BMI, or body mass index, is a measure used to categorize normal, underweight, overweight and obese weights. BMI is calculated by dividing your weight in kilograms by your height in meters squared.
Purpose
BMI is an estimate of the amount of body fat based on height and weight in adults. It is not a direct measure of body fat but correlates with direct measures such as underwater weighing and body scans. It should be used as a screening tool for possible weight problems and risk of developing weight-related disease, not diagnostic. If you have a BMI above normal ranges, you should consult with your doctor for further assessment.
Women
The BMI categories for adult men and women are the same. MayoClinic.com reports that BMI might underestimate obesity in adult women, missing almost half of women in the obese category. This could be because women's body composition tends to have more fat than men. Alternative measures such as skin fold thickness or scans might provide better estimates.
Children
BMI is calculated the same way for children and teenagers, but different BMI categories are used to determine healthy and overweight cutoffs. Age and sex are taken into account because body fat changes with age and differs between boys and girls. The Center for Disease Control publishes for-age growth charts that allow BMI numbers for children to be converted into percentiles specific to age and sex.
Other Exceptions
BMI might not be as accurate an indicator for older people, who tend to have higher amounts of body fat. Conversely, BMI might overestimate obesity in athletes or bodybuilders, who have increased amounts of muscle. Waist circumference might be a better indicator of weight-related risks in these people, since greater amounts of abdominal fat also indicate a higher risk of disease.
Alternative Measures
There are several ways to assess body fat, including skin thickness measurements performed with calipers, bioelectrical impedence, and dual-energy X-ray absorptiometry. These methods require more training for proper use than BMI does and are frequently less available. Factors such as family history also influence your level of risk for weight-related disease. Talk to your doctor if you are concerned about your risk of developing weight-related diseases.
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