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Over half the American population is overweight, and nearly one quarter are obese. People don’t need studies or experts to know that a growing girth will take a toll on their health. But it may take more than a mirror or an arbitrary "goal weight" to get on a leaner road.

Experts have found that body mass index, or BMI may be a better indicator of health risk than absolute body weight. In addition, where you gain the weight may be equally important in determining your risk for certain diseases. Though it is only one of a number of considerations that health experts use to determine these risks, BMI is more highly correlated with body fat than any other measurement of height and weight. Below, Anne Wolf, obesity expert and instructor of research at the University of Virginia School of Medicine, offers simple instructions for determining body mass index, and talks about the usefulness of BMI in assessing health risk and as a goal for weight loss.

What is body mass index, or BMI, and what does it measure?
Body mass index uses a person's height and weight to assess that person's risk of health problems.

How is measuring BMI different from just asking someone to step on a scale?
Weight alone is not a very good measure of body fat. BMI is highly correlated with body fat, and, subsequently, health risk.

Is BMI a new way to assess a person's health risk?
Doctors used to assess a person's "ideal" weight using height measurements, but they then realized that it was sending a very bad message. It implied that each person has an ideal weight for their height, which is not true. In fact, there is a range and it is more useful to understand that by falling into certain categories your risk of diseases goes up. BMI is based on health risk, not on how you look.

How is BMI calculated?
The formula is a little complicated. The first step would be to multiply your weight in pounds by 703. Next, square your height in inches. So if you are 65 inches tall, you're going to multiply 65 by 65. The third and last step is to divide your weight by your height. That will give you your body mass index. For instance, if you are 124 pounds and 5 feet tall (60 inches), your BMI would be 24.2.

And what is considered a healthy and an unhealthy BMI?
A BMI of less than 18.5 would be underweight. A BMI between 18.5 and 24.9 would be considered a healthy body weight. Overweight is categorized as 25 to 29.9, and obesity is categorized as a BMI of 30 and above.

Does BMI account for body proportion?
No. And in fact, one of the other measurements that the National Heart, Lung and Blood Institute recommends using waist circumference with BMI to access your health risk. Experts know that a high waist circumference-or a high amount of abdominal body fat-is very dangerous, and independent of a high body mass index.

So if someone has a body mass index in a healthy range but they have a very high waist circumference, then they still are at risk of developing disease states.

What sorts of health risks does a high BMI pose?
People with high BMI are at very high risk of developing type 2 diabetes and cardiovascular disease. That includes coronary heart disease, hypertension, stroke and enlarged heart.

You're also at a higher risk for insulin resistance, infertility, polycystic ovary syndrome and certain types of cancers, particularly those of the reproductive system.

“We've also seen associations with gall bladder cancer and colon cancer in men,” says Wolf, “as well, and some other smaller types of cancer.”

A high BMI is also associated with structural problems. A lot of people who have a body mass index over 30 have many more hip and knee problems, osteoarthritis and also problems in the hand.

So the health risks are pervasive. They're not just diabetes or heart disease, but the whole body is affected.

Is BMI genetic?
If your family is heavy, you are more likely-for genetic reasons-to be more heavy set than other people. However, genes don't change, and in the last 20 years American body mass indexes have continued to increase dramatically. “It's not a shift in our genetic makeup, it's a shift in our environment,” says Wolf. “We have less physical activity, we are eating more, and we are gaining more weight.”

Is BMI a good measurement of health risk for everyone?
BMI is good for most of the general population. There are, however, people with a lot of muscle-athletes for instance-who have a high BMI. But the high weight part of the equation is more a result of muscle, not fat, and they are not at increased risk of health problems.

There's common sense that has to come through. You can look at someone and say, “They're in really good shape. They have high BMI, but it's because they're fit.” And you won't see a BMI over 30 if someone is in shape.

So what are the major factors that need to be considered in assessing someone's health risks?
“We would measure their weight and their height, calculate their body mass index, and we would measure their waist circumference,” she says. A waist circumference lower than 35 inches for women and lower than 40 inches for men is considered good.

Doctors would also look at family history of disease. “If someone has a lot of type 2 diabetes or cardiovascular disease in their family, that tells me they are at higher risk,” says Wolf. “We also look at their current health. Do they have high blood pressure? Do they have type 2 diabetes or joint pains?”

Doctors would also assess their physical activity patterns and dietary history. “That's a little bit harder to assess, but ideally you want to get a full picture of that person,” she says. “If they are overweight but they are active and they feel really good, they're a lot healthier than the person who is sedentary and thinner.”

What are realistic goals for lowering a high BMI?
The BMI is only related to health risk, not appearance. Health risks start to increase as BMI gets above 25 and dramatically increase with a BMI of 30 and above. Since you can't change your height, the only way to shift to a lower number in the BMI range is by lowering body weight. But doctors know that health risk improves with just a mild weight loss. She says, “even a 5 percent weight loss can result in profound health improvements.”