Body Mass Index

The body mass index ( BMI ) or Quetelet Index is a statistical measure of the weight of a person scaled according to height. It is used as a simple means of classifying inactive individuals of an average body composition according to their body fat content. As a rough guideline for adults a BMI of less than 20 implies underweight, over 25 is overweight, and over 30 is obese. It is calculated by taking the weight of the individual in kilograms and dividing by the square of the height in metres.

It was originally developed between 1830 and 1850 by the Belgian polymath, Adolphe Quetelet during the course of developing "social physics". The BMI is a controversial measurement as it is not intended to make a medical diagnosis, but instead to broadly categorise populations for purely statistical purposes, and its accuracy in relation to actual individual body fat composition can be distorted by such factors as fitness level, muscle mass, bone structure, and ethnicity.

BMI is defined as the individual's body weight divided by the square of the height, and is almost always expressed in the unit kg / m 2 , which is therefore often left out. Hence, using the units in parentheses the BMI value can be calculated as

or (a version adapted for Imperial units):

In physiology the word “weight” means the same as “mass”. The reason height is squared rather than cubed or raised to some other power is simply that, taken over people of different height, the resulting index correlates reasonably well with degree of underweight or overweight. No law of physics or physiological growth is implied.

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