Although the BMI is an unscientific way to characterize a person’s nutritional status, it is nonetheless used as a measure of total body fat mass in medical practice and epidemiological studies, not least because it is easy to calculate and document regularly in personal health records. Typically, doctors use it to categorize patients as “underweight” (a BMI value below 18.5), “normal weight” (18.5-25), “overweight” (25-30), or “obese” (above 30).