HMN 2026: How New body index aims to move beyond BMI and works for babies too

Body Mass Index (BMI) has long been used in public health and clinical settings as a simple tool to classify an individual’s physical status based on their height and weight. Originally developed in the 1830s by Belgian mathematician Adolphe Quetelet, BMI was designed to describe the characteristics of the average man rather than to assess an individual’s health, and it did not account for age, sex, ethnicity or body composition.

For nearly two centuries, BMI has been the go-to measure for healthy body size, but its limitations have sparked a search for a better way to understand the human body. Enter the Consistent Body Mass Index (CBMI), a new approach that questions BMI’s underlying assumption that body weight scales with height squared.

Instead, in this new study, researchers used physics and geometry to show that weight is more naturally connected to height cubed (CBMI=(m/h³)^1/2), because a human body is a three-dimensional structure. By modeling the body as a cylinder, they developed a formula that links weight, height and waist circumference and better represents the body’s proportions.

To see how the model held up in real people, researchers tested it on 400 participants in Istanbul, ranging from newborns to 75-year-olds. CBMI turned out to be a much better way to judge someone’s physical health than standard BMI. The numbers backed it up, too: For the entire group, the correlation coefficient (r) was 0.837, a statistical measure of how closely two things move together. The relationship was strongest in children and weaker in adult men.

The findings were published in Scientific Reports.

In search of a better indicator

After studies began exposing the glaring limitations of BMI, including its failure to account for differences in children, extremely tall or short individuals, and variations in body composition, scientists started exploring better alternatives. One such measure was the Tri-ponderal Mass Index (TMI), which replaces BMI’s use of height squared with height cubed to better account for body size.

While it showed improvement over BMI, TMI still lacked universal applicability and did not perform equally well across all age groups.

Waist circumference emerged as one of the most widely used markers of metabolic health, and for good reason. A WHO expert group found that waist circumference, along with waist-to-hip ratio, could be an important predictor of conditions such as heart disease, hypertension, diabetes and even several cancers.

Despite its clinical importance, waist circumference shares the same fundamental limitation as BMI: Neither measure is truly independent of body size. BMI is calculated as kg/m², while waist circumference is measured in meters, meaning the same number can represent very different health realities depending on the person being measured.

In this study, researchers wanted to combine mathematical theory with real-world measurements to create a more accurate health tool. So, they turned their attention to a geometric ratio, the waist-to-height ratio (w/h), and explored it as a nondimensional measure that incorporates body mass, density and height.

They chose waist-to-height ratio over other nondimensional measures for two key reasons. First, it follows a simple geometric principle of fullness or slimness, where width is compared with length. Second, it has a strong association with obesity-related health risks. To account for the fact that human bodies are not perfectly cylindrical, the researchers introduced a new factor called the form factor (?*). This factor adjusts for differences in body shape and helps make the calculation more realistic.

The researchers included a wide range of participants, from newborn infants just one day old to adults up to 75 years of age. To keep the data balanced, they studied four equal groups of 100 participants each, including adult men, adult women, boys and girls. For each participant, three key measurements were recorded: body weight, height and waist circumference.

Using a mathematical method called the method of least squares, they determined how closely different formulas—CBMI, BMI and TMI—matched the actual body measurements of the 400 participants. They found that CBMI showed greater consistency across different body types and ages.

After establishing that CBMI performed consistently across different age groups, the researchers used the findings to develop new classification charts with cutoff values for normal weight, overweight and obesity.

The researchers also note that while CBMI may not appear dimensionless at first glance, it becomes dimensionless when body density is included in the calculation, making it a measure of body size that is independent of measurement units. However, they emphasize that further research is needed before the formula and the new cutoffs can be applied more broadly across populations.

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