Reccomended preceede reading: What is calorie ?

If you're wondering how many calories you should eat daily, than you're at the right place. Following calculation will be made in two steps:

• BMR estimations
• Activity level appraisal

BMR

Basal metabolic rate (BMR) is the amount of energy expended while at rest (in the post-absorptive[digestive] state). The release of energy in this state is sufficient only for the functioning of the vital organs, such as the heart, lungs, brain and the rest of the nervous system, liver ...
BMR can be measured, but restrictive circumstances are needed (person should be awake, but at complete rest ...).

Currently, the best prediction is the equation develobed by MD Mifflin and ST St Jeor in 1990. And the equation is:



Where m stants for mass (weight), h for height, a for age and s is +5 for males and -161 for female.

Example: A 27 year old woman with the following parameters: weigh = 70kg and height = 163cm.
Resting Metabolic Rate (RMR) = 9.99*70 + 6.25*163 - 4.92*27 - 161 = 1424 kcal per day

Activity level appraisal

To calculate daily calorie needs, this BMR value is multiplied by a factor with a value between 1.2 and 1.9, depending on the person's activity level.

This table gives approximate factor's value depending on activity level.

Activity level	Factor
Sedentary (little or no exercise)	1.2
Lightly active (light exercise/sports 1-3 days/week)	1.375
Moderatetely active (moderate exercise/sports 3-5 days/week)	1.55
Very active (hard exercise/sports 6-7 days a week)	1.725
Extra active (very hard exercise/sports & physical job)	1.9

Continuing our example, lets assume our person has sedentary job and workouts 3-5 times a week, thus the factor will be 1.55.

Daily Calorie needs = RMR * factor = 1424 * 1.55 = 2207 kcal per day.

For more info see: http://en.wikipedia.org/wiki/Basal_metabolic_rate