Basal Metabolic Rate (BMR) is the number of calories your body needs to maintain basic physiological functions at complete rest — breathing, circulation, cell production, and organ function. The BMR formula is the foundation for calculating Total Daily Energy Expenditure (TDEE), which determines how many calories you need to maintain, lose, or gain weight. The most accurate BMR formula in clinical use is the Mifflin-St Jeor equation, which accounts for gender, age, weight, and height.