Smartwatches Significantly Overestimate Calorie Burn, Study Finds

You strap on your smartwatch before a morning run. By the time you finish, the device tells you exactly how many calories you burned—a number that feels precise, scientific, earned. The problem is that number is probably wrong, sometimes wildly so.

Smartwatches rely on standardized mathematical formulas to estimate calorie expenditure, the same equations applied to every wearer regardless of who they are. These algorithms factor in heart rate, movement, and sometimes duration of activity, but they cannot see inside your body. They do not know your metabolic rate—how efficiently your particular physiology converts fuel to energy. They cannot measure your muscle mass, your fitness level, your age, or the dozen other variables that determine how many calories you actually burn during exercise. The watch treats you as an average person, and almost no one is average.

The result is significant error. Daily calorie estimates from the same device can swing by hundreds of calories depending on the activity, the wearer's body, and the particular algorithm the manufacturer chose to embed in the software. Someone using a smartwatch to guide weight loss or fitness decisions is working from a number that may be substantially higher or lower than reality. A person who believes they burned 600 calories during a workout when they actually burned 350 is making fundamentally different choices about diet and exercise than they would with accurate information. The inverse is equally problematic—underestimating burn can lead someone to consume more than they need, undermining their stated goals.

The gap between what the watch displays and what actually happened in your body reflects a hard limit of wearable technology. Heart rate sensors can measure pulse, but pulse alone does not tell you energy expenditure. A person with high cardiovascular fitness might have a lower heart rate during the same workout as someone less fit, yet burn similar or even greater calories. Age, sex, genetics, training history, even the time of day—all shift the relationship between heart rate and calorie burn. Generic formulas cannot capture this variation. They are built for the population average, which means they are built for no one in particular.

Manufacturers know this. The devices come with disclaimers that calorie estimates are approximations. But the numbers appear on the screen with the same confident precision as the time or your step count, and most users treat them as reliable. The watch becomes a source of behavioral feedback—you see the number, you adjust your eating or exercise accordingly—even though that number may be steering you in the wrong direction.

For someone casually tracking activity, the imprecision may not matter much. For someone managing weight, training for performance, or making health decisions based on energy balance, the error becomes consequential. The smartwatch industry has not solved this problem because it cannot be solved at the wrist. Accurate calorie measurement requires either direct physiological testing in a lab or far more sophisticated sensors than current devices carry. Until then, the number on your screen is best understood as a rough indicator of relative effort—useful for comparing one workout to another, but not reliable as an absolute measure of energy burned.