How accurate are fitness trackers?

Are you one of those people that barely leave the house without a fitness tracker around their wrist? So far, I am not infected by this trend even though I was thinking about buying one from time to time. Most of these wearables can measure activity as well as heart rate and energy consumption. But can you really rely on these data? In my recent blog post you will find the answer to this question.


Trust is good, control is better

For customers it is really hard to know how accurately those wearable fitness trackers measure physical data like heart rate or energy consumption. Though you can tell by common sense whether a measured value is totally absurd or not, you do not know it for real. Most producers also do not disclose information about how they actually have tested their devices.

Whether it is for losing weight, preparing for a marathon or being sufficiently active, millions of people are using fitness tracker to improve their health. Most of them trust the data shown on the device’s display. Am I in the perfect heart rate zone? Did I burn enough calories today to really lose weight? That is why it is so important that what is shown on the display is actually true.


Researchers tested Apple Watch and Co. thoroughly

In a recently published study researchers from the Standford University investigated different wrist-worn activity trackers. By doing so they wanted to find out how accurate they measure heart rate and calorie consumption. The scientists studied the following devices: Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and Samsung Gear S2.

Overall, 60 volunteers participated in this study. They wore each of the devices while walking and running on a treadmill as well as while using a bicycle ergometer. In addition, proven medical methods for measuring heart rate and calorie consumption were employed.

One of the researchers in charge stated in an interview: “The heart rate measurements performed far better than we expected, but the energy expenditure measures were way off the mark. The magnitude of just how bad they were surprised me.”


Better not to rely on fitness trackers when measuring calorie consumption

Six of the seven tested devices measured heart rate very accurately. The error was under 5% difference to the real value. However, the measurement of calorie consumption was quite disappointing. None of the tested devices measured it with accuracy higher than 80%. That means that the measured values diverged from the real value at least 20%, sometimes even 100%. How to understand this? If you are burning 200 calories during an activity in reality, the device would show a calorie consumption of 400 calories. With errors like that it is impossible to effectively lose weight. Overall, the Apple Watch was the most accurate device in this study. On the contrary, the fitness tracker Samsung Gear S2 produced the most inaccurate results.

For the researchers from Stanford University it is still unclear why fitness trackers are so bad in measuring calorie consumption. An important factor is however, that heart rate is measured directly by small sensors in the device. This means that there is only a small measurement error that can occur, but no big divergences.

On the other hand, energy consumption is determined indirectly by using a formula. Before using a fitness tracker, you enter your weight and height. In addition, the device gets data about what activity you are doing and maybe even about your heart rate. On the basis of all of this data the fitness tracker determines your energy consumption. The problem is that the programmed formulas are based on statistical mean values and can thus lead to huge variations in individual usage.


In a nutshell

Fitness trackers can be very helpful if you want to keep record of your daily activities in a more objective way. However, you should not put too much trust in the data of your calorie consumption, since current devices are quite inaccurate in this regard. On the contrary, heart rate data are very reliable.

Fitness trackers give a good orientation, but not more. You should better listen to your body and use additional methods like food diaries, body measures and your mirror. In case of doubt: Go the extra lap!




Shcherbina, A., Mattsson, C. M., Waggott, D., Salisbury, H., Christle, J. W., Hastie, T., Wheeler, M. T., Ashley, E. A. (2017) Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort. Journal of Personalized Medicine, 7(2), 3.

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One comment

  1. 1

    A fitness tracker is a device which is always good for the tracking the physical activity of any person. I always like to purchase a tracker while doing my all day to day activity including zyming. i will surely purchase a new fitness tracker soon as i have losted my last and missing that too. Thank you for the awesome information given by you…

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