Your body uses energy all day long: to breathe, think, move, and even to sit around and do nothing. The BMR Calculator will calculate your Basal Metabolic Rate (BMR); the number of calories your body uses just to stay alive (at resting state).
Because your BMR decreases as you age, it becomes harder to eat whatever you want and stay slim. Likewise, depriving yourself of food in hopes of losing weight also decreases your BMR, working against your health and fitness efforts.
However, a regular routine of cardiovascular exercise and strength training can increase your BMR, improving your health and fitness even when your body's ability to burn energy gradually slows down.
So find out the number of calories your body uses per day at resting state. Calculate your BMR here by entering your height, weight, age and gender.
Once you know your BMR, you can calculate your Daily Calorie Needs based on your activity level using the Harris Benedict Equation. This will give you an idea of how much calories to consume per day to maintain your current weight, or to lose or gain weight.