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BMI: Everything You Need to Know About Body Mass Index

Body Mass Index – or BMI calculator – is a popular and deeply flawed tool to gauge obesity and health; but why is it so inaccurate?



BMI Body Mass Index
Image Credit: Pexels

Clinical obesity represents a growing cause for concern in the U.S., the developed world, and beyond. Now, in America specifically, childhood obesity is being diagnosed with alarming frequency. One tool heavily relied upon by doctors to determine if a patient qualifies as overweight or obese by clinical standards is the Body Mass Index or BMI. However, this simple calculation which has become a cornerstone in determining one’s health just might be cracked with new research showing the unreliability of this pervasive measurement. Meanwhile, superior measurements exist that provide a more precise description of body fat content and thus a more accurate overall sense of a patient’s degree of obesity.

The Intricacies of BMI

Since the index’s publication in 1972, the BMI has been used to measure body fat and overall health in an individual. Its popularity has much to do with ease of use and cost effectiveness. If you question the measurement’s actual significance outside the doctor’s office, know that BMI has been utilized by U.S. employers to provide financial incentives for employees to lower their BMI.

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This is because one’s weight issues or propensity for health complications are considered by health insurance companies. Poor health means more doctor visits; the more one visits a clinic or doctor’s office the more their health insurance provider must pay.

How the BMI Calculator Works

BMI is determined by two factors, an individual’s height and weight. To calculate it, one must divide their weight in kilograms (kg) by the square of their height in meters (m²). If this jargon seems a bit confusing, just take your height and weight and convert your recorded measurements to the units just mentioned.

  • BMI is a calculation known for dividing people into one of four categories:
  • Those with a score less than 18.5 are classified as underweight.
  • Those with a score between 18.5 and 24.9 are considered normal or healthy weight.
  • Individuals with a score of 25 to 29.9 are considered overweight.
  • Lastly, individuals with a score of 30 or greater are deemed obese.

It’s important to note, however, that basing one’s health on their height and weight alone fails to account for body’s bone, muscle, or fat proportions.

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Muscle, for example, is a tricky topic. An individual who is particularly muscular, with a very small percentage of body fat, possessing a BMI between 25 and 30 are likely to be considered overweight. This is almost certainly the case for many professional football players. Nevertheless, that’s not an accurate assessment of the individual’s health status.

Lean muscle, or muscle that is independent of, as well as not covered by, fat and fat itself weighs the same pound for pound. Their composition, on the other hand, greatly differs. According to the personal health columnist for The New York Times Jane E. Brody, fat takes up around “four times the space of muscle tissue” (para. 4).

This makes muscle about four times as dense, or thick, as fat tissue. It’s because of body composition nuances such as this that BMI runs into a little bit of trouble.

BMI Calculation Accuracy and Reliability

In a 2016 paper published in the International Journal of Obesity, A. Janet Tomiyama et al. from the University of California, Los Angeles (UCLA), identified BMI as a flawed approach to determining one’s health status.

The paper cited a study also conducted by Tomiyama et al. assessed 40,420 individuals, aged 18 and above, who were a part of the 2005 to 2012 National Health and Nutrition Examination Survey (NHANES).

It’s important to note that this survey at the time contained “nationally representative prevalence data on cardiometabolic health.”

The study found that around 29 percent of individuals classified as obese, and even about 16 percent of those said to have type 2 or 3 diabetes were cardiometabolically healthy.

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Conversely, over 30 percent of individuals said to be normal weight or healthy were cardiometabolically unhealthy. Talk about a dramatic twist!

Additionally, Medical Daily writer Jalessa Baulkman stated that using BMI to gauge health has led to over 54 million Americans being misclassified as unhealthy. It’s because of this that Tomiyama and company (2016) suggested that policymakers consider the consequences of relying solely on this measurement.

The same goes for employers and physicians as well. Ultimately, BMI should be used as a screening tool when trying to determine body fatness, but not as a definitive diagnostic tool for actual health. Since obesity is an ever-increasing health concern, it’s still important that individuals and health professionals correctly measure a patient’s body fat.

In light of the established inadequacies inherent to BMI, what are more accurate metrics to gauge obesity? Below are some alternatives that can be used to more accurately determine overall body fat composition and health status in an individual.

Alternatives to BMI

Tape Measure

A waist circumference, or the distance around your waist, that equals half your actual height can be one indication of good health, as this measurement can be an indication of excess abdominal fat. For example, if you happen to be five foot, eight inches tall (68 inches), then having a waist circumference of 34 inches or smaller would be considered healthy.

According to the Centers for Disease Control and Prevention, excessive abdominal fat is potentially dangerous because it places one at greater risk for developing high blood pressure, Type II Diabetes, and coronary artery disease. To determine waist circumference, one must stand straight and wrap a tape measure around their mid-section.

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The tape measure should be just above one’s hip bones and along the belly button. The person being evaluated should not suck in their gut during this process. Furthermore, taking the average of about three different measurements should give you a pretty reliable number.

Skin Calipers

A skin caliper is a device pulls subcutaneous fat, or fat that lies directly under the skin, away from muscle through use of tongs situated at their ends. A skin-fold assessment can be completed using several different parts of one’s body, such as the abdominals, back, thighs, chest, and arms.

An equation is used to determine body-fat percentage from the recorded measurements. It’s important that a practicing professional conduct this test to produce more accurate results.

Hydrostatic Weighing

This method is also referred to as “underwater weighing.” According to exercise physiologist and fitness consultant Elizabeth Quinn, this method determines one’s “total body density using Archimedes’ Principle of displacement” (Quinn, 2017, para. 1).

Hydrostatic weighing works by comparing a person’s normal body weight on dry land to their body weight when completely submerged in a large tank of water. Accordingly, hydrostatic weighing is one of the most accurate assessments of body fat composition with a very small margin of error. As a result, it’s considered the gold standard for body composition assessment.

Bioelectrical Impedance Analysis

This method measures body composition by sending an electrical current through the body. The faster the current, the less body fat an individual has. Body fat, or adipose tissue, causes greater resistance for an electrical current than fat-free mass, and therefore slows the rate at which the current travels. Bioelectrical Impedance Analysis has been shown to be fairly accurate when estimating body fat.

Air Displacement Plethysmography

Like underwater weighing, air displacement plethysmography, also known as BOD POD, follows a similar technique except with air. As a result, this method has gained popularity among body composition researchers due to the non-invasive test-procedure as well as the lack of technical expertise required.

Researchers David A. Fields, Paul B. Higgins, and Gary R. Hunter have described the BOD POD as “a single fiberglass unit composed of two chambers” (p. 1).

The test chamber accommodates an individual during testing while the reference chamber contains instrumentation that measures “changes in pressure between the two chambers” (p. 1).

Therefore body composition is essentially calculated by measuring the amount of air before and after an individual enters the BOD POD along with the density of air.

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Duel Energy X-Ray Absorptiometry

Also referred to as “bone densitometry,” this method produces pictures of a person’s body internally by using a very small dose of ionizing radiation. Dual energy x-ray absorptiometry measures lean body mass, fat mass, and bone mineral density. Through this method, technicians can determine the breakdown of body composition for each section of one’s body.

Final Thoughts

While the public perception of the desirable body size may have shifted to be more diverse and has started to include more plus-sized physiques, being clinically overweight or obese can still lead to major health complications.

Positive body image aside, having an excess of body fat may cause issues such as high blood pressure, diabetes, and heart disease. The best thing one can do is determine if their unique physique is, in fact, conducive to good health. Fortunately their are many tools other than the potentially  misleading BMI that can be used to answer that question.

Write to J. Stokes at


  • Baulkman, J. (2016, February 06). Normal Body Mass Index Doesn’t Mean You’re Healthy, Study Says; Millions Of Americans Mislabeled As Obese. Retrieved March 17, 2018, from
  • Brody, J. E. (2010, August 31). Weight Index Doesn’t Tell the Whole Truth. Retrieved March 16, 2018, from
  • Fields, D. A., Higgins, P. B., & Hunter, G. R. (2004). Assessment of body composition by air-displacement plethysmography: influence of body temperature and moisture. Dynamic Medicine, 3(1), 3.
  • Quinn, E. (2017, November 30). How to Measure Body Fat With Hydrostatic Underwater Weighing. Retrieved March 17, 2018, from
  • Tomiyama, A. J., Hunger, J. M., Nguyen-Cuu, J., & Wells, C. (2016). Misclassification of cardiometabolic health when using body mass index categories in NHANES 2005–2012. International Journal of Obesity, 40(5), 883.

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Jared Stokes is a Master Personal Trainer with an EdD in Health Education from Columbia University in New York City. Jared is the Co-Founder and current CEO of K3mistry Productions; a multifaceted organization that teaches and promotes advanced media literacy. Furthermore, Jared is Co-Host for the weekly AVIOT Podcast (Atypical Voices In Our Time). Jared Stokes has been TGNR's Health & Fitness contributor since November 2015.