The Basal Metabolic Rate (BMR) is the rate at which energy is consumed by the organs of the humans and animals at the time of total rest. It can be measured in terms of per square meter of body surface per hour in human beings. It is directly proportional to the body mass and si inversely proportional to the age.
To know more about BMR, read this link : http://en.wikipedia.org/wiki/Basal_metabolic_rate