BMI Defined
BMI stands for Body Mass Index, also known as the Quetelet index. It is a heuristic device that combines body weight and height of an individual to tell them something about their health. It does NOT measure body fat directly, but BMI has been correlated to body fat assuming an average body composition. Because it is easy to calculate, it is very commonly used to identify weight problems in individuals. There is another commonly used term, the BMI Prime. This is a comparison of a person’s BMI to the maximum healthy BMI (25 in the United States).
To calculate your BMI:
BMI = weight in lbs. x 703 / ((height in inches) x (height in inches))
To calculate your BMI Prime:
BMI Prime = (BMI Score) / 25
The following table gives good reference for the different BMI levels:
| Category | BMI range | BMI Prime |
| Severely underweight | less than 16.5 | less than 0.66 |
| Underweight | from 16.5 to 18.4 | from 0.66 to 0.73 |
| Normal | from 18.5 to 24.9 | from 0.74 to 0.99 |
| Overweight | from 25 to 29.9 | from 1.0 to 1.19 |
| Obese Class I | from 30 to 34.9 | from 1.2 to 1.39 |
| Obese Class II | from 35 to 39.9 | from 1.4 to 1.59 |
| Obese Class III | over 40 | over 1.6 |
BMI Shortcomings
BMI is one of the most commonly used tools for medical professionals when talking to people about their weight because it is simple to use and understand, and provides a logistical platform for all clinicians to begin from. Be wary when using this method yourself. BMI is an excellent tool if you are of average height, and average body composition. If however, you are very tall or very short, the BMI will not be an accurate reflection of your health. If you are really tall, your BMI may be too low compared to the charts declaring over or under weight. Conversely, if you are really short, your BMI may read too high. The other way the BMI can be misleading is if you have very high or low lean muscle mass. A person with a lot of lean muscle mass will almost always be declared overweight based on the BMI, which makes it a very misleading statistic. In short, be careful when using this heuristic, but do use it to get a basic understanding of where you should be, but if you are very tall or short, or lift a lot of weights, maybe take the value with a grain of salt.
