What Is the Purpose of BMI?

The body mass index categorizes people into four weight classes. Your BMI determines whether you are underweight, normal weight, overweight or obese. Although BMI offers a starting point for determining relative body composition, don't get frustrated if you wind up in the obese category and are in tiptop shape.

The Formula

You can determine your BMI using the Centers for Disease Control and Prevention's formula: your weight in pounds divided by the square of your height in inches and multiplied by 703. For example, if you stand 5 foot 3 inches tall -- 63 inches — and weigh 110 pounds, your BMI is 19.48, putting you in what the CDC considers to be a healthy range..

Healthy Range

Of the four BMI categories, only one is normal weight. Pat yourself on the back if you fall into this category. The “normal weight” for BMI ranges from 18.5 to 24.9. Falling into another category could mean you need to change your eating and exercise habits to reach a healthier weight.

Unhealthy Categories

Falling into the "underweight" category does not necessarily mean you are unhealthy -- active people with low body fat can easily fall into this category. The "overweight" and "obese" categories potentially signify a greater issue, but sometimes physically fit, very muscular people land in these categories.

Accuracy

Overall, BMI provides a good general measure of a person's body composition, but it is not foolproof. It does not always account for healthy people who are very tall. It also doesn't take into account the higher body fat required for a normal healthy female. For example, having large breasts could put you in the obese category. Discuss your BMI with your doctor, who can give you the best sense of how well the figure does or doesn't reflect your overall fitness.