At the center of a recently released study is an abbreviation that has been around since the 1800s, but many people have no idea what it means, or whether or not it matters.

BMI, the body mass index, is universally accepted by physicians as a reasonable indicator for body-fat percentage — although it does not measure actual body fat. Doctors and trainers say an individual’s BMI is an incomplete measure of their overall health and fitness, but for many people that rank higher on the spectrum, the number can be reduced with diet and exercise.

The obesity study published recently by the Trust for America’s Health and the Robert Wood Johnson Foundation used BMI to project a drastic increase in health-care costs if obesity rates in the U.S. continue to climb at current rates.

BMI is derived via a formula that divides a person’s weight by their height squared, multiplied by 703. (No need to be a math whiz to do this; there are plenty of resources online or at your doctor’s office.) The resulting number is the BMI: below 18.5, underweight; 18.5-24.9, normal; 25-29.9, overweight; 30 and above, obese.

A number of factors make the BMI formula an inexact indication of how overweight someone is. Women tend to have more body fat than men; body-fat percentage may increase with age; and very fit athletes may have a higher BMI due to increased muscle development. The standard BMI chart is not used for children under 4 feet, 1 inch, and it does not factor in other health risks such as high blood pressure, high glucose, smoking and family history.

The medical community still sees the indicator’s value.

“BMI is the national standard,” according to Dr. Antoinette DeIngeniis-DePasquale, who treats overweight patients at the Medi Weight Loss Clinic in Wayne, N.J. “And it makes it much easier for us to see who is obese and who is at risk.”

DeIngeniis-DePasquale does note, though, that BMI is only part of the picture. Although it is routinely used at her clinic, she says, it is part of an overall evaluation, based on many other factors.

One reason for that is that BMI isn’t accurate in all cases. As certified fitness trainer Gary Schulman of Oradell, N.J., is quick to say, “I’m 5 foot, 5 inches and weigh 160. And I’m not overweight.” Even if his BMI says that he is.

Schulman, who also is an instructor at Faustini’s Institute of Martial Arts in Oradell, isn’t anti-BMI, but says, “it bothers me that they don’t have a footnote in there, explaining that the BMI should be combined with your muscle mass. Most family doctors don’t gauge your actual body-fat percentage, which can be measured with a skin fold caliper or an electronic scale that measures fat through low-grade impulses that shoot through your feet and hands. There is another method, hydrostatic water testing, which I’ve never used, but it’s supposed to be very accurate.”