The formula for calculating BMI is simply your weight in kilograms divided by your height in meters squared. This formula is used for everyone regardless of gender or age. If you prefer, you can us U.S. measurements to calculate your BMI. Simply divide your weight in pounds by the square of your height in inches and multiply by a conversion factor of 703.
What It Measures
BMI is not a direct measure of body fat, but a metric that correlates with body fat across the general population. If you have a BMI of 30, for instance, then it means you have 30 kilograms of fat per meter of height squared. While BMI doesn't measure body fat directly, it correlates with body fat in postmenopausal women. A study published in the October 2002 academic journal, "Obesity Research," found a strong association between BMI and body fat percentage in postmenopausal women. This suggests that in general, postmenopausal women with a higher BMI will have a higher percentage of body fat.
BMI is used to classify the weight status of individuals. According to the National Institutes of Health, a BMI of 18.5 to 24.9 is considered normal. Anything below this range is considered underweight. A BMI of 25 to 29.9 is considered overweight and anything above this is categorized as obese. But according to the "Obesity Research" study, these classifications "may be misleading" for postmenopausal women. The researchers recommend that the NIH use a BMI of 25 or above as an indicator of obesity in postmenopausal women, rather than the current standard of 30.
Because BMI is not a direct measure of body fat you should consult your physician to find out if your weight is healthy. Your doctor will look at factors affecting your weight, such as body composition. Your doctor will be better able to interpret your individual BMI and, if necessary, may perform a direct test of your body fat -- such as a skinfold test -- to get a more accurate picture of your health.