BMI Demystified: What is BMI and How Does it Work?

In our guides on erectile dysfunction, blood pressure and overall health, we occasionally make reference to studies that link body mass index (BMI) with conditions like ED and lower levels of testosterone in men.

BMI is a system of measuring body mass that’s calculated by comparing your overall weight to your height. BMI is widely used in medicine as a screening tool for conditions like obesity. It’s a simple, effective method that provides accurate results for most people.  

While BMI has its advantages, it’s not a perfect system. There are certain situations in which your body mass index might not provide an accurate picture of your health. Below, we’ve “demystified” BMI to explain what it is, what it isn’t and what your BMI information means for your health.

What Exactly is BMI?

BMI, or body mass index, is a simple mathematical system used to calculate whether you’re at a healthy weight, underweight, overweight or obese based on your height and body weight.

Because it only requires two measurements, BMI information is quick and easy to calculate. Search online for “BMI calculator” and you’ll find numerous results, all of which let you quickly calculate your BMI by typing your weight and height into an online form.

If you’d like to calculate your BMI, you can use this calculator from the National Heart, Lung and Blood Institute to get a quick, accurate result. 

The equation used to calculate BMI is extremely simple, making it easy to adjust for imperial or metric data. Using pounds and inches, the BMI formula is weight / (height X height) X 703. For meters and kilograms, the formula is weight (kg) / [height (m)]2. Neat little trick, right?

This simplicity has several advantages. It makes BMI ideal for implementing at scale, such as in a medical study. It also makes BMI cheap and easy to calculate. Because it isn’t complicated, BMI information also serves as an easy number to remember, for both doctors and patients.

However, BMI has some significant disadvantages. The first is that it only takes body weight into account, not body composition. This means that people with a high amount of body fat and little muscle mass, who are “skinny fat,” often have an optimal BMI score.

It also means that muscular people, who have a high body weight due to muscle mass but might have very little body fat, are often grouped in with overweight and obese people.

Finally, BMI can be particularly inaccurate for people on either extreme in terms of height. Both short people and very tall people often have BMI scores that aren’t fully accurate as metrics for determining their health.

BMI is Designed to Provide Accurate Measurements at Scale

We’ll get into these disadvantages in more detail a little bit later. However, it’s important to first cover why BMI is the way it is, as well as the major scientific advantages provided by BMI as a metric.

On the whole, BMI is designed to provide accurate measurements at scale, meaning it can be quickly used to measure rates of obesity in large groups of people.

This makes BMI great for scientific studies of the general population. Using BMI, researchers can quickly calculate obesity rates for thousands of people at once using people’s height and weight data, while also accounting for some estimated margin of error.

Compared to other methods of measuring obesity, this saves massive amounts of time. It’s also significantly cheaper than more complicated body composition tests. The end result is cheaper, faster scientific research and a greater volume of scientific studies into obesity.

BMI Doesn't Account For Body Composition

The first—and biggest—disadvantage of BMI is that it doesn’t account for body composition. This means that if you’re very lean or muscular, your BMI results may not provide an accurate picture of your general health.

For example, a muscular bodybuilder or athlete could weigh 220 lbs while measuring in at 5’10” in height. Using a standard BMI calculator, this person would get a BMI result of 31.6—high enough to place them in the “obese” BMI range.

Even a less muscular person, weighing in at 200 lbs while measuring 5’10” in height, is classed as “overweight” using BMI.

In general, if you have a lot of muscle mass and a relatively low body fat percentage, BMI isn’t a reliable metric. Because the BMI system only factors in body mass, it can’t differentiate between muscle and fat, and consequently won’t provide accurate results for people with a lot of muscle mass.

If you’re a muscular person and would like to learn more about your body composition, your best option is to measure your body fat and muscle mass percentage using a more accurate system, such as DEXA or hydrostatic body composition analysis.

BMI Misses Normal Weight Obesity, or “Skinny Fat”

Another disadvantage of BMI is that it often provides inaccurate results for people with “normal weight obesity.” This condition involves having a normal or “healthy” body weight, albeit with a higher than optimal percentage of body fat.

If a person has very little muscle mass, they can remain in the “healthy weight” BMI range despite having similar body composition to an obese person.

People with normal weight obesity are occasionally referred to as being “skinny fat,” since their overall body weight is closer to a skinny person than an obese person. Despite having a normal body weight, these people often have a higher-than-normal risk of conditions like heart disease.

If you have a normal BMI result but feel concerned about your body composition, it’s best to talk to your doctor about using a more detailed test to measure your body fat percentage and overall body composition.

BMI Isn’t Optimal for Very Tall or Very Short People

As we said above, BMI is designed specifically to provide accurate data on body weight and obesity in large samples of the general population. However, its third disadvantage is that it doesn’t provide accurate data for people who are very tall or very short.

The formula used to calculate BMI doesn’t take into the extra skeletal weight of extremely tall people. This means that tall, fit and healthy people often get BMI results that place them in the “overweight” or “obese” BMI ranges. 

For example, a man measuring in at 6’6” in height with a weight of 230 lbs scores 26.6 using BMI—enough for him to be considered overweight.

BMI also has disadvantages for short people. A 2013 review of the BMI system from Oxford University found that the BMI system inaccurately measures short people, causing them to be “misled into thinking that they are thinner than they are.”

As a general rule, if you’re less than or approximately five feet in height, or taller than six feet, it’s often worth taking your BMI results with a grain of salt and talking to your doctor about an alternative method of measuring your body composition.

Should You Take Your BMI Results Seriously?

While the BMI system isn’t perfect, it’s still a useful tool for measuring body weight in relation to height.

One way to think of BMI is as a signal of potential weight problems. If you have a BMI that puts you in the overweight or obese BMI ranges, it’s worth seeking additional information—either via a body composition test or advice from your doctor—on what it could mean for your health.

Likewise, an overly low BMI score that puts you in the underweight category can also signal that it’s worth talking to your doctor about the potential effects your weight could have on your health and wellbeing.

This article is for informational purposes only and does not constitute medical advice. The information contained herein is not a substitute for and should never be relied upon for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.