There is no definite answer. If it is there, it is rather relative. Most people know the number of calories in the food items they buy by reading the label. Nevertheless, if you ask them how many calories they should eat per day, most of them have no clue. Of course, if you have to probe further and would like to know how many calories they should eat in order to increase their weight or maintain it at the existing level or reduce it, do not expect any valid answer.

However, you can use a rule of thumb to get the approximate number of calories you need per day. For Instance, assume your weight is 200 pounds. Multiplying the number by 10, you get 2000 (that is, 200 X 10 = 2000). It means you should eat 2000 calories per day to maintain your current weight. Eat less to lose weight and more to gain it. If you consume 3500 fewer calories per week at the rate of 500 calories per day, you are likely to lose one pound of weight.

Even when you are resting (or sleeping), your body needs some energy (calories) to carry out all its vital functions like those of the lungs, heart, kidneys and so on. The minimum number of calories required for sustaining life in an individual when resting or sleeping is calculated with the help of a formula. It is called the Basal Metabolic Rate (BMR). Children, youth, tall and lean people and pregnant women have higher BMR. If you have a fever or are under some stress, your BMR comes down. Fasting raises it. In other words, BMR changes as the variables change.

The Harris-Benedict Equation is more accurate than the rule of thumb for giving the number of calories you should eat per day, as it takes into consideration your activity level also, along with your height (in inches), weight (in pounds), age (in years) male or female, which are the bases for calculating your BMR. First calculate your BMR and multiply it with the appropriate activity factor in order to arrive at the number of calories you should eat per day to maintain your present weight. For instance, if your BMR is 2000 and the activity factor applicable is 1.2 (assuming you lead a sedentary life, with little or no activity), the number of calories you need per day is given by 2000 X 1.2 = 2400. This number naturally increases as the activity level increases. The other activity factors for progressively higher levels of activity are 1.375, 1.55, 1.725 and 1.9. Several websites offer help in calculating your BMR as well as your daily calorie-intake, if you supply all the necessary details.

Every individual is unique in respect of calorie needs. As there is no uniform number of calories that all healthy individuals should eat everyday, and as most people are confused about it, the US Department of Agriculture has thrown some light on the issue. Its chart gives you a general idea about your caloric needs.

For specific advice regarding the calorie-intake applicable in your particular case, you must consult your doctor or dietician.