Whether you want to gain weight, lose weight or maintain your current weight, you need to know how many calories you should eat on a daily basis. To calculate this, you need to know the number of calories you burn each day while resting and during physical activity. Mark D. Mifflin and colleagues published an article in a 1990 issue of “The American Journal of Clinical Nutrition” that provides formulas for these calculations. Once you figure out how many calories you need to consume to achieve your goal, you simply need to monitor your daily food and drink intake closely.
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Weigh yourself in pounds on a reliable scale. Convert your weight to kilograms by multiplying your weight in pounds by 0.4536. For example, if you weigh 150 pounds, multiply 150 by 0.4536 to get 68 kilograms.
Measure your height in inches. Convert your height to centimeters by multiplying your height in inches by 2.54. For example, if you are 66 inches tall, multiply 66 by 2.54 to get 168 centimeters.
Calculate your resting energy expenditure--the calories you burn per day while resting--using the following formula if you are female:
(10 × weight in kilograms) + (6.25 × height in centimeters) - (5 × age in years) - 161;
or this formula if you are male:
(10 × weight in kilograms) + (6.25 × height in centimeters) - (5 × age in years) + 5.
For example, using the weight and height from above, if you are a 40-year-old female, your resting energy expenditure equals (10 × 68) + (6.25 × 168) - (5 × 40) - 161, which comes to 1,769 calories per day.
Multiply your resting energy expenditure by a physical activity factor to calculate your total energy expenditure. If you exercise less than three times per week, use 1.12 for your physical activity factor if you are female and 1.11 if you are male, according to Heather Fink, Lisa Burgoon and Alan Mikesky, authors of “Practical Applications in Sports Nutrition.” If you exercise three to five times per week, use 1.27 if you are female and 1.25 if you are male; and if you exercise more than five times per week, use 1.45 if you are female and 1.48 if you are male.
Using the same example, if you exercise three to five times per week, your total energy expenditure equals 1,769 × 1.27, or 2,247 calories per day, which is the number of calories you should eat to maintain your weight of 150 pounds.
Subtract 500 to 1,000 calories from your total energy expenditure if you want to lose about 1 to 2 lb. per week, according to William McArdle, Frank Katch and Victor Katch, authors of “Sports and Exercise Nutrition.” Conversely, add 500 to 100 calories to your total energy expenditure if you want to gain 1 to 2 pounds per week. Therefore, following the same example, if you want to lose 1 to 2 pounds per week, eat between 1,247 and 1,747 calories per day.