Your tracker and dashboard show an estimated number of calories burned based on your BMR (Basal Metabolic Rate), which we calculate using the height, weight, age, and gender information that you provided when you set up your Fitbit account. If your tracker measures heart rate, the calorie burn estimate also takes heart rate into account. Note that calorie tracking for the following day begins at midnight and incorporates the calories you burn while sleeping.
When you sync your tracker, Fitbit replaces your estimated calorie burn with your tracker's data. If you manually log activities, the calories burned by during those activities are taken into account as well.
When you haven't synced your device or logged any activities, Fitbit tries to guess how many calories you have burned if you got out of bed, got dressed, went to your day job, came home, and did nothing much more than walk to your car.
Once you start logging activities, Fitbit stops estimating and uses the data you've provided instead. The more you wear your tracker, the more accurate your calorie burn data will be.
January 6, 2015