Is Body Mass Index (BMI) Accurate for Measuring Health?

BMI (body mass index) has long been a standard way to assess whether a person is at a healthy weight. But many health experts believe the metric is outdated.

Keep reading for a better understanding of BMI, including its uses and limitations, as well as what it can tell you about your weight and overall health.

BMI is a health metric calculated using a person’s height and weight, explains Wajahat Mehal, MD, professor of medicine and director of the Yale Weight Loss Program in New Haven, Connecticut.

Adults can measure their BMI by taking their weight in pounds, dividing that value by their height in inches squared, and multiplying the latter value by 703. Alternatively, they can use the BMI calculator from the Centers for Disease Control and Prevention.

Here’s what the various BMI numbers mean:

• Less than 18.5: Underweight
• Between 18.5 to 24.9: Normal
• Between 25 and 29.9: Overweight
• More than 30: Obesity

BMI has long been a staple in health assessments. However, it is far from perfect, and as the years have passed, more and more science has emerged that reveals the flaws of this approach.

BMI Is an Inaccurate Way to Predict Disease Risk

BMI Can Misclassify Muscular People as Having Obesity

Although a high BMI is associated with having a high amount of body fat, BMI doesn’t directly measure body fat percentage.

“BMI is essentially a surrogate for adiposity or percent body fat, which is the actual risk factor,” says William Yancy Jr., MD, internal medicine doctor and obesity medicine specialist at Duke Health in Durham, North Carolina.

“Because BMI can’t differentiate between fat and muscle, it can misclassify very muscular people as overweight or obese,” Dr. Yancy says.

Two examples: The clearly fit and muscular USA rugby star Ilona Maher revealed she had a BMI of 29.3 in a TikTok video that has more than 1.2 million views.

When Arnold Schwarzenegger was crowned the Mr. Universe of the bodybuilding world, he also had a BMI of around 30, which would be in the “obesity” category, says Dr. Mehal.

“Clearly, his weight was due to muscle — he had very little body fat. But that’s the rare exception in today’s world. For most people who have a high BMI, it’s because they carry too much adipose tissue,” he says.

BMI can also underestimate risk in people with very low musculature, such as people with sarcopenia (a type of muscle loss that happens with aging or immobility), says Yancy.

“There is definite truth that the BMI was tested and validated in a white male population,” says Sara Bleich, PhD, professor of public health policy at the Harvard T.H. Chan School of Public Health in Boston.

Yet Dr. Bleich explains that BMI is still useful as a tool because of the way it has evolved over time. “If BMI were only used in its original form and not changed, then we’d have to worry,” she says.

There are so many different studies that have looked at elevated BMI risk in many different populations, and what it uniformly shows is that a higher BMI is bad for you, says Bleich.

“A major challenge is that, statistically, it’s normal in this country to be overweight, but each day when you’re walking around with excess weight, it’s increasing your risk for chronic conditions,” she says, adding that that’s true no matter your race.

Because BMI is so easy to measure accurately in mere seconds, it will likely continue to be used in studies and healthcare screenings, says Mehal.

It’s also useful because it can be repeated reliably, says Yancy.

“In other words, if you measure someone’s weight or height two times, you are likely to get a very similar measurement both times, and different people using the same equipment are likely to get the same result,” he says.

The same can’t be said for some things that are harder to measure — for example, how much muscle a person has — which would be a better indicator of a person’s metabolic health, says Mehal.

But just because BMI is simple to measure doesn’t mean a person should let it define them or their health. “It’s just one number out of so many numbers that you can use to gauge a person’s health,” says Mehal.

The same should be said for weight, he adds. “We tend to over-obsess about people’s weights, when there are lots of other indicators of health,” Mehal adds.

Health measurements that we can use along with BMI to calculate health risks include waist circumference, blood pressure, cholesterol, and medical history, he says, adding, “It’s not either-or.”

• BMI is a simple and widely used metric to assess whether someone is at a healthy weight.
• BMI can sometimes misclassify individuals as having obesity, especially those with a lot of muscle, and it doesn’t predict disease risk as accurately as other metrics that factor in criteria like fat distribution.
• Despite its limitations, BMI remains valuable in healthcare settings for tracking trends and diagnosing obesity, but it should be used alongside other health indicators.