BMI

By Karen Asp

A healthy weight goes beyond the number on the scale: There's also your body mass index (BMI), which is a measurement that determines if you're at an appropriate weight for your height by estimating your body fat. To calculate your BMI, your height and weight are entered into a mathematical equation and the resulting number reveals whether you’re underweight, normal weight, overweight or obese.

Though BMI is generally a reliable approximation of body fat, it doesn't always tell the whole picture. "BMI doesn't differentiate between body fat and lean muscle mass," says Wayne Westcott, Ph.D., fitness research director at the South Shore YMCA in Quincy, Mass. and author of several books on fitness. BMI might be a poor indicator of body fat in athletes, people who have a muscular build and those who are heavily engaged in strength training because their dense muscle mass may classify them as overweight when they aren’t. On the flip side, it might underestimate body fat in older individuals and people who have lost muscle mass.

Even with its drawbacks, however, BMI can give you a rough idea of how healthy you are. "You can use BMI to help you evaluate whether you need to make lifestyle changes," Westcott says.

You can determine your BMI by using a calculator, where you'll be asked to plug in your height and weight.

Or you can figure out your BMI with this formula:
(Weight in pounds) divided by [(height in inches) x (height in inches)] x 703

Once you've determined your BMI, use the following chart to determine where you fit in:

Below 18.5: Underweight
18.5 - 24.9: Normal
25 - 29.9: Overweight
30 and above: Obese

If your BMI is 25 or above and you're not an athlete or you don't have a muscular build, you could be at risk for numerous health conditions including heart disease, type 2 diabetes. Talk with your physician about losing weight. On the other hand, if you're underweight, consult with your physician to determine if and how you should gain weight. For women, being underweight can interrupt the menstrual cycle and cause bone loss.