Measured or estimated?
No watch on your wrist can measure the energy you burn directly. In a lab that only works through your breath: from oxygen uptake and carbon dioxide output, researchers calculate how much energy your body uses (indirect calorimetry).
Your smartwatch or sports watch measures something else, whether it comes from Garmin, Apple, Polar or another brand: heart rate, movement and, depending on the sport, pace or power. From that and your profile (age, weight, height, sex) it calculates an estimate. The calorie number is always the output of a model, never a measurement.
What studies show
- A 2020 systematic review looked at 158 studies of nine brands of sports watches and fitness trackers. For energy expenditure, no brand was accurate. For heart rate, Garmin and Apple were among the most accurate brands (Fuller et al. 2020).
- In a Stanford University study, 60 people wore seven wrist devices while sitting, walking, running and cycling, with their energy expenditure measured in the lab. No device achieved an energy expenditure error below 20 percent. The devices were most accurate for cycling and least accurate for walking. No Garmin watch was part of this study (Shcherbina et al. 2017).
Both papers show the same pattern: good watches now measure heart rate fairly reliably, but deriving calories from it remains imprecise. Newer models may do better, but independent studies on them are rare.
Where the errors come from
- The model barely knows you. Age, weight and sex explain only part of the differences between people. How fit you are and how efficiently you move varies from person to person.
- Wrist heart rate varies. A loose strap, cold weather or a lot of wrist movement can disturb the optical sensor. A wrong heart rate gives a wrong calorie number.
- The sport matters. When you ride with a power meter, the work you did is known, which is a much better basis. How to calculate from it yourself is explained in How many calories do you burn cycling?
Total, resting and active calories
Brand apps such as Garmin Connect or Apple Health split your daily burn into resting calories and active calories. Apple calls them Resting Energy and Active Energy. Resting calories roughly match your basal metabolic rate, the energy you use even without moving.
The calories of a recorded activity usually include the part you would have burned anyway during that time. If you add all of them to a daily target that already includes your basal metabolic rate, you count that part twice. That is roughly 1 kcal per kilogram of body weight per hour, so about 70 kcal for one hour of training at 70 kg.
How to use the numbers well
- Use the number for comparison: a session showing 800 kcal was very likely harder than one showing 400 kcal, even if neither number is exact.
- Don't add your full training burn to your daily target. That is why HabitHelm adds only part of it. How this works is explained under Training day calories.
- Check over a few weeks how your weight develops on your target. That is the best way to tell whether the numbers fit you.
- Workouts from watches that upload their activities to Strava, such as Garmin, Polar, Suunto or Coros, reach HabitHelm via Strava. Setup is explained for Garmin under Connect Garmin.
Sources
- Fuller D, Colwell E, Low J et al. (2020): Reliability and Validity of Commercially Available Wearable Devices for Measuring Steps, Energy Expenditure, and Heart Rate: Systematic Review. JMIR mHealth and uHealth 8(9):e18694. doi.org/10.2196/18694
- Shcherbina A, Mattsson CM, Waggott D et al. (2017): Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort. Journal of Personalized Medicine 7(2):3. doi.org/10.3390/jpm7020003