How Accurate Is the “Calories Burned” Number on Your Fitness Tracker?

Your watch or band finishes a workout and cheerfully announces how many calories you burned. It feels precise, and it is tempting to treat it as a number to “eat back.” The research says it is friendlier to think of it as a rough guess, and there is plenty to love about what trackers do well.

What the study tested

In a well-known 2017 study in the Journal of Personalized Medicine, Stanford researchers tested seven popular wrist-worn devices on 60 volunteers doing walking, running and cycling. Participants also wore lab equipment that measures heart rate and energy use directly, which is the reference standard. The team then checked how far each device was from the lab numbers.

Heart rate was great. Calories were a rough guess.

• Heart rate: six of the seven devices were within about 5% of the reference, which is accurate enough for most uses.
• Energy expenditure (calories burned): the typical error ranged from about 27% for the best device to about 93% for the worst. None of the seven had an error under 20%.

That does not make a tracker useless. The calorie figure is simply an estimate built from your heart rate, movement, age, weight and sex using a formula, and real energy use varies from person to person in ways a wrist cannot see.

The study used 2017 devices, and newer ones have improved. But the basic challenge, estimating energy use from a wrist, remains.

How to use the number sensibly

• Skip the “eat back” habit. If the number is 30% too high, eating it back can erase a day’s progress.
• Compare you with you. A tracker is more reliable for trends, such as whether you did more this week than last, than for single sessions.
• Lean on heart rate. It is a useful measure of effort. Our zone 2 explainer shows how to use it.
• Keep your own estimate in perspective. Food tracking is an estimate too, as our guide to calorie-tracking apps shows. Two uncertain numbers do not make one precise one.
• Let results guide you. If your weight and energy are trending the way you hoped, the exact calorie count matters much less.

Better uses for a tracker

• Steps and active minutes, as we discuss in the 10,000-steps myth.
• Resting heart rate and heart-rate trends over weeks.
• Sleep time, though it is weaker at sleep stages. See our sleep tracker guide and the wearables overview.

Putting it together

Wrist devices measure heart rate well and calories burned loosely. Enjoy the calorie figure as a loose estimate, and let it cheer you on rather than tell you to eat more. For medical questions about exercise and heart rate, talk with a clinician.

Sources

• 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, 2017 (PMC5491979).
• Healio, “Fitness trackers inconsistently collect data” (2017), summary of the same study.