A calorie is a unit of energy. They apply to anything containing energy including food.We need them to survive. The number of calories in a food is a measure of how much potential energy that food possesses.

Foods are a compilation of three building blocks, carbohydrates, fats and proteins. So if you know how many carbohydrates(4 calories), fats(9 calories) and proteins(4 calories) are in any given food, you know how many calories that food contains.

You may calculate the daily calorie needs with formulas below:
Adult male: 66 + (6.3 x body weight in lbs.) + (12.9 x height in inches) - (6.8 x age in years)
Adult female: 655 + (4.3 x weight in lbs.) + (4.7 x height in inches) - (4.7 x age in years)