Is BMI a lot of BS?

Is BMI a lot of BS? I don’t think so, but I also recognize that it’s a tool that’s only effective when used with other tools. Just like a hammer isn’t much of a tool without a saw to cut the wood to the correct size, and a nail for the hammer to drive into the wood, you can’t do much using just BMI. Body Mass Index (BMI) is a contested method of determining a healthy weight for adults. It’s a calculation using metric height and weight.

Body Mass Index (BMI) is a person’s weight in kilograms divided by the square of height in meters. BMI doesn’t specifically measure body fat, but a high BMI can be an indicator of a lot of body fat. It’s not a reliable way to measure body fat. There are more reliable ways to determine how much fat a person is carrying on his body than using BMI.

Skinfold thickness measurements, bioelectrical impedance, densitometry (underwater weighing), dual energy x-ray absorptiometry (DXA) and other methods are all ways to directly measure body fat. BMI is an alternative to direct measures of body fat. As an alternative, it’s okay, but so far from perfect that BMI alone should never be the only way an individual sets a weight goal.

Many, but not all doctors use BMI to advise patients if their weight puts them at risk for a number of metabolic syndrome diseases.

BMI replaces the old Metropolitan Weight Tables originally introduced in 1943 and updated slightly in 1983. Those health charts weren’t great either, but it’s human nature to want to simplify and categorize that which is neither simple nor can be categorized accurately.

The theory behind BMI is that adults of a certain height have a weight range where there are the fewest early deaths within that range.

The calculation, based on metric height and weight is arranged by indexes. When the body mass index number is too low, there are health risks that are associated with being too thin. When the index number is too high, the health risks are associated with being slightly overweight to morbidly obese. In short, the index number indicates how likely an individual is to suffer health problems related to body weight. That’s fine.

Within the BMI, indexes between 18.5-24, is considered to be a “healthy weight range.”

This is when using the BMI becomes a problem. This healthy range not only is used to identify a weight range that’s healthy because there are fewer weight-related disease within that range, but also just to determine whether one is overweight, obese, and morbidly obese. This isn’t fine.

Screen Shot 2016-05-24 at 10.23.44 AM

The BMI is fine as one tool to decide whose weight is fine and whose weight may not be when applied to the population in general.  

When BMI is applied to a group of people, it's a fairly accurate way to gauge health of the group. Within the group, however, BMI may yield an inaccurate conclusion about how fat an individual may be.

When BMI is applied to a group of people, it’s a fairly accurate way to gauge health of the group. Within the group, however, BMI may yield an inaccurate conclusion about how fat an individual may be.

It’s not fine on an individual, person-by-person basis, and it’s not fine when it’s the only tool. Classifying an individual as underweight, overweight, or obese using just this formula ignores the obvious. Some people have greater lean tissue mass (muscle) and a lower percentage of body fat. More muscle, less fat makes a lean individual deceptively heavy,


Athletes and body builders may have a height/weight ratio that makes their BMI more than “Normal or Healthy” weight. There is nothing unhealthy or abnormal for those people and they are not overweight or obese.

A BMI higher or lower than the normal weight doesn’t mean much by itself. In addition to BMI, there are other important health numbers to consider, including but not limited to, waist-to-hip ratio, heart rate, blood pressure, and blood cholesterol levels. This information is necessary to get an accurate assessment. Doing it any other way makes BMI just a bunch of BS.

Jackie Conn

About Jackie Conn

Jackie Conn is married and has four grown daughters and four grandchildren. She is a Weight Watchers success story. She's a weight loss expert with 25 years of experience guiding women and men to their weight-related goals. Her articles on weight management have been published in health, family and women's magazines. She has been a regular guest on Channel 5 WABI news, FOX network morning program Good Day Maine and 207 on WCSH.