Before embarking on any weight-loss program, it's important to know where you're starting from so you can figure out how far you have to go. Your body mass index is a ratio of your height and weight that helps estimate your body composition, or degree of fatness. For a more detailed picture, use a body composition scale or have your body fat percentage measured by your doctor or trainer. Once you know where you are, you can calculate your calorie needs.
BMI
Divide your weight in pounds by your height in inches squared, then multiply by 703 to find your BMI. A BMI of 18.5 to 24.9 is normal, and 25.0 to 29.9 is labeled overweight. Anything below 18.5 is considered underweight, and anything over 30.0 is considered obese. Because it is so easy to calculate and applies to a wide variety of people, the BMI is the scale most widely used by doctors to determine your body composition, but it is not accurate for everyone.
Body Fat Percentage
Because your BMI only takes your height and weight into account, it may give inaccurate results to those with abnormally high or low muscle mass. For example, a weightlifter who is five feet nine inches tall may weigh 200 pounds because of a high percentage of muscle mass. At that weight, his BMI would classify him as overweight, even though he may only have 10 percent body fat. For people like this, a body fat percentage estimate is a more accurate way to gauge body composition. Skinfold measurements are taken by a professional, and fed into a formula that produces a number that corresponds to a scale. Anything over 32 percent for women or 25 percent for men qualifies as obese, while anything under 10 percent for women or 2 percent for men registers as underweight.
Calculating Calorie Needs
To calculate the amount of calories your body needs as it is now, divide your weight in pounds by 2.2 to convert to kilograms, and multiply your height in inches by 2.545 to convert to centimeters. Multiply your weight in kilograms by 9.99 to find A, multiply your height in centimeters by 6.25 to find B, then multiply your age by 4.92 to find C. Add A and B, then subtract C. Women should subtract an additional 161, while men should add an additional 5. The result is the number of calories you need to maintain your weight. Multiply your total by 1.5 to 1.7 if you are moderately active to compensate for the number of calories you burn in daily activity.
Creating a Deficit
To lose weight, you must create a 500-calorie-per-day deficit. You can do this by burning an extra 500 calories through exercise, or by eating 500 fewer calories. The best, most permanent results are usually gleaned by combining both approaches with a healthy, balanced diet and daily moderate exercise. This allows you to eat more food so you don't feel so deprived, and it helps you turn exercise into a lifelong habit. A regular physical activity regimen is the key to permanent weight loss.
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