My fitness trainer is morbidly obese — at least according to his body mass index (BMI), a measure of height and weight that many people use as a health gauge. But this old-school measure is entirely imprecise. A new study reveals that using BMI incorrectly labels more than 54 million Americans as “unhealthy,” even though they are not. In fact, since muscle weighs more than fat, individuals who are particularly lean and muscular can have BMIs that classify as obese; while slender folks with too little muscle have beautiful BMIs. Weight and height are just two of numerous biomarkers to consider when assessing your health trajectory, but if calculators are your thing, check out A Body Shape Index, or ABSI. The measure can reveal more about your risk for dangerous, life-threatening diseases than your body mass index (BMI), weight or waist size alone.