If nearly two thirds of Americans are overweight, and one third of those individuals are obese, just where do you fit in? And what does your weight mean for your health?

For medical professionals, body mass index, or BMI--a measurement of your weight in relation to your height--is one of the best ways to assess the impact of weight on future health risk. Being overweight means having a BMI of 25 or greater. To earn the dubious distinction of being obese, your BMI must be 30 or greater.

To calculate your BMI, divide your weight in pounds by your height in inches, squared, and then multiply the result by 703. You can also calculate it online at http://nhlbisupport.com/bmi/bmicalc.htm

For example, a man five feet nine inches tall who weighs 149 pounds would have a BMI of 22, which is considered a normal, healthy weight. If he gained 20 pounds, he would be considered overweight. And if he gained an additional 34 pounds, tipping the scales at 203 pounds, he would be considered obese.

Experts at the Duke Diet & Fitness Center, one of the nation's oldest and most successful weight-management centers, say moderate health risks begin at a BMI of 26 to 27. That's a level that would make a person 20 percent overweight. For people with higher BMIs, the higher the BMI, the greater the risk of developing additional health problems.

Besides premature death, high blood pressure, high cholesterol, diabetes, and heart disease, says Howard Eisenson, a physician who is the director of the Duke Diet & Fitness Center, those additional health problems include angina, heart failure, respiratory problems, sleep apnea syndrome, asthma, and a poor capacity to perform any sort of exercise.

"You're less able to function and enjoy hobbies, and the activities of daily living are more limited," he says. "Just about every organ system can be affected. You can have problems with urination, frequency of urination, urgency of urination, urination incontinence, gastroesophageal reflux, indigestion, swelling of legs, skin infections, skin breakdown, and gall stones. Cancers are also more prevalent in people who suffer from obesity. It's quite a huge list."

Because BMI is a crude estimate and doesn't take into account an individual's relative proportion of muscle mass and body fat, it isn't a good guide for lean football players and other athletes with muscular builds, most of whom would be classified as obese from their height and weight measurements. Nor is it necessarily accurate for the elderly and others who have lost a significant proportion of their muscle mass and can be considered underweight by BMI standards while having a high percentage of body fat.

A better method of estimating the degree of leanness or fatness for those individuals is a skin fold caliper test, a common procedure used by fitness professionals to measure body-fat percentages. Body-fat measuring scales and other instruments that gauge an individual's body-fat percentage using an electrical current are generally much less accurate because their estimates vary widely depending on whether you're fully hydrated or not.

If you're obese or significantly overweight, like most Americans, Eisenson advises against striving too quickly to achieve what the BMI charts might suggest as your ideal body weight. "People who are significantly overweight are not served well by the message of 'here's your ideal body weight,' because it's so challenging," he says. "It's really not realistic for most people who are significantly overweight to achieve that body weight. Ultimately, they become discouraged and so does their health-care provider. I think people in the pursuit of an ideal weight often subject themselves to extremes. A more honest message, a more realistic message is: Let's try to achieve a healthier weight."

-- Kim McDonald