There was a time, before Fitbits, when nobody knew quite how many calories they were burning on a daily basis. Sure, you could calculate a rough estimate based on your body size, sex and age, but the idea that a gadget on your wrist could tell you was revolutionary. It was also wrong. Even as fitness wearables have gotten more advanced, they’re still wrong. Most trackers figure out when your body is moving and by how much. This is the basic idea behind how trackers detect how many steps you’re taking. Then there’s the heart rate sensor that uses your heart rate to make a calculated guess. Whatever the source of the data — heart rate, movement, or a combination — the gadget uses a formula to calculate how many calories it thinks you’re burning. Unfortunately, the fitness tracker doesn’t actually know how many calories you’re burning. It’s calculating a probable number based on incomplete information. You may get different step counts if you put a gadget on your right wrist versus the left wrist, and the optical heart rate sensors may be less accurate on darker skin. Now for the really bad news. A recent study that looked at a variety of devices found that they are all wrong more times than they’re right.
• Garmins underestimated calorie burn 69% of the time.
• Apple watches overestimated calorie burn 58% of the time.
• Polar devices overestimated calorie burn 69% of the time.
• Fitbits underestimated calorie burn 48% of the time and overestimated 39% of the time.
