Basal Metabolic Rate, known as BMR, is the rate at which the body expends energy to maintain essential life functions while at complete rest. The majority of these essential functions continue mostly unnoticed—things like the heart beating, the lungs inhaling and exhaling air, the kidneys filtering waste, new cell creation, maintaining proper body temperature, and more. A BMR calculator measures the sum of kilocalories required to maintain these essential body functions. On average, this sum is approximately 60-70% of a person's total daily energy expenditure.

Because BMR measures these activities while at complete rest, a true calculation of BMR can only be done after a 12-hour fast and a good night's rest and in the absence of any physical activity or emotional excitement, as these factors can temporarily influence metabolic rate.