You may be aware of Body Mass Index (BMI), but you might not know exactly how it works. BMI is a calculation that divides a person’s weight (in kilograms) by their height (in meters) squared. The result is then used to categorize individuals as underweight, normal weight, overweight, or obese. While it seems straightforward, this method has been widely criticized by experts in the health community. 

BMI = weight (kg) / height²
According to most criteria accepted around the world:

• A BMI of 18.49 or below means a person is underweight
• A BMI of 18.5 to 24.99 means they are of normal weight
• A BMI of 25 to 29.99 means they are overweight
• A BMI of 30 or more means they are obese

BMI’s Outdated Origins The foundation of the BMI was developed between 1830 and 1850 by Adolphe Quetelet, a Belgian astronomer and statistician. He created a formula to assess the average body shape of a population, not to measure an individual’s health [1]. The modern term “Body Mass Index” wasn’t coined until 1972 by physiologist Ancel Keys, who argued for its use as a simple tool for population-level studies, acknowledging its limitations for individual assessment [2]. Despite its historical context and known flaws, BMI remains the go-to method for health professionals to quickly categorise individuals.

In 2013, Prof. Nick Trefethen, a mathematician from Oxford University wrote a letter to The Economist, in which he questioned the usefulness of the current BMI formula, calling it a “bizarre measure.” He argued that the formula leads to confusion and misinformation. The height term, he says, divides the weight by too much when people are short, and by too little when they are tall. The BMI calculation does not measure overall lean muscle or overall fat content.

Trefethen points out that any calculation that assigns one number to a person will not be perfect. Humans are too complex to be described by a single figure, we are three-dimensional beings.

Why BMI is Inaccurate The BMI calculation’s primary flaw is that it fails to distinguish between body fat and lean muscle mass. Muscle is denser and weighs more than fat, leading to significant misclassifications. For example:

• An Olympic athlete who is 1.83 meters (6 feet) tall and weighs 96 kilograms (211 lbs) could have a BMI of 28, placing them in the overweight category.

• A sedentary person of the same height, weighing 92 kilograms (203 lbs), would have a BMI of 27, technically considered overweight but with a lower BMI than the athlete, despite having less muscle and a higher percentage of body fat.

This issue is particularly pronounced in athletes and bodybuilders, whose high muscle mass often classifies them as “overweight” or “obese” even when they are in peak physical condition with very low body fat.

Furthermore, BMI can also mask health risks in individuals who appear to be at a “normal” weight. A person can have a low body weight but a high percentage of visceral fat, the dangerous fat that wraps around internal organs [3]. This type of fat is strongly linked to an increased risk of serious medical conditions, including heart disease, type 2 diabetes, Alzheimer’s, stroke, and high cholesterol [4]. BMI provides no information about fat distribution, which is a more critical indicator of health risk than overall weight alone.

More Accurate Ways to Measure Health

Given the inadequacies of BMI, many health and fitness professionals are turning to more accurate methods that measure body composition—specifically, the ratio of body fat to lean mass. While no method is 100% perfect, these provide a much more conclusive view of an individual’s health.