Revised: April 6, 2005

How to Calculate Your Body Mass Index (BMI)

The body mass index, or simply BMI, was first adopted as a medical standard, in 1985. It has been debated, argued and attacked and defended - depending on whatever agenda you may be following - throughout the lay press and the medical community since. However, it does remain a valuable guideline for quantifying the ultimate goal, which is body fat. Measuring body fat can also be done using skinfold measurements, using calipers. If you take the sum of the skinfold measurements at 4 locations - biceps, triceps, subscapular, and hip. Body fat can then be estimated from standard tables.(1) Of course, in the laboratory, there are much more accurate ways available to measure actual body fat. These include bioelectric impedence and, the "gold standard," underwater weighing.(2) But these are not practical for large groups of people.

Step One: Compute height into inches and divide by 39.4 to convert to meters. For example: 5 feet 8 inches equals 68 inches, and 68 inches divided by 39.4 equals 1.7 meters. Square that number by multiplying it by itself:

In the above Example, for instance: 1.7 times 1.7 equals 2.9 meters squared.

Step Two: Divide weight by 2.2 to convert to kilograms: 140 pounds divided by 2.2 equals 64 kilograms. Divide the kilograms by the height in meters squared: 64 kilograms divided by 2.9 meters squared equals a body mass index of 22.

Interpretation: A BMI of 27 or more is considered overweight and is consistent with being 20% over the normal weight. The accepted definition of obesity is a BMI of 30 or more; it is 30 per cent overweight.For those of us who have trouble with kilograms and centimeters, let's use the good USA measures: pounds and inches. To calculate your BMI using pounds and inches:

Step 1: multiple your height, in inches, times your height in inches

Example: if you are 5 foot 2 inches tall, your are 62" tall; 62 X 62 = 3844

Step 2: divide the weight in pounds by the results from step 1

Example: if you weight 200 pounds, divide 200 by 3844 to get 0.052

Step 3: Multiply the result from step 2 times 703

Example: 0.052 times 703 = 36; your BMI is 36

The whole formula is: BMI:

Weight in pounds x 703
(Height in inches)²

Or, if you don't want to go through all the math, simply click here click here for an online calculator for your BMI.

Category BMI Underweight Below 18.5 Normal 18.5 - 24.9 Overweight 25.0 - 29.9 Obesity 30.0 and Above

 

BMI is a reliable indicator of total body fat, which is related to the risk of disease and death. BMI correlates, in large populations, with body fat. You can calculate predicted percent body fat from a given BMI by the following equation:

Percent Body fat = 1.2 (BMI) + 0.23 (age) - 10.8 (gender) - 5.4

Where gender = '1' for men and '0' for women. If you are a 40 y.o. female with a BMI of 35, your predicted percent body fat is:

Percent Body Fat = (1.2 times 35) + (0.23 times 40) - (10.8 times 1) - 5.4

Percent Body Fat = 42 + 9.2 - ((10.8) - (5.4))

Percent Body Fat = 35

Now, the percent body fat calculated from BMI is just an estimate, but it does seem to have some degree of reliability, at least as an average value.(3)

The BMI is relevant for both men and women but it does have some limits. The limits include:

• It may overestimate body fat in athletes and others who have a muscular build (for example, Shaquille O'Neal, the All-Star center of the NBA, is - by current BMI measures, "obese." But, at 7' 3" and 330 lbs, he is hardly average. As are 60 percent of the players of the NFL. But it does serve as a guide for we normal folks.
• It may underestimate body fat in older persons and others who have lost muscle mass.
• It may misclassify people with short legs relative to their height (I like to think I am in this group; I am assigned a BMI of 30; but I think it is because I have a short-for-my-height inseam of my pants. I have short legs. But, I am OK with my weight and height. My waist-to-hip ratio, by the way is 0.84).
• Appropriate weight gain during pregnancy varies and depends upon initial body weight or BMI level.  Pregnant women should contact a health professional to assure appropriate weight gain during pregnancy. 

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"L'uomo con la pancia è un uomo importante"

(Italian saying: "A man with a tummy is an important man")

Another useful measure of medical risk from obesity is the waist circumference, which determines fat distribution. To determine your waist circumference by placing a measuring tape snugly around your waist. It is a good indicator of your abdominal fat which is another predictor of your risk for developing risk factors for heart disease and other diseases. This risk increases with a waist measurement of over 40 inches in men and over 35 inches in women

Finally, the measurement of the waist-to-hip ratio can further determine your risk from being overweight. According to the FDA, research suggests that more important than the amount of extra weight a person carries is where it is located. Waist-to-hip ratio can be calculated by dividing the number of inches around the waistline by the circumference of the hips. For example, someone who has a 27-inch waist and 38-inch hips would have a ratio of 0.71. A woman whose ratio is 0.8 or higher would be at high risk of weight-related health problems, as would a man whose ratio is 0.95 or above. Numerous studies show that fat in the hips and thighs is less health-threatening than abdominal fat. While other fat cells empty directly into general circulation, the fatty acid contents of abdominal fat cells go straight to the liver, by way of the portal vein, before being circulated to the muscles. This process interferes with the liver's ability to clear insulin from the bloodstream. As blood levels of insulin increase, muscles and other cells become insulin-resistant, and blood glucose levels rise as a result. In response, the pancreas cranks out more insulin, prompting the autonomic nervous system (which controls heart rate, blood pressure, and other vital signs) to produce norepinephrine, an adrenalin-like chemical that raises blood pressure. This sets the stage for the development of diabetes, hypertension, and heart problems. (4)

BMI misclassifies about 5% of people. As I have already mentioned, I am firmly convinced I am one of these. Skinfold measurements misclassify 3-4% of people as does bioelectric impedence. While "obesity-activists" always jump to say "we told you so!" when a study comes out like the news reports which stated that Shaq O'Neal was "obese," for large populations and the general public, the BMI remains a useful tool for assessing a person's weight-related medical risks.

The debate of "BMI versus fitness" continues with two recent research studies reported by the National Institutes of Health.
The two recent studies examining the combined effect of BMI and physical activity on health reached different conclusions. One, a prospective study of 37,878 healthy women enrolled in the Women’s Health Study (WHS) looked at the association between BMI, physical activity, and the development of type 2 diabetes. Each year for 7 years, researchers gathered information on the women’s BMI and physical activity levels, and whether they had been diagnosed with type 2 diabetes in the last year. They divided study participants into six groups based on BMI and activity levels: normal-weight active, normal-weight inactive, overweight active, overweight inactive, obese active, and obese inactive.

As expected, when the researchers looked at BMI and physical activity levels separately, they found that women who were overweight or obese had a higher risk of developing type 2 diabetes than normal-weight women, and inactive women had a higher risk than active women. However, when they examined the effects of BMI and physical activity together, increased activity within the same BMI group—normal-weight, overweight, or obese—offered a negligible reduction in the risk of developing type 2 diabetes, but increased BMI, regardless of activity level, led to a significantly higher risk.

A second study seemed to give oppostite results. This study looked at the relationship of BMI and physical fitness to the development of cardiovascular disease in women. Researchers assessed BMI and fitness among 906 women with suspected coronary artery disease (CAD) who were enrolled in the Women’s Ischemia Syndrome Evaluation (WISE) for diagnostic testing. The women completed two questionnaires, one to assess fitness based on self-reported ability to perform various activities that correlate with treadmill test results, and one to assess average physical activity levels at home, work, and leisure.

Both high BMI and low fitness and activity scores were associated with CAD risk factors such as hypertension, diabetes, and dyslipidemia. However, diagnostic tests showed no difference in the presence or severity of CAD across BMI categories. In contrast, women with low fitness scores were significantly more likely to have obstructive and severe CAD. Follow-up over the course of 4 years revealed that these women suffered more adverse events including stroke, congestive heart failure, and death, regardless of BMI.

In discussing the two studies, researchers from the Cooper Institute, a research and education center in Dallas that focuses on physical activity and health, noted differences in the studies that could partially account for the divergent results. Each one measured different outcomes—development of type 2 diabetes versus adverse cardiovascular events. The diabetes study followed healthy women; the CAD study followed women with suspected heart disease. Each study used different measures of self-reported physical activity with potentially unequal levels of accuracy.

Regardless of the differences in study methods and results, the Cooper Institute researchers concluded that physical activity is the common denominator in achieving both increased fitness and long-term weight management. Rather than focus on the “fit vs. fat” debate, they suggest that the medical community support physical activity to promote both health and weight control.

Clearly, the old Metropolitan Life Insurance weight tables used in medical evaluations for years are definitely not realistic in establishing desireable weights and goals. To be a more useful indicator of health risks, experts advocate broadening the definition of obesity by using three criteria:

1. weight for age and height (rather than for gender and height)
2. waist-to-hip ratio
3. presence of such weight-related health problems (such as hypertension, hyperlipidemia, or diabetes)

As a population-based tool for large studies and epidemology, the BMI serves its purpose. But it should be used, in private practice, as part of the evaluation of the overweight patient. Obesity risks can be further modified by other risk factors. For an evaluation of what your weight might mean to your overall health, talk to your doctor. They can give you guidance as to whether you are at an increased risk and if you should lose weight.

1. Durnim and Worersley; British Journal of Nutrition 32:77-97 (1974)
2. Lukaski, HC; Methods of assessment of human body composition: Traditional and new. American Journal of Clinical Nutrition 46:437-456 (1987)
3. Srinivas N, Saisudha K, Chinnala KM: Obesity: An overview on its current perspectives and treatment options; Nutrition Journal 2004, 3:3
4. Brook RD, Bard RL, Rubenfire M, Ridker PM, Rajagopalan S. Usefulness of visceral obesity (waist/hip ratio) in predicting vascular endothelial function in healthy overweight adults. Am J Cardiol. 2001 Dec 1;88(11):1264-9.