Body mass index, or BMI, is a measure of health that uses a mathematical formula comparing height to weight. A BMI score indicates whether a person is underweight, healthy, overweight or obese. Although BMI does not exactly measure body fat, your BMI score does correlate to your body fat percentage, which is how it determines if you are healthy. Body fat percentage is how much of your weight comes from fat. Waist and neck measurements are another method to estimate body fat percentage.
BMI
Calculate BMI by first weighing yourself and then multiplying your weight in pounds by 703. Use a tape measure to measure your height in inches. Divide the sum of the first calculation by your height in inches. Take the sum of that measurement and divide it again by your height in inches. The sum is your BMI score. Your score puts you into a category that ranges from underweight to morbidly obese. If your score is lower than 18.5 you are underweight. A healthy score is between 18.5 and 24.9. A score of 25 to 29.9 means you are overweight. A score of 30 to 39.9 means you are obese. A score of 40 or higher means that you are morbidly obese.
BMI Exceptions
The BMI score indicates health for most people, but there are exceptions. Bodybuilders have more muscle than the average person, and since muscle weighs more than fat, this offsets a BMI score so that a bodybuilder appears overweight. Seniors also get inaccurate scores. A score of 25 to 27 is a healthy score for seniors. Body mass index is also not appropriate for children. A pediatrician can determine if a child is overweight for her age.
Measurements for Men
Measuring the waist and neck allows men to use a formula to determine if they are at a healthy weight. You can use this method separately or along with BMI to see if you need to lose or gain weight. Use a tape measure to determine height in inches, width of the neck just below the larynx and waist at the belly button. Take measurements on bare skin and keep the tape as horizontal as possible. Plug your waist, neck and height measurements into a formula to determine body fat percentage: 495/(1.0324-0.19077(log(waist-neck))+0.15456(log(height)))-450. This answer gives you a body fat percentage.
If you prefer not to use this calculation or if it is not giving you a reasonable answer, use an online calculator instead by putting your measurements into the appropriate boxes. For men, a body fat percentage of 2 to 4 percent is essential and anything below that is underweight. A body fat percentage of 6 to 13 percent is ideal for athletes. In shape men have a normal body fat percentage of 14 to 17 percent. Eighteen to 25 percent is acceptable for men. Twenty-six percent or higher indicates obesity for men.
Measurements for Women
Women use a similar formula for determining body fat using their height, waist and neck measurements. If you are a woman, measure your hips as well at the largest point of the glutes and then input your measurements into a slightly different formula: 495/(1.29579-0.35004(log(waist+hip-neck))+0.22100(log(height)))-450.
You can use an online calculator if you prefer. Women need more body fat than men. A woman's essential fat is 10 to 12 percent. A percentage of 14 to 20 percent is ideal for women who are athletes. Fit women may have a body fat percentage of 20 to 24 percent. A percentage of 25 to 31 percent is acceptable, but higher than 32 percent indicates obesity.
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