What is My BMI? (And Does it Matter?)

If you’re trying to maintain a healthy weight, you’re probably familiar with the concept of body mass index (BMI). BMI is the default measure for assessing weight status. 

But it’s not the only measure. It’s probably not even the best. 

For instance, bodybuilders are often classified as “obese” by the BMI chart.[*] If you’re telling me the Terminator is obese, I’m not buying. 

Other measures—like body fat percentage and waist circumference—may track more closely with consequential health outcomes[*], but they still aren’t perfect. The ideal ranges are based on population averages, and your mileage may vary. 

Keep reading to learn the basics of BMI, how it relates to health, BMI vs. body fat percentage and waist circumference, and how to think about healthy weight.  

What Is BMI?

Body mass index (BMI) is a metric meant to determine if your weight is healthy. The calculation entails dividing your weight in kilograms by your height in meters squared—or by dividing your weight in pounds by inches squared times 703—but save yourself the hassle and use this BMI calculator instead. 

With your BMI in hand, you can see where you fall on this BMI chart[*]:

• Underweight: BMI less than 18.5
• Normal weight: BMI 18.5 to 24.9
• Overweight: BMI 25 to 29.9
• Obese: BMI over 30 

Furthermore, obesity is often subdivided into categories:

• Class 1: BMI of 30 to <35
• Class 2: BMI of 35 to <40
• Class 3: BMI of 40 or higher. Class 3 obesity is sometimes categorized as “severe” or “morbid” obesity.

The chart, as you can see, wants your ideal BMI to be between 18.5 and 24.9. That’s what’s considered a healthy or “normal” weight. 

What if you aren’t in that range?

BMI-Related Risk Factors

Someone is considered obese when their BMI exceeds 30. Being obese, in turn, is correlated with a multitude of potential health risks. 

Many researchers now consider obesity to be a chronic disease.[*] Whether or not you agree, the data shows that being obese increases one’s risk for other chronic diseases like diabetes, heart disease, cancer, sleep apnea, high blood pressure, and neurodegenerative disease.[*] In fact, diabetes is so closely related to obesity that researchers have coined the term “diabesity” to describe the excess accumulation of body fat seen in people with type 2 diabetes.[*] 

Conversely, a BMI of under 18.5 is also correlated with health risks. Being underweight has been linked to:

• A 19.7% higher risk of heart disease compared to normal-weight people.[*] (Note: overweight people had a 50% higher risk and obese people a 96% higher risk).
• Increased fracture risk[*]
• Increased dementia risk[*]
• Sarcopenia (age-related muscle loss)[*]
• Increased all-cause mortality risk (more so than being obese, according to a Swedish study)[*]

Ideally, you want to avoid both extremes of the BMI chart. There are problems, however, with relying solely on BMI to assess healthy weight. 

Does BMI Accurately Reflect Healthy Weight?

On a population level, BMI is a decent measure of weight status. Those with BMIs over 30 tend to be obese people with excess body fat who would benefit from weight loss—and those with BMIs under 18.5 tend to be underweight people who would benefit from weight gain. 

But for very muscular people, the BMI chart is fairly useless. Since muscle is denser than fat, a bodybuilder could easily have a BMI of 35. Does this person need to lose weight? Probably not. 

Also, a “normal” BMI doesn’t guarantee good health. A person with a normal BMI can still have significant amounts of visceral fat around their organs. In some ways, this presentation (called “skinny fat”) denotes higher health risks than obesity.[*] 

It appears there may be a genetic component here. For example, Asian people were noted to have more visceral fat, cardiovascular disease, and metabolic disease at BMIs that would be considered normal for white people.[*] For example, lower cut-offs (i.e. a BMI of >27 being considered “obese”) have been suggested for certain populations.[*]

The takeaway is that BMI is a useful but flawed measure. Let’s look at some other measures now.  

BMI vs. Body Fat Percentage vs. Waist Circumference

The problem with obesity is an excess of body fat, not an excess of body weight. Because of this, it makes sense to measure body fat.

Measuring body fat percentage (ideally with a DXA scan, the gold standard of body composition testing) not only measures adipose fat (fat under the skin) but also visceral fat. This offers a better picture of your overall metabolic health than BMI. 

What’s an ideal body fat percentage? Most sources say men should be between 10-22% and women between 20-32%, but within these ranges, there’s no consensus on what’s best for health and longevity.[*] 

For instance, if a woman goes from 40% body fat to 30% body fat, that’s almost surely beneficial. 

But what if she goes from 30% to 20%? We don’t know. It probably depends on the person. 

Along with body fat percentage, waist circumference is also probably superior to BMI for assessing healthy weight. When you use waist circumference, you won’t misclassify any bodybuilders as obese. 

In one analysis, waist circumference was a strong predictor of high blood pressure, high cholesterol, and diabetes in almost 15,000 people.[*] To minimize these risks, the NIH advises us to strive for waist circumferences under 40 inches (for men) and 35 inches (for women). 

What’s the Best Way To Assess Healthy Weight?

If you had to choose, body fat percentage and waist circumference are probably better measures than BMI. They’re better indicators of how much body fat you’re gaining or losing. 

Even so, it’s possible to over-emphasize these metrics. They’re important for tracking your health progress, but they’re not everything. 

Your body fat percentage says little about your sleep quality, energy levels, exercise performance, and mood. You need to assess these crucial metrics separately. 

If these metrics are improving, it probably means your diet and lifestyle are on point. And when your diet and lifestyle are on point, your health tends to handle itself.