Body Mass Index

Comments Off on Body Mass Index | April 18, 2012

Body mass index (BMI) is a popular method doctors use to quickly estimate if an individual is underweight, normal weight, overweight or obese. Its usefulness has its limitations because BMI is calculated by dividing weight in kilograms by height in meters squared.

Calculating Your BMI

Use the following equation to calculate your BMI score. You must convert your weight to kilograms and height to meters.

To convert your weight to kilograms (kg), divide weight in pounds (lbs) by 2.2 lbs/kg.

To convert your height to meters (m), divide your height in inches by 39.24.

BMI = weight ÷ (height x height)

BMI scores are expressed as a unitless number interpreted as:

BMI Score Interpretation
Less than 18 Underweight
18-24 Normal Weight
25-29 Overweight
30-39 Obese
40 and Higher Morbidly Obese

BMI Limitations

BMI scores are not gender dependent.

A woman with a BMI of 24 may have an acceptable ratio of body fat whereas a man of the same height and weight with the same score may be overweight since men normally have less body fat than women.

There is also controversy about the validity of a person with a BMI of 24 being classified as having normal weight whereas a score of 25 is considered overweight.

Percentage of body fat is a more accurate measure of a person’s health, but it is also more time consuming and expensive. Doctors can calculate a patient’s BMI score in seconds.

BMI scores have limited usefulness and should be used as a guide.

BMI scores do not take physical conditioning into account, your score can be a less accurate measure of your health than looking at yourself in a mirror. You can’t hide excess body fat.

If you perform strength training on a regular basis to develop muscle mass, BMI scores do not distinguish between muscle and fat. Muscle is denser than fat so a higher than normal weight do to muscle can produce a BMI score indicating an overweight condition. Most doctors are intelligent enough to distinguish between muscle and excess fat. Flab is fat.

Doctors are human and make mistakes. If your doctor is basing a diet and exercise program solely on your BMI score, and you are concerned about your scores accuracy and proposed treatment discuss your concerns with your doctor.