Is BMI Accurate? What Is It and Is It Useful?

When you go in for a wellness exam, oftentimes the first things the medical assistant does is take your height and weight. With this data, the computer spits out your body mass index (BMI) number.

Your BMI could be anywhere from less than 18 to over 40, indicating where you fall on a scale of healthy body weight. This number is also supposed to give an idea of your risk for disease and death based on your weight. 

That seems like pretty important info, but how accurate is BMI for predicting your health? 

As it turns out, this frequently used tool gives useful insight into health risk, but leaves much to be desired in terms of personalization. We’re digging into the pros and cons of BMI, including alternative measurements that might provide a better picture of health.

First, how does BMI work? Well, it’s a measurement of a person's weight in kilograms divided by height in meters squared.

BMI has been used as a health measurement since the mid-19th century. 

Its invention is credited to a Belgian mathematician named Adolphe Quetelet. He was apparently fascinated with the idea of using statistics to establish measurable characteristics of the "normal man." 

BMI went through some rebranding over the years and, ultimately, it was physiologist Ancel Keys who officially coined the term “body mass index” in 1972.

Ironically, none of those involved in creating BMI were healthcare professionals. But fast forward to today and — despite criticisms about its limitations — BMI remains a standard screening tool to measure body composition and assess disease risk in global healthcare settings. 

Want to discover your BMI? Check out the Hers BMI Calculator.

BMI Chart

When your BMI is calculated, that number falls into a specific category that indicates how “normal” or healthy your weight is for your height. 

Here’s a standard BMI chart:

• Underweight: BMI less than 18.5

• Normal weight: BMI 18.5–24.9

• Overweight: BMI 25–29.9

• Obesity Class I: BMI 30–34.9

• Obesity Class II: BMI 35–39.9

• Obesity Class III (Extreme Obesity): BMI 40 or greater

If you don’t fall within the “normal” weight category, your healthcare provider might recommend certain health and lifestyle changes. 

Having an elevated BMI is associated with a higher risk of conditions like heart disease, gallstones, sleep apnea, osteoarthritis, high blood pressure, type 2 diabetes and certain cancers.

Additionally, BMI can be an insightful data point for whether someone should be considering weight gain or weight loss.

Is BMI Accurate?

BMI is a widely used method for assessing weight status, but its accuracy is limited in certain scenarios.

While BMI correlates with health risks associated with obesity at the population level, it may misclassify people with high muscle mass or different body compositions. 

In other words, BMI has pros and cons. Ideally, it should be used alongside other clinical assessments for a comprehensive evaluation of someone’s health status and disease risk.

With all the tools available online, you might be wondering, are BMI calculators accurate? If you want to calculate your BMI outside of the doctor’s office, there’s no shortage of online options. If you want to use these, do sowith caution and remember, it’s only one number in a much larger puzzle to your health.

Is BMI reliable? It depends. There are several advantages to using BMI as a health measurement tool: 

It’s Simple and Widely Accepted

BMI only requires a person's weight and height — AKA it’s easy to use in numerous settings. It’s also widely accepted and recognized by healthcare professionals, organizations and institutions.

It’s a Quick Screening Tool

BMI serves as an initial screening tool for identifying people who might be classified as underweight, overweight or having obesity. It helps healthcare providers quickly assess weight status and identify potential health risks as part of their initial screening process. 

It Provides Insight into Health Risks

Research shows that higher BMI values are associated with an increased risk of type 2 diabetes, cardiovascular disease, certain cancers and even death. BMI can therefore provide valuable insights into someone’s overall health and disease risk.

In fact, in a retrospective study assessing 103,218 deaths, researchers found that those with a BMI of 30 or higher who were classified as having obesity had a 1.5-2.7 times higher risk of death over 30 years. Additionally, a meta-analysis of randomized controlled trials observed a 15% reduction in all-cause mortality when adults with obesity lost weight. 

It Can Help Guide Patient Care

While not diagnostic on its own, BMI provides valuable information to help healthcare providers care for us. It can help initiate conversations about weight management, lifestyle modifications and preventive healthcare strategies.

It’s Cost-Effective

Compared to more complex measures of body composition, such as dual-energy X-ray absorptiometry (DEXA) — which shoots X-ray beams at your bones — or underwater weighing, BMI is a cheaper tool for assessing weight trends on a large scale.

It’s a Component of Preventive Care

By identifying people at risk for obesity-related health conditions early, BMI can allow for early intervention and preventive care strategies. 

While there are plenty of reasons why BMI continues to be a widely used screening tool, there’s also no lack of criticism around using it. For instance, some people wonder, does BMI account for muscle mass and is BMI accurate for everyone?

Not necessarily. Some reasons why BMI is flawed include: 

It Has Limited Accuracy

BMI may not accurately reflect body composition differences among populations, ethnicities, ages and genders.

It may underestimate adiposity in older adults and overestimate it in athletes with high muscle mass. Basically, BMI and muscle mass struggle to get on the same page.

The usefulness of BMI for measuring weight-related health is also limited for shorter people.

It Doesn’t Differentiate

One of the problems with BMI is that it doesn’t differentiate between fat mass and lean body mass. This means it could falsely classify people with high muscle mass as overweight or having obesity.

It also doesn’t account for factors like waist circumference, body fat distribution or metabolic health, which are key factors in obesity-related health risks.

It’s Not Helpful for Certain Populations

BMI may not be appropriate for measuring weight in pregnant women, elderly adults or individuals with significant muscle mass, which can lead to inaccurately interpreted health risks.

It May Have Ethnic and Gender Biases

BMI cutoff points may not be appropriate for all ethnic groups and genders, as body composition varies across populations. Using universal BMI categories may lead to disparities in healthcare outcomes.

It Doesn’t Measure Body Fat Distribution

BMI doesn’t provide information about body fat distribution. Central adiposity, or abdominal obesity, is strongly associated with increased health risks, but BMI alone doesn’t capture this information.

It Doesn’t Measure Fitness

BMI doesn’t account for physical fitness or cardiovascular health. These are crucial components of overall health and may influence disease risk independently of weight status.

It Has the Potential for Stigmatization

Relying too much on BMI as a measure of health may contribute to weight-based stigma and discrimination, overlooking other aspects of health and well-being beyond weight status. 

Simply labeling someone as having obesity doesn’t provide encouragement for healthy weight loss. Rather, people with obesity are more likely to internalize this stigma, making them less confident in their ability to lose weight.

It Can Miss Important Health Risks 

Some individuals labeled as "normal weight" based on BMI may still have an increased risk of obesity-related health conditions because of things like abdominal obesity or other abnormalities in their health, leading to potentially overlooked health risks.

While BMI gets the most air time, there are alternative measurements that can be used to help evaluate body composition and related health risks, like:

• Waist-to-Hip Ratio (WHR): WHR assesses body fat distribution by comparing the circumference of the waist to that of the hips. A higher WHR indicates more central (abdominal) obesity, which is associated with increased health risks such as heart disease and type 2 diabetes.

• Waist-to-Height Ratio (WHtR): WHtR is calculated by dividing waist circumference by height. It takes into account central obesity relative to overall body size. It’s also been shown to be a better predictor of cardiovascular and mortality risk than BMI. 

• Body Fat Percentage: Body fat percentage measures the proportion of fat mass relative to total body weight. It offers a more direct measurement of body fat compared to BMI. Body fat percentage can be measured with skin calipers, bioelectrical impedance analysis (BIA) or DEXA.

• Visceral Fat Measurement: Visceral fat, the fat stored around internal organs, is strongly linked to metabolic health and disease risk. It can be measured with magnetic resonance imaging (MRI) or computed tomography (CT).

• Body Shape Index (ABSI): ABSI takes into account your waist circumference, height and weight to estimate visceral fat and overall risk of death. It offers a more comprehensive assessment of body composition and fat distribution compared to BMI.

• Lean Body Mass: Assessing lean body mass, which includes muscle, bone and organs, provides additional information about body composition beyond just your fat mass. Understanding lean body mass can differentiate between people with a similar BMI but different body compositions. 

• Physical Fitness Assessments: Evaluating physical fitness abilities like endurance, muscular strength and flexibility helps paint a good picture of your health beyond just a number on the scale. 

• Metabolic Health Markers: Monitoring metabolic health markers like blood pressure, blood sugar levels, lipid profile and inflammatory markers can tell you a lot about health status independent of weight. 

Is BMI outdated? Yes and no. Is BMI a good indicator of health? Also yes and no.

BMI has over 100 years of history as a standard health assessment tool. But while BMI can tell you where your weight falls compared to what’s “normal,” it fails to account for other critical details of your health.

Some key takeaways about BMI: 

• It provides a simple and quick assessment of weight status compared to what’s “normal” for your height.

• It provides insight into potential health risks associated with obesity, which can help identify people at risk of weight-related conditions.

• It lacks accuracy in differentiating between fat mass and muscle mass, which could falsely diagnose people with high muscle mass as having obesity. It also doesn’t account for variations in body composition, waist circumference, race, age or people of shorter stature.

The bottom line? It’s best to take BMI with a grain of salt and use it alongside other measurement tools for a better picture of overall health. After all, it was invented over a century ago by a couple of men with no healthcare background — and likely no eye for the details that play into our individual health stories.