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Fitness trackers cannot directly measure how many calories a person burns. They estimate the figure from movement, heart-rate readings and personal data such as age, weight and sex, and studies cited by Lifehacker found that those estimates are often substantially wrong.
An estimate built from incomplete data
Accelerometers help a watch infer whether someone is walking or running, while optical heart-rate sensors can provide another input when movement is difficult to interpret. The device then applies a proprietary formula. That process can be affected by how the tracker is worn, the activity being performed and whether the wearer’s movements match the pattern expected by the algorithm.
The underlying measurements also have limitations. A tracker may miss steps when a hand stays on a shopping-cart or treadmill handle, and the source notes that optical heart-rate readings can be less accurate on dark skin. Calorie burn is not measured directly, so errors in the inputs carry into the final estimate.
Research finds inconsistent results
A 2020 study covering devices from Apple, Garmin, Polar and Fitbit judged a reading accurate only when it was within 3% of a laboratory measure. Garmin devices underestimated calorie burn 69% of the time, while Apple watches overestimated it 58% of the time. Polar devices overestimated 69% of the time; Fitbits underestimated 48% and overestimated 39%.
Other research cited in the report found poor accuracy across the Apple Watch 6, Fitbit Sense and Polar Vantage V during sitting, walking, running, cycling and strength training, with variation coefficients from 15% to 30%. A 2025 review of Apple Watch studies found calorie estimates were off by about 18% on average. A 2026 study also reported that results became less accurate as participants’ body-fat percentage increased, with the direction and size of errors varying by device.
What the numbers are useful for
The findings make calorie readouts a weak basis for deciding how much to eat. Lifehacker’s report suggests using body-weight trends alongside food intake instead: changes in weight can indicate whether current intake is too high or too low, allowing adjustments without treating the tracker’s calorie figure as a measured fact.
Source: Your Fitness Tracker Has No Idea How Many Calories You’re Burning
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