The BMI and what it means

BMI, or the body mass index, is a numerical measure of body fat in humans. It is calculated taking into account a person’s weight and height. Originally known as the Quetelet index, BMI does not measure the percentage of fat in the human body.

The body mass index was derived in the nineteenth century by Belgian polymath Adolphe Quetelet. It is calculated by dividing the individual’s body weight by the square of his/her height. Thus, the unit of measure is kg/m2. A BMI chart is sometimes used to determine the BMI. In this chart, weight is taken on the horizontal axis and height on the vertical axis. Contour lines are used for different BMI values.

BMI Categories

Simply understood as a measure of an individual’s fatness (or thinness), BMI allows physicians and healthcare professionals address weight related problems. However, BMI was never meant to be used for medical diagnosis. It was meant to indicate whether a person fell into one of the four categories:

  • Underweight: BMI below 20
  • Optimal weight: BMI of 20 to 25
  • Overweight: BMI above 25
  • Obese: BMI above 30
  • Morbid obesity: BMI above 40

According to the World Health Organization (WHO), a BMI of below 18.5 suggests malnutrition or health problems like eating disorder. A BMI of less than 17.5 is an informal indicator of anorexia nervosa (a disorder in which a person refuses to eat due to the fear of gaining weight). A BMI of around 15 is considered to be an indicator for starvation.

According to the US National Health and Nutrition Examination Survey conducted in 2007, as many as 59% of men and 49% of women in the US had a BMI above 25. Morbid obesity was found in 2% of American men and 4% of women. Comparing this with the survey conducted in 2007, it was found that the percentage of American men with a BMI of above 25 had risen to 63%. Moreover, 26% of these men were in the obese category.

BMI in Children

The BMI for children is different from that for adults. While the calculation remains the same, it is compared to values that are typical for other children who are of the same age and gender. Instead of a BMI demarcating overweight, optimal weight and underweight, a percentile is calculated, which allows comparison.

  • Underweight: BMI less than the 5th percentile
  • Overweight BMI between the 85th and 95th percentile
  • Obese: BMI of above the 95th percentile