Body mass index is a commonly used method for assessing overweight and obesity, which have been linked to health problems including cardiovascular disease, diabetes, fatty liver disease and several types of cancer. BMI is calculated by dividing a person’s weight by the square of their height. Standard BMI categories are as follows:
- Underweight: less than 18.5
- Normal weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Obesity: 30 to 39.9
- Severe (“morbid”) obesity: 40 or greater.
Although BMI is widely used, many experts think it is not the best measure of overweight and obesity—or the best predictor of resulting health problems. Invented nearly 200 years ago, BMI was not intended as a diagnostic tool, and it does not reflect evolving understanding about weight, fat and associated health risks. For example, BMI does not account for differences among racial and ethnic groups, and it does not distinguish between fat and denser muscle tissue, so it can classify slender athletes as overweight.
Overreliance on BMI matters because the measure is often used to recommend lifestyle changes, prescribe treatments for obesity and determine insurance coverage. This is a growing concern as bariatric surgery and weight loss medications, such as Wegovy (semaglutide) and Mounjaro (tirzepatide), come into wider use.
Waist circumference and the ratio of waist to hip measurements is a better way to assess abdominal obesity, indicating accumulation of deep visceral fat surrounding the internal organs. Having an “apple-shaped” body is considered to be a greater health risk than having a “pear-shaped” body with more fat around the hips and thighs. Various techniques are used to measure body composition, or the relative proportions of fat, muscle and bone. Researchers are developing new methods to better predict health risks associated with excess weight and fat.