Calories are the foundation of any successful diet. A calorie is actually a unit of energy -- one calorie is the amount of energy needed to raise the temperature of 1 gram of water by 1 degrees Celsius. In terms of what this means for you, if you eat more calories than you burn you'll gain weight, and if you eat fewer than you burn, you'll lose weight. When it comes to working out calorie intake, one of the main factors to consider, along with your goals, is your body weight.

## General Guidelines

As a rough estimate, the United States Department of Agriculture recommends that women between the ages of 19 and 30 consume 1,800 to 2,400 calories daily, while women between 31 and 50 should consume 1,800 to 2,200 per day. Eat more if you're active and less if you're sedentary. This is still a generic guideline though and doesn't factor body weight into the equation.

## Calories by Body Weight

For a more personalized calorie intake, Harvard Health Publications recommends consuming 15 times your body weight in pounds in calories daily to maintain your weight. This would mean consuming 1,800 calories each day if you weigh 120 pounds. Alternatively, calculate your basal metabolic rate -- the number of calories you need daily just to survive -- then multiply that by your activity level. The formula for women is BMR = 655 + ( 4.35 x weight in pounds ) + ( 4.7 x height in inches ) - ( 4.7 x age in years ). If you're sedentary, multiply your result by 1.2. Increase this to 1.275 if you're lightly active, 1.55 if you're moderately active, 1.725 if you're very active and 1.9 if you're extra active.

## Calorie Intake Relative to Goals

These formulas all give you your maintenance calorie level -- the number of calories you'd need to consume each day to maintain weight. This is fine if you're just looking to keep an eye on your intake and maintain general health. If you're looking to lose weight you'll need less than this, though, and for weight gain you'll need more. Going 500 calories below your maintenance level each day will lead to 1 pound of weight loss per week, and going 500 calories over each day will lead to 1 pound of weight gain each week.