The body mass index, a calculation based on your height, weight and age, is too simplistic, new research shows. The number can be especially misleading in children and the elderly.

BMI, which health statisticians use to determine whether a person is normal, overweight or obese, can’t make the distinction between lean body mass and fatty tissue. So two people could be the same height and the same weight but have very different amounts of body fat. Or another way to look at it, some adults can weigh the same now as they did 20 years ago, and yet have much fatter waistlines.

Science writer Jane Brody writes about the overused and misunderstood number in the New York Times.

An individual’s BMI is calculated by dividing the square of your weight (in kilograms) by your height (in meters). That’s all. The problem is, one kilogram of muscle takes up less space than one kilogram of fat. So you can be quite lean and yet heavy for your height if you’re particularly muscular. Or you can be quite fat for you height and not have the reflected in your BMI.

For kids, Brody writes, BMI can’t account for the change in relationship between lean body mass to height as they get older. Is a child who develops a high level of lean body mass (muscles!) going to be cause for concern to the doctor who only looks at BMI? What about the child developing a lot of fatty tissue but comes in weighing less than the muscular kid (and winds up staying under the doc’s radar).

Experts in the piece say BMI is, however, a good tool for determining the overall health of a population. It’s just bad at determining the health of an individual. Better are body fat pincher tests, particularly around the belly, or waist-to-hip ratios (again, for adults).

I’ve been ignoring BMI for years, frustrated by the calculation for exactly the reasons described. Also? When a BMI determines Brad Pitt and George Clooney are obese? Yes, time to find a rubric that carries a little more, um, weight.

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