One of the purposes of food -- probably the most important -- is to provide calories to the body. Calories are energy which the body can burn to carry out its functions. For us, the important question is: how many calories should we get in a day? The standard, one-size-fits-all answer is, "2,000 calories." But it's more accurate to say that the answer depends on who you are. For example, a huge male lumberjack typically needs more calories than a small, elderly woman. Some people's caloric needs might be as low as 1,200 calories; while others (typically extremely active individuals) could need as much as 5,000 calories.
The variables typically taken into consideration in estimating one's caloric needs are sex, age, height, weight, and physical activity level. However, these variables do not account for all variance, so estimates may not be accurate. This site contains an online calculator that will estimate your caloric needs; it is based on good science. Click 'Nutrient Calculator' at the bottom of the page, or go to Nutrient Profile Calculator.