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Fitbit estimates your Base Metabolic Rate (BMR) based on your stats (gender, age, height, weight) and accumulates this starting at midnight throughout the day. BMR is what you burn just to stay alive (breath, think, digest, pump blood...). When you wear the unit for activities it adds those to the BMR to give you a total calorie expenditure. If you don't wear the unit, it just keeps adding the BMR calories to your total.
Warren | Cincinnati, OH
Versa Lite, Ionic, Charge3, Inspire HR, Blaze(retired), Alta( retired),- Pixel 3
Take a look at the Fitbit help site for further assistance and information.