There are a number of factors that you need to take into consideration to determine how many calories a person should eat to lose weight, so there is no one single number that will work for everyone. It's possible, however, to come up with an estimate for how many calories you need to maintain your weight, which you then adjust to figure out how many calories you should eat to lose weight at the rate of three pounds a week.
Caloric Needs Calculations
You can use equations to calculate how many calories you need a day. Usually, these start with an estimate of your basal metabolic rate -- which is how many calories your body needs when you're resting. Then, multiply this by another number, based on your activity level to account for the extra calories you'll burn through your daily activities. Your BMR varies, based on your age, gender, height and weight. There are a number of different equations for this purpose, and each of these will have slightly different results.
For women, the BMR = 655.1 + (weight in kilograms x 9.6) + (height in centimeters x 1.8) − (age in years x 4.7)
For men, the BMR = 66.47 + (weight in kilograms x 13.7) + (height in centimeters x 5) − (age in years x 6.8)
Using the Harris Benedict formula, people who don't exercise should multiply their BMR by 1.2 to get an estimate of how many calories they need each day to maintain their weight.
Those who participate in light exercise need to multiply their BMR by 1.375, and those who exercise three to five days a week should multiply their BMR by 1.55. The multiplier for very active people is 1.725, and for very active people with a physical job, the multiplier is 1.9.
The BMR times the number for your activity level gives you your daily calorie needs.
The Minimum Number of Recommended Calories
It isn't a good idea for women to eat less than 1,200 calories each day or for men to eat less than 1,800 calories a day, as this could cause your metabolism to slow down. It's also difficult to fit in the recommended nutrients on a diet that's has fewer calories than this. Should you need more of a calorie deficit for you to lose three pounds a week, you'll need to burn those calories by exercising more or settle for a slightly slower weight loss so you can stay healthy.
Creating a Caloric Deficit
Each pound of weight loss requires a 3,500 calorie deficit, which would mean cutting 500 calories a day for one pound of weight loss. Thus, once you've determined approximately how many calories you need to maintain your weight, you'll need to create a caloric deficit of 1,500 calories a day to lose three pounds a week.
So, if you're a moderately active man between the ages of 36 and 40 and need about 2,600 calories a day to maintain your weight, you'll need to eat 1,800 calories a day and then find a way to burn another 700 calories each day through exercise to lose three pounds a week.
Losing this amount of weight a week would be very hard for sedentary women over age 25, as they typically need only about 1,800 calories a day to maintain their weight. It would mean cutting 600 calories a day and burning 900 calories through exercise. In this situation, it's more realistic to aim for a weight loss of slightly more than 1 pound a week.
It's always a good idea to exercise during times of weight loss, as a combination of cardio and strength training will help limit the loss of lean muscle mass. Without participating in these types of exercise, as much as 25 percent of any weight you lose will come from muscle, rather than from fat. The Centers for Disease Control and Prevention recommend that adults get at least 150 minutes of moderate-intensity cardiovascular exercise each week, and do strength training exercises at least two times a week that work all the large muscles in the body, including the back, abdomen, legs, arms, shoulders, hips and chest. Before starting an exercise regimen, check with your doctor.