Calculate BMR : The basal metabolic rate formula takes several variables like height, weight, age and gender to calculate your calorie needs as if you did not do any exercise
Calculate daily calorie requirement: Taking both BMR & activity level of an individual following formula gives a fair estimate of once daily calory requirement: http://www.muscleandstrength.com/tools/bmr-and-daily-calorie-calculator.html
However there is another way out to calculate our calorie requirement that goes as follows:
Harris Benedict Formula
To determine your total daily calorie needs, multiply your BMR by the appropriate activity factor, as follows:
If you are sedentary (little or no exercise) : Calorie-Calculation = BMR x 1.2
If you are lightly active (light exercise/sports 1-3 days/week) : Calorie-Calculation = BMR x 1.375
If you are moderatetely active (moderate exercise/sports 3-5 days/week) : Calorie-Calculation = BMR x 1.55
If you are very active (hard exercise/sports 6-7 days a week) : Calorie-Calculation = BMR x 1.725
If you are extra active (very hard exercise/sports & physical job or 2x training) : Calorie-Calculation = BMR x 1.9
Calorie needs to lose weight : The calorie deficit can be achieved either by calorie-restriction alone, or by a combination of fewer calories in (diet) and more calories out (exercise). This combination of diet and exercise is best for lasting weight loss. Indeed, sustained weight loss is difficult or impossible without increased regular exercise.
As a guide to minimum calorie intake, the American College of Sports Medicine (ACSM) recommends that calorie levels never drop below 1200 calories per day for women or 1800 calories per day for men. Even these calorie levels are quite low.
An alternative way of calculating a safe minimum calorie-intake level is by reference to your body weight or current body weight. Reducing calories by 15-20% below your daily calorie maintenance needs is a useful start. You may increase this depending on your weight loss goals.
Calorie needs to gain weight: If you want to gain body weight, you need to consume more calories than you burn. One pound of body weight is roughly equivalent to 3500 calories, so eating an extra 500 calories per day will cause you to gain one pound a week.
For optimum health, if you increase your calories to gain weight then (health permitting) gradually increase your level of physical exercise in order to maintain or increase your lean body mass. The benefits of exercise on physical and mental health are well documented and shouldn't be ignored.