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The algorithm keeps track of your Base Metabolic Rate (BMR), those calories you burn just by living. It uses your stats (gender, height, weight...) to make the estimate. You can check the accuracy by going to various BMR calculators on the Web. In any case, even when not wearing the unit it keeps track of your BMR and continues to add it throughout the day.
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.
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