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 you provided us with when you set up your Fitbit account.
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 doing those will be 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.
If that you find that your calorie burn is just as high or higher on days when you are less active, it may be because you have calorie estimation enabled. Calorie estimation is used when no activities are logged or data is synced.
To disable calorie estimation:
Log into your dashboard.
Click the gear symbol in the upper right.
Scroll down the page to Preferences.
Change the Calorie Estimation setting to Disable.