Body Mass Index (BMI) is a metric used for measuring health. It’s calculated by dividing a person’ weight in kilograms by the square of the person’s height in meters. If you prefer to use English units, it’s your weight in pounds divided by the square of your height in inches, then multiplied by 703.

BMI doesn’t scale well. A tall man with the exact same build and body composition as a shorter man will have a higher BMI. Secondly, the measure ignores variation in body shapes. Some people are slender, others are stocky. Moreover, people carry fat in different places. Subcutaneous fat just below the skin is generally not associated with a steep rise in mortality, while abdominal fat is. Finally, BMI does not differentiate between fat and muscle mass. This glaring drawback means that many muscular athletes are considered overweight, or even obese. ¹

BMI is one of the most widely used measures of obesity. But it’s apparent that it is flawed in more than one way. Being overweight may not be as unhealthy as it was 40 years ago. Researchers have found a BMI of 27 is linked to the lowest rate of death, but someone with a BMI of 27 is currently classified as being overweight.

Here are the CDC numbers for BMI:

Below 18.5—underweight
18.5-24.9—normal or healthy weight
25.0-29.9—overweight
30.0 and above—obese

Researchers looking at 120,528 people from Copenhagen found the lowest risk of having died from any cause was a BMI of 27 in people in the 2003-2013 group. The study showed that in this time frame, there was no difference between the death rates of people with a BMI of 18.5 to 24.9 (healthy), and those with a BMI of 25 to 29.9 (overweight), which were 4 per 1,000 per year for both groups. ²

In what could be the death knell for BMI, research out of UC Santa Barbara and UCLA reveals that millions of Americans labeled overweight or obese based on their BMI are, in fact, ‘perfectly healthy.’ The findings suggest that 34.4 million Americans considered overweight by virtue of BMI are actually healthy as are 19.8 million who are considered obese. According to Jeffrey Hunger, a doctoral student in UCSB’s Department of Psychological & Brain Science, and a co-author of the paper, BMI is a deeply flawed measure of health. “In the overweight BMI category, 47 percent are perfectly healthy,” he said. “So to be using BMI as a health proxy, particularly for everyone within that category, is simply incorrect. Our study should be the final nail in the coffin for BMI.” ³