Body Mass Index (BMI)

Body mass index (BMI) or Quetelet Index is a statistical measure of the weight of a person scaled according to height. It was invented between 1830 and 1850 by the Belgian polymath Adolphe Quetelet during the course of developing "social physics".

The BMI has become controversial because many people, including physicians, have come to rely on it for medical diagnosis - but that has never been the BMI's purpose. It is meant to be used as a simple means of classifying sedentary (physically inactive) individuals with an average body composition. For these individuals, the current value settings are as follows: a BMI of 18.5 to 25 may indicate optimal weight; a BMI lower than 18.5 suggests the person is underweight while a number above 25 may indicate the person is overweight; a BMI below 15 may indicate the person has an eating disorder; a number above 30 suggests the person is obese (over 40, morbidly obese).

In physiology the term “weight” is used interchangeably with “mass”. For a given body shape and given density, the BMI will be proportional to height e.g. if all body dimensions increase by 50%, the BMI increases by 50%.

Generally, a BMI chart displays calculated BMI as a function of weight (horizontal axis) and height (vertical axis) using “contour lines” for different values of BMI or colors for different BMI categories.