The most common mistake in fitness: eating against a number that was never real. Let's examine what your watch actually knows and what it guesses.
What do validation studies say?
A recent meta-analysis on the Apple Watch found a mean absolute percentage error of 27.96% in estimating energy expenditure[1]. A comparative device review recorded only moderate accuracy: 56.63% overall, with clear variation between brands[2].
Accuracy also varies by activity: for walking and running the error averaged about 31%, jumping to roughly 52% for cycling — likely because sitting during exercise confounds the algorithms[3].
energy estimation
vs cycling
— by contrast
Why the error?
Because calculating energy burned is complex: it includes activity plus basal functions like sleeping and breathing, and devices estimate it without knowing your muscle mass or movement efficiency[4]. They measure movement and heart rate, then guess the rest with a general equation.
But what your watch genuinely does well
- Step counting: relatively accurate, and it is the metric the evidence links to lower mortality (7,000 steps).
- Resting and exercise heart rate: a reliable fitness marker for personal tracking.
- Consistency: if it errs by the same amount every day, it is excellent for day-to-day comparison.
The golden rule
Don't "eat back" burned calories. If your watch says 600, the real figure might be 400 — and eating 600 against it turns your workout from a helper into a blocker. Treat exercise as a health and muscle gain, and leave the calorie budget to your plate (calculate it here).