While 2,000 calories may be the default number of calories that daily nutritional values are based on, the number of calories you actually need each day depends on your gender, weight, height, age and activity level. The HarrisBenedict formula can be used to determine the estimated number of calories you need each day to maintain your weight.
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Calculate your basal metabolic rate (BMR) using the following equation: 655 + (4.35 x your weight in pounds) + (4.7 x your height in inches)  (4.7 x your age in years) if you are female. If you are male, use the equation: 66 + (6.23 x your weight in pounds) + (12.7 x your height in inches)  (6.8 x your age in years) to determine your BMR. Your BMR represents how many calories your body requires to maintain basic functions.

Calculate how many additional calories your body requires based on your activity level. If you are sedentary or engage in very little activity, multiple your BMR by 1.2. If you are lightly active or exercise lightly between one and three times a week, multiply your BMR by 1.375. If you get moderate levels of exercise, multiply your BMR by 1.55. Multiply your BMR by 1.725 if you are very active and exercise six or seven days a week. If you exercise extremely hard, are in an athletic training program or have a job that consists of hard labor, multiply your BMR by 1.9. The result from multiplying your BMR by your activity factor is the total number of calories you need to consume each day to maintain your current weight.

Subtract 500 from the total number of calories you must consume each day to maintain your weight if you are trying to lose weight. A deficit of 500 calories a day equals a deficit of 3,500 calories a week, which is equivalent to one pound. To lose weight at a slower rate, subtract 250 calories from your daily caloric needs.

Add between 250 and 500 calories to your estimated daily caloric intake if you want to gain weight. An extra 500 calories each day will lead to weight gain at a rate of one pound per week.
Tips & Warnings
 Multiply your BMR by the activity factor that accurately describes your activity level. Do not underestimate or overestimate your activity level.
References
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