What Is a BMI Ratio?

The National Institute of Diabetes and Digestive and Kidney Diseases estimates that 66 percent of the U.S. adult population is overweight or obese. The body mass index (BMI) is a standard measure of the body's mass and fat ratio used to screen people for health risks. Before you undertake any changes, knowing your BMI can provide a helpful starting point.

History of the BMI

Study of the body's fat, weight and height are not new, but the search for a reliable measure became increasingly important as the life insurance industry and medical concerns about weight and disease grew. As early as 1897, R.L. Livi published a formula in his "indice ponderale" that became known as the ponderal formula, which was weight divided by height squared. Adolphe Quetelet later devised a formula that relied on weight divided by height cubed, but it did not refer to body fat or mass. In the July 1972 issue of "Journal of Chronic Diseases," Ancel Keyes and associates analyzed the reliability of these formulas and concluded that weight divided by height squared was the best indicator of body density, calling it the "body mass index." The BMI was best at estimating body mass given the differing proportions of bodies.

How the BMI is Computed

The BMI is computed using the formula decided upon by Keys -- weight divided by height squared times a newer conversion factor. This makes it easier to work with the number. For example, a 5-foot-5-inch adult woman weighing 150 lbs. would be 150 lbs. divided by 65 x 65 inches, or 4,225, times the conversion factor 703, resulting in a BMI of 24.95, or 25 rounded.

Examples of BMI Categories

Currently, the BMI numbers for adults are divided into four categories. A BMI of below 18.5 is considered underweight. Between 18.5 to 24.9 is normal. A BMI of 25 to 29.9 is overweight. More than 30 is obese. Thus, a woman with a BMI of 25 is considered overweight. There are individual variations that do not match these categories like athletes whose BMI may be due to more muscle. Generally, women have more body fat than men.

Significance of the BMI

Use of the BMI is common because it is a relatively reliable indicator of body fatness that is calculated the same way for adults 20 years old and older. For children and teens, interpretation of BMI is gender and age-specific. It is inexpensive and easy to use as a screen for health problems that can lead to further testing, if warranted.

Health Risks Indicated by a High BMI

As a screening method, use of the BMI can lead people to take action for prevention or reduction of risks for serious illnesses and conditions. Among the many conditions related to high-body density are type 2 diabetes, liver disease, high blood pressure, coronary heart disease, stroke, sleep apnea and cancers of the endometrial tissue, breast and colon.