Basal metabolic rate is the amount of energy the body expends at complete rest to maintain basic physiological functions—breathing, circulation, cell production, and temperature regulation. It is usually measured in the morning after an overnight fast, in a thermally neutral environment, with the person awake but motionless.

BMR accounts for the largest share of daily calorie expenditure in most people. It varies with body size, composition, age, and sex. Muscle tissue burns more energy at rest than fat tissue, so two people of the same weight can have different rates. BMR declines gradually with age, partly because of loss of muscle mass. Thyroid hormones are major regulators; both under- and over-activity of the thyroid produce corresponding changes in metabolic rate.

Knowledge of BMR helps in designing weight-management programs and in assessing nutritional needs during illness. Indirect calorimetry—measuring oxygen consumption—remains the gold-standard method, though equations based on height, weight, age, and sex provide useful estimates.

Even when the body appears still, it is continuously spending energy simply to stay alive. That expenditure is the basal metabolic rate.