Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — just to keep your heart beating, lungs breathing, and cells functioning. BMR represents 60–75% of your daily calorie burn, making it the cornerstone of every diet and nutrition plan. This guide compares the two most common BMR formulas (Mifflin-St Jeor and Harris-Benedict), shows how to calculate yours step by step, and explains why your BMR isn't fixed.