When it comes to BMI—the most common measure of obesity—the price is right.

BMI, or body-mass index, is calculated by dividing weight in kilograms by height in meters squared. A result less than 18.5 is considered underweight, according to the Centers for Disease Control and Prevention; 18.5 to 24.9 is healthy; 25 to 29.9 is overweight; and 30 or higher is obese.

Medical experts rely on BMI to assess the prevalence of obesity because it’s simple, cheap and generally accurate.

But the index doesn’t distinguish fat from muscle or other tissue, and that causes some people to be placed in the wrong category.

“We have two different groups that tend to be misclassified,” said Luiz Guilherme Grossi Porto, a professor of physical education at Universidade de Brasília and a visiting scientist and collaborator at Harvard University.

Very muscular people can be mistaken for obese, he said, while those who are tall with a high percentage of body fat can be erroneously categorized as healthy. Certain groups also are more likely to be misclassified. According to research by the World Health Organization, Asians typically have a higher percentage of body fat than white people of the same age, sex and BMI.

A BMI of 30 is strongly correlated with a higher risk of diseases such as heart disease, but because the measure doesn’t account for body composition, someone who is 6 feet tall and weighs 221 pounds will have a BMI of 30 whether they are toned and muscular or soft and flabby.

“You can have two people with the same BMI, but one has 50% body fat and the other has 30% body fat,” said Holly Lofton, director of the medical weight management program at NYU Langone Health.

A woman with more than 32% body fat and a man with more than 25% is considered obese.

To test the accuracy of BMI in a physically active population, Dr. Porto and his research colleagues verified the BMI classifications of 3,822 military firefighters by also measuring their body-fat percentage. 

In total, BMI correctly categorized the firefighters as obese or not obese 85.8% of the time—but there were differences among subgroups. Those who weren’t obese were accurately classified 93% of the time, while those who were obese were accurately classified only 47.4% of the time.

Still, the group’s overall prevalence of obesity was similar according to both measures, with BMI pegging it at 13.3% and body-fat percentage at 15.9%.

“Every body composition measure has pros and cons,” Dr. Porto said. “But BMI has some advantages. It’s very, very low cost. It’s easy to measure. The margin of error in terms of the population prevalence is very low. It’s a very good index for obesity prevalence.”

The CDC has used BMI to track obesity in the U.S. since at least 1985, most recently estimating the prevalence at 42.4%.

Part of the reason the measure endures, according to Dr. Lofton, has to do with health insurance. “Insurance will only provide treatment for patients with certain BMI,” she said, although a better, and equally low-cost, gauge of obesity is the medical waist measurement, which is less likely to mistake a muscular person for someone who is unfit.

“In the medical community, the waist is at the top of the pelvis, immediately above your hip bones,” Dr. Lofton said. “If that measurement is greater than 40 inches for males or 35 inches for females, it more directly correlates with obesity.”

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A related measure uses the waist-to-hip ratio, dividing the circumference of the waist at its smallest point by the circumference of the hips at their widest. According to the World Health Organization a waist-to-hip ratio greater than 0.85 for women and 0.9 for men indicates obesity.

More accurate measures of body fat are also more expensive and require specialized equipment.

Hydrostatic weighing is one of the most precise. “It involves dumping you in a tank of water completely naked,” Dr. Lofton said.

Bioelectrical impedance, a feature now available in some bathroom scales, sends a painless impulse through the body that penetrates various tissues at different speeds, allowing it to detect and measure the percentage of fat. Dual-energy X-ray absorptiometry, the same tool used to detect bone density, can also measure body fat.

For the cruder measures, Dr. Lofton urges caution.

“BMI and waist circumference are not diagnostic tools for disease risks,” she said. “If you go online and find your BMI is high, the next step is to go to a medical professional and see what that means for you and your health risk.”

Write to Jo Craven McGinty at Jo.McGinty@wsj.com