The body mass index (BMI) is a method of comparing a person's weight and height. It is used to estimate a person's healthy body weight and is one of the most common methods for doing so. The BMI is also used to assess a child's physical development. The limitations of the BMI should be clearly understood to avoid using it inappropriately.
Calculation
The BMI is calculated by dividing the subject's weight by the square of the subject's height. It is usually measured in units of kilograms per meters squared and a conversion factor must be applied if the measurements are made in other units. A BMI measured in pounds per inches squared must be multiplied by 703. A BMI measured in pounds per feet squared must be multiplied by 4.88.
Usage
The BMI is intended to be used as a simple method of establishing an individual's weight classification. The use of the BMI is most appropriate in population studies as a convenient method of estimating body fat percentage. This will continue to remain an important application of the BMI as obesity becomes more prevalent in developed countries.
Interpretation in Adults
The World Health Organization (WHO) classifies people with a BMI below 18.5 as underweight and those with a BMI greater than 25 as overweight. WHO also classifies people with a BMI greater than 30 as obese. Note that WHO doesn't consider age or gender when making weight classifications in adults, although some other sources do.
Interpretation in Children
The BMI is also used to compare children of similar age and gender by converting the BMI into a percentile score. This score indicates the percentage of children with the same age and gender that have a lower BMI than the test subject. A BMI percentile ranking of less than 5 indicates the child is underweight, a percentile ranking of 85 to 95 indicates the child is overweight and a percentile ranking greater than 95 indicates the child is obese.
Limitations
The BMI is inappropriate for individual diagnosis and doesn't actually measure body fat, although it's widely used for these purposes. It also assumes the individual has a sedentary lifestyle and an average frame. A person with a large frame or significant muscular development can be classified as overweight. Similarly, a person with a small frame can be classified as underweight.
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