How Accurate Is Body Mass Index, or BMI?

Medically Reviewed by Louise Chang, MD on February 12, 2008
4 min read

What's your number -- under 25 or over 35? Body mass index (BMI) may not be a term that's on everyone's lips, but it's important for your health to understand what it is and to know your number.

Essentially, BMI is a simple mathematical formula, based on height and weight, that is used to measure fatness. You should be aware of your BMI because of the health risks of being overweight (that is, having a BMI of 25 or over). According to a report in the August 2006 New England Journal of Medicine, excess body weight during midlife is associated with an increased risk of death.

On the other hand, being too thin and having a BMI that's below the healthy range (18.5 to 24.9) can also be a health concern.

Many health care experts think BMI is a useful tool to measure weight and health risks, but others question its accuracy. Some believe a better way might be to take out the tape measure and check your waist circumference. Or is there a place for both methods?

In June 1998, in an effort to make sure doctors, researchers, dietitians, and government agencies were all on the same page, the National Institutes of Health announced its BMI guidelines. They replaced the old life insurance tables as a method to gauge healthy weight.

To calculate your BMI, divide your weight in pounds by your height in inches squared, then multiply the results by a conversion factor of 703. For someone who is 5 feet 5 inches tall (65 inches) and weighs 150 pounds, the calculation would look like this: [150 ÷ (65)2] x 703 = 24.96. An easier way is to use WebMD's BMI calculator.

(This BMI calculator is for use in the evaluation of adults, not for children.)

According to the NIH definitions, a healthy weight is a BMI of 18.5-24.9; overweight is 25-29.9; and obese is 30 or higher.

BMI is the measurement of choice for most health professionals.

''I think BMI is a very good and easy screening tool,'' says obesity expert, Cathy Nonas, MS, RD, a spokeswoman for the American Dietetic Association.

But while it is a simple, inexpensive method of screening for weight categories, it is not a diagnostic tool. Health professionals need to do further assessments to fully evaluate health risks. These assessments would include measurements of body fat percentage, diet history, exercise patterns, and family history.

Further, BMI does not take into account age, gender, or muscle mass. Nor does it distinguish between lean body mass and fat mass. As a result, some people, such as heavily muscled athletes, may have a high BMI even though they don't have a high percentage of body fat. In others, such as elderly people, BMI may appear normal even though muscle has been lost with aging.

Take for example, basketball player Michael Jordan: ''When he was in his prime, his BMI was 27-29, classifying him as overweight, yet his waist size was less than 30,'' says Michael Roizen, MD.

That's one reason some experts think waist circumference can be a better overall health measurement than BMI.

Another is that your health is not only affected by excess body fat, but also by where the fat is located. Some people gain weight in their abdominal regions (the so-called ''apple'' body shape.) Others are ''pear-shaped,'' with excess weight around the hips and buttocks. People with apple shapes are at higher risk for health problems associated with being overweight.

''Fat around your waist is more biologically active and can do more damage to your body than weight around your hips," says Roizen, co-author of You: On a Diet. "The data show that waist circumference is more reliable and more closely correlated with diseases associated with obesity.''

According to the National Institutes of Health, a bigger waist circumference (greater than 40 inches for men and 35 inches for women) is linked to a higher risk of type 2 diabetes, high blood pressure, abnormal cholesterol levels, and heart disease when BMI is 25 to 34.9.

To properly measure your waist, no math is needed. Just use a soft tape measure around your bare midsection at your belly button. Find your upper hip bone, and measure around the abdomen above the bone. The tape should be snug, but not dig into your skin.

Nonas argues that waist circumference is not a better tool than the BMI ''because we do not have good criteria or cut points for levels of overweight, obesity, age or height.'' She also thinks that properly measuring the waistline is a little more difficult than measuring height and weight.

One thing that experts agree on is that weight is only one factor in our risk for disease. When it comes to evaluating weight and its impact on health, your percentage of body fat, waist circumference, BMI, and physical activity patterns are all important.

The National Heart, Lung, and Blood Institute recommends that health care providers assess BMI, waist circumference, and any other risk factors for obesity-related conditions. Combining all of the information provides the best assessment.

The first step toward shrinking your waistline and getting your BMI in line is to start eating a healthier diet and getting regular exercise. Preventing any further weight gain and slowly reducing weight into a healthier range is an excellent goal.

And while you might want to lose more, dropping as little as 5%-10% of your body weight can bring dramatic improvements in blood pressure, blood cholesterol, and blood sugar.

Nonas recommends four steps to a healthy lifestyle:

  • Being physically active.
  • Making healthy food choices.
  • Avoiding overeating.
  • Scheduling an annual physical examination.

''These are the vital parts to maintaining a long and healthy life,'' she says.