Your BMR (Basal Metabolic Rate) is the estimated minimum level of energy required to sustain the body's vital functions when at rest. The Harris-Benedict formula is used to calculate the BMR (Basal Metabolic Rate).
This is how many calories your body needs when at reast to maintain its current weight.
It varies from person to person. But this calculator gives you an idea.
http://www.fitwatch.com/qkcalc/bmr.html