Basal metabolic rate (BMR) is the amount of energy expended daily by humans at a neutrally temperate environment while in the post-absorptive state. The release, and using, of energy in this state is sufficient only for the functioning of the vital organs, the heart, lungs, nervous system, kidneys, liver, intestine, sex organs, muscles, and skin.
Determining how many calories your body needs to maintain itself on a daily basis is generally determined by a predictive equations. Historically, the most notable one was the Harris-Benedict equation, which was created in 1919.
I found the use of a device that measured activity and recorded calorie intake to be very helpful.