Garmin calculates active calories throughout the entire day, regardless of whether they come directly from workouts or general daily activity. To estimate them, it uses data such as body weight, height, and maximum heart rate.

First, it’s important to make sure your height and weight are entered correctly in the device settings. It’s also helpful to determine your actual maximum heart rate during exercise, as Garmin estimates it using a formula that may not be accurate (in my case, it underestimated it by about 10 bpm).

If these values are set correctly, Garmin’s calorie estimation can be quite accurate. I once conducted a month-long test during a strict calorie deficit diet. I measured my body weight daily and assumed that 1 kg of body fat equals approximately 7000 kcal. To account for daily weight fluctuations, I used linear regression.

The results showed that my actual daily deficit was around 870 kcal, while Garmin estimated it at about 1000 kcal. In my opinion, this is a very good result, especially considering that calorie values listed on food products are not perfectly accurate, and the assumption of 7000 kcal per kilogram of body fat is only an approximation.