Body Mass Index (BMI)

The Body Mass Index is an indirect measure of an individual's body fat content, by means of a correlation with height and weight. In other words, the person's body fat is not directly measured, rather an indication is obtained by entering the height and weight into a formula. That formula is as follows:

BMI = Weight / Height²

That is to say, the Body Mass Index is the person's Weight divided by the square of that person's Height. If the data is given in metric units (kilograms for weight and meters for height) then the formula can be used directly as above. However, if one is using pounds for weight and inches for height, the formula is modified as follows with a conversion factor of 703.

BMI = [Weight / Height²] x 703

For adults who are 20 years of age or older, the results of this body mass index calculation can be interpreted in the following way. If the Body Mass Index is below 18.5, the individual is thought to be underweight. If the BMI is between 18.5 and 24.9, the person is thought to be in the normal or healthy weight range. If the Body Mass Index is between 25 and 29.9, the individual is assessed as being overweight, and if the BMI is 30 or more, the person is considered to be in the obese category. It is important to note that in the case of younger people below the age of 20, a different scale of interpretation is used.

While the BMI is only an indirect measure (it does not involve any direct physical measurement of body fat) it is by far the most convenient and inexpensive approach, requiring only scales and a tape measure. It is important to measure the height and weight accurately. With regard to height, shoes should be removed, and any hair ornaments or braiding removed as well, to ensure the accuracy of the height measurement. The individual should be standing on a non-carpeted floor, against a flat wall. The height should be ascertained to within 1/8 of an inch. With regard to the weight, it is recommended that digital scales be used. The scales should be on a firm, non-carpeted floor. Once again, footwear needs to be removed. Any heavy items of clothing should be removed prior to weighing. The individual should stand in the middle of the scales.

There are some reservations about the universal applicability of body mass index measurement. For example, women tend to have higher body fat than men at the same BMI. Similarly, older individuals tend to have more body fat than younger people at the same body mass index. Many have also argued that waist measurement is a more useful indicator. However, despite these reservations, BMI continues to be a useful measure, not only of overweight and obesity, but also of risk of obesity-related medical conditions. These include diabetes, high blood pressure, abnormal cholesterol levels, gallbladder disease, heart disease and stroke, and osteoarthritis.

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