https://www.scientificamerican.com/article/is-bmi-an-accurate-way-to-measure-body-fat/

Body Mass Index (BMI) is a mathematical formula that divides a person's weight by the square of their height to arrive at a number that falls into one of these bodyweight categories:

Beyond assigning one of these categories, a high BMI can be also an indicator of high body fat and therefore can be used to screen for certain weight levels that could lead to health problems. Even though it is often used as one, it is not a true diagnostic of body fatness or of an individual’s overall health.