Basal metabolic rate (BMR) is the average daily amount of energy (calories) that is used by a body when it is at rest. Heredity, height, body composition, and age determine a person's basal metabolic rate. It is also known as basal energy expenditure (BEE).
The energy in a person's BMR is used to keep body temperature normal and to power the heartbeat, breathing, and other organ functions. Your BMR uses up about two-thirds of your daily calories.
The best measure of BMR is taken when you are not digesting any food or doing any activity, usually right after waking up for the day.
Another option is to measure resting metabolic rate (RMR), also known as resting energy expenditure (REE). This is very similar to BMR, but the test can be done with fewer restrictions.