You can use the Harris-Benedict formula to determine your daily calorie need. You need to know your current height and weight, plus you'll have to gauge your daily activity levels.
You can use my online calculators:
Or you can do the math yourself:
First, determine your basal metabolic rate:
Women: 655 + (4.35 x weight in pounds) + (4.7 x height in inches) - (4.7 x age in years)
Men: 66 + (6.23 x weight in pounds) + (12.7 x height in inches) - (6.8 x age in years)
Next, take that number and multiply it by the number that corresponds to your daily activity level:
- Sedentary (little or no exercise) - 1.2
- Lightly active (light exercise/work 1-3 days per week) - 1.375
- Moderately active (moderate exercise/work 3-5 days per week) - 1.55
- Very active (hard exercise/work 6-7 days a week) - 1.725
- Extra active (very hard exercise/work 6-7 days a week) - 1.9
The final number you have is the number of calories you need per day to maintain your current weight. If you want to gain weight, you'll need to consume more calories, or burn fewer calories; and if you want to lose weight, you'll need to eat fewer calories or burn more calories by increasing your physical activity.
Academy of Nutrition and Dietetics. "In non-obese individuals, what is the prediction accuracy and maximum overestimation and underestimation errors compared to measured resting metabolic rate when using the Harris-Benedict formula?" Academy of Nutrition and Dietetics Evidence Analysis Library. Published March 2005.
"Food Intake Patterns." United States Department of Agriculture. Center for Nutrition Policy and Promotion. Accessed May 10, 2011. http://www.mypyramid.gov/downloads/MyPyramid_Food_Intake_Patterns.pdf.