By: Joann Bally CSCS
We’ve discussed BMI, or Body Mass Index, before, but it’s being used in the news so much lately, that it seems time to explain it again.
Body Mass Index is a population screening tool, not something that tells you definitively whether you’re fat. Researchers use it in large studies because it is easy and cheap—all they need to know is the person’s height and weight. Sometimes they never even see the person, so take their word for it. From this they draw conclusions on what percentage of the study population is obese or the relationship between a given BMI range and a certain disease or condition.
A BMI of less than 18.5 is considered to be underweight. A desirable BMI is 18.5 to 24; 25-29 is overweight; 30 and over is obese. A number of 40 or more signifies severely obese—about 100 pounds or more overweight.
The Body Mass Index is a poor indicator of body fat percentage. Fitness professionals generally prefer body fat percentage to BMI as a measure of fitness, but it’s harder to measure. Athletes and other fit and/or muscular individuals may have a high BMI but a low percentage of body fat. This is one drawback of using BMI on an individual basis. Another is that it doesn’t differentiate between men and women. Also, some studies indicate that a somewhat higher BMI may be desirable in some populations, such as the elderly.
To find out your body fat percentage, you may need to find a facility that offers hydrostatic (underwater) weighing, or a device called a Bod Pod. Bielectrical impedance can be accurate, but it requires some preparation. Skinfolds, measured with calipers, can be accurate but only if the technician is skilled, which the trainer at your local gym may not be. Calipers can’t be used on some very obese people. If you have a scale or other device that purports to measure body fat, first see if it’s consistent—body fat doesn’t change radically in a few days—then use it to gauge your progress, rather than worrying about the exact numbers.
To find your BMI, divide your weight in kilograms by your height in meters squared. To avoid the metric system, multiply your weight in pounds by 703, then divide by your height in inches and divide that answer by your height again.
BMI Calculator Values For Adults
HEALTHandFITNESS.com 
