Calorie intake to gain weight

In order to gain weight, you need to eat 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.

Calorie intake to lose weight

There are approximately 3500 calories in a pound of stored body fat.

So, if you create a 3500-calorie deficit you will lose one pound of body weight. (On average 75% being fat and 25% lean tissue).

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.

If you want to lose fat, a useful guideline for lowering your calorie intake is to reduce your calories by at least 500, but not more than 1000 below your maintenance level.

You should never drop your calorie intake below 1200 calories per day for women or 1800 per day for men without a physician recommendation and follow-up since those numbers are already 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

Remember that a healthy weight loss is no more than 1 pound (0.5 kg) per day.