatches
The international standard for medical-grade pulse oximeters requires accuracy within 3% of an arterial blood gas test, which is the gold standard. Some FDA-cleared wrist oximeters meet this threshold across the 70% to 100% SpO2 range under controlled lab conditions. But real-world performance on consumer smartwatches is more variable.
A Lancet analysis found the Apple Watch had an average difference of 0.8% compared to a standard fingertip oximeter in patients with chronic lung disease, with readings occasionally swinging as far as 4.1% in either direction. A multicenter hospital study testing the Apple Watch Series 7 and the Withings ScanWatch against ward-based oximeters in COVID-19 patients found overall accuracy of 84.9% and 78.5%, respectively. More telling was how well each watch caught genuinely low oxygen levels: the Apple Watch detected clinically significant hypoxia only about 35% of the time, while the Withings ScanWatch caught it about 69% of the time.
Those numbers highlight an important pattern. Smartwatches perform reasonably well when your oxygen is normal, confirming readings in the 95% to 100% range. They become less reliable precisely when accuracy matters most: when oxygen drops into a concerning range. Some researchers have shown that machine learning algorithms can narrow the error to around 2.9% compared to fingertip oximeters, but these improvements haven’t yet been validated against arterial blood gas testing.
What the Reading Actually Tells You
Your smartwatch displays a number labeled SpO2, which stands for peripheral oxygen saturation. It represents the percentage of hemoglobin molecules in your arterial blood that are carrying oxygen. A reading of 98% means 98 out of every 100 hemoglobin molecules passing through your wrist capillaries are bound to oxygen.
Most healthy people at sea level will see values between 95% and 100%. Readings consistently below 95% can signal that your lungs or circulation aren’t delivering enough oxygen, which is relevant in conditions like COPD, sleep apnea, pneumonia, or heart failure. A single low reading on a smartwatch isn’t cause for alarm, since motion, fit, or sensor noise could be to blame. But if you see repeated readings below 90%, or if low readings coincide with symptoms like shortness of breath or confusion, that’s worth following up with a fingertip oximeter or a healthcare provider who can run a proper arterial blood gas test.
Many watches also take background readings overnight, which can help spot patterns like repeated oxygen dips during sleep. These trends are more useful than any single snapshot, because they smooth out the noise of individual measurements. Think of your smartwatch SpO2 feature as a screening tool that can flag trends worth investigating, not a diagnostic instrument that replaces clinical equipment.