Derek Burnett
Derek Burnett is a Contributing Writer at Bottom Line Personal, where he writes frequently on health and wellness. He is also a contributing editor with Reader’s Digest magazine.
In the world of weight loss, health, and fitness, you’ll often hear people talking about body mass index (BMI)…what theirs is, what it means in obesity medicine, and what’s a good BMI versus a bad one. As you become familiar with your BMI, it’s important to have some context around this measurement so you can understand its implications for your own health.
This measure is used to estimate whether someone is carrying too much fat for their frame. It’s arrived at using a simple calculation…You take the person’s body weight in pounds by height in inches squared and multiplying the resulting number by a conversion factor of 703. So if two people each weigh 210 pounds, then chances are the one who is six and a half feet tall is probably carrying less fat than the one who’s only five feet tall, and this will be borne out in their BMIs. The tall person in this example has a BMI of 24.3, which is considered normal. The short person’s BMI is 41, considered obese.
Many doctors like using BMI as a measure of body fat because it’s intuitive and easy to understand. Conceptually, there are only two dimensions to consider, which makes it a very accessible tool for the general public.
The concept of the body mass index was developed in the 1830s, not by a medical doctor but by a Belgian mathematician, sociologist, and astronomer named Lambert Adolphe Jacques Quetelet. Quetelet was not particularly concerned about obesity when he came up with the BMI formula. Instead, as a social scientist, he was seeking to arrive at a model for “the average human man,” and set out to collect BMI data on a broad population. While not quite a eugenicist (the study of selective breeding), Quetelet thought to use the average BMI as a measure of “perfection,” with any significant deviations from it constituting “deformity and disease.” While he made some effort to collect a representative sample of BMIs, his study population fell far short of universality.
In the decades that followed, the idea of weight-versus-height as a predictor of health and longevity took root in the insurance and medical fields. In 1972, for instance, with heart disease and obesity on the rise, American dietician Ancel Keys, who is also credited with popularizing the Mediterranean diet, published a paper with colleagues concluding that Quetelet’s simple measure was as good as any other for predicting obesity health risk, and termed it the “body mass index.”
With Keys’ support and certification, the body mass index developed into even broader use. By 1985, the National Institutes of Health had incorporated the measure into its definitions of obesity and overweight. Since then, it has become a staple for doctors and trainers, as well as patients concerned about their weight. However, in 2023, the American Medical Association began discouraging doctors from over-relying on BMI to assess their patients’ health, urging them to also consider other measures of obesity risk in their total assessment of an individual’s health, including the amount of visceral body fat and waist circumference.
Unlike in the days of Quetelet and Keys, you don’t need a doctor or strong math skills to learn your BMI, nor do you need to know your height and weight in pounds and inches. There are a number of BMI calculators on the internet from reputable sources that let you enter your measurements. Here are three:
BMI has always had its detractors, but in recent years more and more experts have pointed out its shortcomings. The primary arguments against it center on the idea that it is too blunt of an instrument to give a reliably accurate, individualized assessment of risk.
Muscle versus fat. One of the biggest problems with BMI is that it misclassifies people who carry a lot of muscle. Because muscle weighs more than fat, plugging body weight into this simple calculation can result in the misclassification of a very healthy person as being dangerously obese. It’s not hard to imagine two men sharing the same height and weight despite the fact that one is lean and muscular while the other carries a large pot belly. To the algorithms of a health system looking only at BMI, both will be flagged as overweight, despite the actual low body fat of the bodybuilder.
Race, ethnicity, age. BMI-based recommendations have traditionally been based on data that skews white and European, but mounting research suggests that people of some ethnicities, like Pacific Islanders, are healthier at slightly higher BMIs, while others, like Japanese people, are healthier at slightly lower BMIs. And for older people, a slightly higher BMI is generally associated with better health.
Individuals versus population. While critics acknowledge that BMI is a crude tool for assessing the health risk of an entire population, the problem, they say, is when doctors and patients take it as gospel at the individual level.
Other health measures
Because of the limitations of BMI as a measure of actual health risk, experts will sometimes look to other measurements to try to get more granular about the amount of fat a patient is carrying.
Waist circumference. As the name suggests, this is simply a measurement of your size at the waist. It can make a good complement to BMI, since where you carry your fat matters. Abdominal fat secretes more inflammation and hormones into your bloodstream than fat carried elsewhere on your body and is thus considered more dangerous. If someone had an okay BMI but was carrying a lot of belly fat, they might not understand their risk if all they relied on was their BMI. Conversely, a very muscular person on a short frame might have a BMI that looked alarming on paper, while carrying no excess abdominal fat. Men are considered to have abdominal obesity if their waist circumference measures 40 inches or more. For women, that figure is 35 inches.
Waist-to-height ratio. This measure is even simpler than the BMI and has been found to be superior to it as an indicator of cardiometabolic risk. You simply divide your height by your waist circumference. As a general rule, you want the result to be 0.5 or less. In other words, keep your waist circumference half your height, or less.
Caliper method. Frequently used by bodybuilders, this method of estimating body-fat percentage requires some specialized knowledge, and requires two people. Using calipers, the amount of fat under the skin is measured at various sites around the body (including some you can’t reach yourself). Those figures are then entered into a set of formulas to generate a body-fat percentage.
DXA. For people who want to be very scientific about their body composition, dual energy x-ray absorptiometry (sometimes called DXA) is a non-invasive, quick, pain-free measure of bone density, lean muscle mass, and fat tissue in the body. You simply lie on a table while an x-ray arm passes over your body. The readout of your results includes the percentage of body fat, skeletal muscle mass, visceral adipose (abdominal) tissue, and several other measures related to body composition. It’s not usually covered by insurance unless you have osteoporosis. Out of pocket, expect to pay between $150 and $500, depending on location. You can get a DXA scan through a lab company or a special clinic.
