The calories you need daily depends partly on genetics but also on your stage of life, age, activity level, size, body composition and gender. Larger and more muscular people generally need more calories; this is why men tend to need more. If you're in adolescence or pregnant, your calorie needs also go up. An easy formula can help you estimate your basal metabolic rate -- the number of calories you need to breathe, pump blood and have functioning organs. Then use a multiplier that takes into account your activity level to determine your total daily energy expenditure. Consume calories equal to this expenditure to maintain your weight.
Determine Your BMR
Measure your height in centimeters by multiplying your height in inches by 2.54. Next, determine your weight in kilograms by dividing your weight in pounds by 2.2. Men will then plug these numbers into the Harris-Benedict equation: 66 + (13.7 x weight in kilograms) + (5 x height in centimeters) - (6.8 x age). Women use a slightly different equation: 655 + (9.6 x weight in kilograms) + (1.8 x height in centimeters) - (4.7 x age). A 40-year-old, 140-pound woman who is 5 feet 4 inches, for example, has a BMR of 1,374 calories per day.
Factoring in Physical Activity
Multiply your BMR by a number to factor in physical activity. For sedentary people, who only conduct activities of daily living -- such as bathing, cooking and walking around their home or office -- use 1.2; for those who do light exercise one to three times per week, use 1.375; for people exercising three to five times per week, use 1.55; those who work out or do sports six to seven times per week can use 1.725; and those who perform one or more daily workouts and have a physically demanding job can use 1.9. In the previous example, if the woman jogged three times per week and had a desk job, she will need an estimated 1,889 calories -- 1,374 calories times 1.375 -- to maintain her weight.