Basal Metabolic Rate (BMR) is an estimate of how many calories your body burns at complete rest just to maintain basic functions like breathing, circulation, and cell production. This calculator uses the Mifflin-St Jeor equation, which is generally considered more accurate than older formulas like Harris-Benedict, and factors in your age, sex, height, and weight in either metric or imperial units. Your BMR is typically the starting point for calculating your total daily calorie needs.