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 But many health experts believe the metric is outdated. It sometimes misclassifies people with distinctive body types, and isn’t always an accurate predictor of disease risk. Still, it’s not going anywhere anytime soon.

What Is BMI?

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.

• Less than 18.5: underweight
• Between 18.5 to 24.9: healthy weight
• Between 25 and 29.9: overweight
• More than 30: obesity

Drawbacks of Using BMI

BMI has long been a staple in health assessments. However, it is far from perfect, and as the years have passed, more and more research has emerged revealing 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, an 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, Dr. Mehal says.

“Clearly, his weight was due to muscle — he had very little body fat. But that’s the rare exception in today’s world,” he says. Most people with a high BMI carry too much adipose tissue (body fat), he adds.

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), Yancy says.

BMI Doesn’t Account for Racial or Ethnic Variations in Body Types

“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 remains useful as a tool because of how it has evolved. “If BMI were only used in its original form and not changed, then we’d have to worry,” she says.

Many different studies have looked at elevated BMI risk across different populations, and the consistent finding is that a higher BMI is bad for your health, Bleich says.

“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.

BMI May Be Here to Stay

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

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

“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, Mehal says.

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,” Mehal says.

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

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.”