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What numbers can you trust on your wearable device?

According to a recent article in the Journal of Personalized Medicine, your wearable device may not be as accurate as you think.


Researchers looked at the Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio ALPHA 2, PulseOn, and Samsung Gear S2. Their primary mandate was to evaluate each device in terms of how accurately it measured heart-rate and number of calories burned.


If you wear your device to monitor your heart rate, you’re in luck. The researchers found an acceptable range of error for each of the devices tested. On average, the error was below 5%. In other words, if your heart rate registers 100 beats per minute, in reality it could be anywhere between 95 and 105 beats per minute. The Apple Watch had the lowest error at 2.5%, while the Samsung Gear 2 had the highest at 6.8%. Wearing a strap around your chest is always the most accurate way to measure your heart rate, but a 5% error may not make or break your decision.


On the other hand, if you wear your device to help count calories expended throughout the day, you’re out of luck. The error in estimating energy expenditure averaged about 27%. For walking and running, the error reported for calories burned was between 28% and 35%. For sitting, surprisingly, the range of reported error was between 48% and 57%. In fact, one watch was reported to have a 92.6% error in estimating calories burned!


So… If you are out running and your wristband tells you (based on your age, height, and weight) that you burned 500 calories during your session, in reality you could have burned as few as 365 calories or as many as 635 calories. The error really starts to add up when you look at the total number of calories burned at the end of the day.


How the outcomes of this research affect you (or don’t) depends on why you use a wearable device to begin with. Always start with the why.

·      If you wear your device to increase your overall fitness, strap it on. Your fitness will improve if you use the heart-rate function to monitor and adjust the intensity of your exercise sessions.

·      On the other hand, if you wear your device to help create a caloric deficit, you may be better off looking at your nutrition (as long as you don’t create error here) and ignoring the calorie read-outs on your watch.


I meet a lot of people who use these devices to monitor energy in and energy out. Here’s my rule of thumb: If, over time, your wearable device shows a calorie deficit and you’re not losing weight, then most likely you are not in a calorie deficit.


Now that you know that all of the devices tested were inaccurate at reporting calories burned, don’t let this known error mask the truth. It’s not you, it’s your device. The inaccuracy of your device may make you feel like you’re the issue–like something is wrong with your body–when in fact, something is wrong with the device.


If you’re struggling to see change, if your body has hit a plateau, or if you’re trying to sort through the confusion, FitMetabolism can help. Remember, weight is biopsychosocial, not just bio. You have to look at all the moving parts, not just calories burned, especially as reported by a device that is most likely not accurate!



Shcherbina A. et al. “Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort.” Journal of Personalized Medicine, vol. 7, no. 2, 2017.


Jason Hagen