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Basal Metabolic Rate (BMR) is the amount of daily energy expended by animals (or humans) at rest.
This means that energy is expended only for the functioning of the vital organs, no any activity is performed. Imagine that you do nothing but you still loose the energy in form of body heat.
It is believed that if man consumes less energy than his BMR, body starts to accumulate kilocalories for future expenses, so instead of losing weight you're gaining it.
This calculator provides estimation for BMR, given the height, weight, age and sex.