If you’re shopping for a new smartwatch, you’ve probably come across the terms SpO2 sensors, pulse ox, or blood oxygen levels. SpO2 sensors measure your oxygen saturation in the blood – or, more simply, the amount of oxygen you have in your blood. In fact, these sensors and stats are included in most modern smartwatches and fitness trackers. The only problem is that not every wearable maker uses these sensors in the same way.
Some smartwatches passively measure your SpO2 while you sleep, while others let you take a direct measurement. But don’t worry, we’ll go over what SpO2 sensors are, how they work and what their limitations are. We’ll also go over how to set up SpO2 readings on some of the more popular wearables that support this metric.
How SpO2 Sensors Work
You may be familiar with pulse oximeters, which are: clips you wear on your finger that measure your blood oxygen saturation. These devices became more widely known during the early days of the pandemic, as there were low oxygen levels a common symptom for many people with COVID-19. This led to pulse oximeters becoming an indispensable item.
SpO2 sensors in smartwatches work similarly to the photoplethysmography (PPG) sensors used to measure heart rate. PPG or optical heart rate sensors work by shining a green light into your skin to determine your heart rate based on the light that is reflected back. The difference is that instead of green light, smartwatch SpO2 sensors shine red and infrared light into your skin. Then, based on how light is reflected, the devices use an algorithm to estimate your blood oxygen levels.
A “good” score is generally defined as anything above 95 percent, but results above 90 percent are also considered normal. Every wearable device maker also defines what a “good” or “normal” measurement is, so be sure to read all of the included explanations thoroughly.
Limits of Portable SpO2 Sensors
There are many reasons why wearable device manufacturers have started adding these sensors to their devices. Most have added it as a factor in determining your sleep quality. Others, such as Fitbit and Withings, have added SpO2 sensors as a possible way to identify conditions like sleep apnea.
But while fingertip pulse oximeters are a Class II Medical Devicesmartwatch SpO2 functions are intended only for the common good. As such, they generally do not require FDA approval. (However, the Withings ScanWatch has received FDA approval for its SpO2 feature to detect breathing disorders during sleep.) Blood oxygen readings from the wrist are also usually less accurate than the fingertip. That’s because while fingertip pulse oximeters shine light through your entire finger, pulse sensors use the less reliable method of measuring the reflection of light.
In short, you should not use portable SpO2 measurements for any medical purpose. You should also not place too much importance on these measurements. The best they can do is give you an idea of what your baseline is. Even then, you should expect shaky readings from time to time.
Now that we are aware of the limitations, here’s how to use SpO2 settings and sampling on various portable platforms.
Apple introduced SpO2 sensors in its Series 6 Watch. That means if you want to take SpO2 measurements, you need a Series 6 or Series 7. The Apple Watch SE does not contain SpO2 sensors.
On the Apple Watch you can check your oxygen level in your blood within 15 seconds. If you wear the device to sleep, it will also record background readings while you snooze.
To set up Blood Oxygen on your Apple Watch:
- Open the Health app on your iPhone.
- Press To leaf through tab.
- Scroll down to Breathing†
- Tap Blood Oxygen†
- Tap Switch†
To perform position measurements:
- Open the Blood Oxygen app on the Apple Watch.
- Tap Get started†
- Wait 15 seconds.
- Tap Finished†
Please note that sample measurements can be finicky. Try to remain very still while measuring, with your arm resting on a table or on your lap. Also make sure your belt is snug and that the sensor array is making good contact with your skin.
To enable or disable background measurements:
- Open the Settings app on the Apple Watch.
- Scroll down to Blood Oxygen†
- Turn on or off Blood Oxygen Measurements for periodic daily measurements.
- Turn on or off In sleep focus for nighttime readings.
- If you want to suppress readings during the movies or other events, you can use the In theater mode institution.
Fitbit was one of the first to integrate SpO2 sensors with the Fitbit Ionic in 2017. However, it wasn’t until 2020 that it started actively recording the stat. Every current Fitbit device, except the Inspire 2, can track your blood oxygen levels.
Unlike the Apple Watch, you can’t run an SpO2 sampling on Fitbit devices. The statistic is much more passive. You don’t need to do anything to enable the metric, but here’s how to view your SpO2 baseline.
To view your nighttime SpO2 metric:
- Press Today tab in the Fitbit app.
- Press sleep tile.
- Tap a day’s sleep log to view the Estimated Oxygen Variation graphic.
- For Fitbit Premium users, tap the Restoration tile in a sleep log to see the EOV graph.
To view your SpO2 trends on the Fitbit Charge 4, Charge 5, Luxe, Sense, Versa 2, and Versa 3:
- You must have a Fitbit Premium subscription.
- Make sure your device is up to date.
- Wear your device for 24 hours.
- The next morning, sync your device.
- Press Today tab.
- Press Health Statistics tile.
- Scroll to Oxygen saturation (SpO2).
You can also install the SpO2 watch face to view the information on your wrist.
- Press Today tab.
- Tap your profile picture.
- Tap the picture of the device where you want to install the watch face.
- Tap Gallery†
- Navigate to the bells tab.
- Look for the SpO2 dial. You can tap Show all to see the full list.
- Select the SpO2 dial.
- Tap to install†
Keep in mind that after your daily sync, it may take an hour or two for the watch face to display your SpO2 data on the wrist.
If you have the Sense of Versa 3, you can also enable SpO2 measurements in the background through the SpO2 Tracker app. To install:
- Follow the first four steps in the last section.
- Look for the SpO2 tracker app.
- Tap to install†
Garmin devices call SpO2 Pulse Ox. There are several settings for how often your tracker or smartwatch measures blood oxygen levels. You can also view your blood oxygen levels through the Pulse Ox widget on supported devices.
To read out a sample:
- Navigate to your device’s menu.
- Press Pulse Ox widget.
- Sit still while it calculates your blood oxygen level.
To turn blood oxygen tracking on or off:
- Open the Garmin Connect app.
- On Android, select the three dashes icon in the top left corner.
- On iOS, tap More bottom right.
- Tap Garmin devices†
- Select Track activity†
- Tap Pulse Ox†
- Turn the on or off Sleep tracking or Follow all day settings.
To view Pulse Ox sleep stats:
- Follow the first three steps in the last section.
- Tap Health Statistics†
- Select sleep†
- You will see three tabs: Stages, Pulse Ox, and Breath.
- Tap Pulse Ox†
Samsung Galaxy Watch 4 and Watch 4 Classic
Like the Apple Watch, the Samsung Galaxy Watch 4 and Watch 4 Classic allow you to take samples. For passive sleep measurements, you’ll need to manually enable that setting, as it’s off by default.
To add the Blood Oxygen tile:
- Swipe left from the main screen to access your tiles.
- If you don’t already have the Blood Oxygen tile, swipe all the way to the left.
- Tap Add tile†
- Scroll until you find Blood Oxygen.
- Press Blood Oxygen tile to add it to your watch.
To take samples:
- Swipe left on your watch until you reach the Blood Oxygen tile.
- Tap Measuring unit†
- Scroll through the on-screen instructions.
- Tap OK†
- Stay still while you read.
To enable blood oxygen readings during sleep:
- Open the Samsung Health app on the watch.
- Scroll down to Settings†
- Tap Sizes†
- Tap Blood oxygen during sleep†
- Switch on Measure constantly†
As you can see, SpO2 features on smartwatches are still in their infancy. We may see more advanced uses of these sensors in the future and will update this guide accordingly. In the meantime, it’s worth remembering that SpO2 functions are currently meant to provide additional context regarding your baselines and how your body recovers from exercise — and not much more.