Study Finds That Your Fitness Trackers Are Terrible at Counting Calories

Fitness trackers are so inaccurate that they may be sabotaging your workout goals.

Fitness trackers are incredible devices. Simply strap a watch-like device on your wrist and you have access to a wealth of data on your activity, workouts, heart rate and even the amount of calories you burn.

But can such a small device actually be accurate? A recent study out of Stanford University found it's a bit of a mixed bag.

The team at Stanford assessed several popular fitness trackers currently on the market: The Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and the Samsung Gear S2.

Sixty subjects wore the devices through a standardized workout that involved walking, running and cycling to test accuracy at different intensities and in activities.

The researchers found the trackers did an excellent job of measuring heart rate. The Apple Watch scored the best with an error rate of 2.0 percent, while the Samsung Gear S2 scored the worst with an error rate of 6.8 percent.

Considering these devices are worn on your wrist, this level of accuracy is impressive. A few percentage points is worth the tradeoff of having the convenience of wearing a device on your wrist over being hooked up to an ECG machine with electrodes attached to your chest. That said, accuracy during strength training workouts drops quickly once you hold onto a weight or your wrist bends like during a Push-Up.

But when it comes to measuring the amount of calories you burn, things aren't so hot.

The FitBit Surge scored the best with a 27.4 percent error rate, whereas the PulseOn had a staggering error rate of 92.6 percent. Oddly enough, the devices were more accurate when walking and running than sitting.

Overall, the Apple Watch scored the highest while the Samsung Gear S2 scored the lowest.

This is a troubling but not terribly surprising discovery. Fitness trackers are only able to estimate the amount of calories you burn based on your height and weight, heart rate and activity level. That simply isn't a ton of data to work with. The gold-standard metabolic cart measures oxygen intake to provide an accurate assessment of how many calories you burn during exercise.

Problem is relying on an inaccurate calorie number can totally mess up your goals. If it overestimates how many calories you burn, you might not be doing enough activity to actually lose weight. If it underestimates the amount of calories you burn, you might not be eating enough food to offset your workout and maintain your weight or add muscle.

If you consistently do the same type of workout, you might be able use the calorie readout to compare individual training sessions. Outside of that, it's virtually worthless and actually hurts your progress, not help it.