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Sections: Physiology

Body Mass IndexBody mass index (BMI) is a value derived from the mass (weight) and height of a person. The BMI is defined as the body mass divided by the square of the body height, and is expressed in units of kg/m2, resulting from mass in kilograms and height in metres.

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Adolphe Quetelet, a Belgian astronomer, mathematician, statistician, and sociologist, devised the basis of the BMI between 1830 and 1850 as he developed what he called "social physics".

The BMI is expressed in kg/m2, resulting from mass in kilograms and height in metres:

I = m/h2

  • m – mass in kilogramm
  • h – height in meters,

The WHO regards an adult BMI of less than 18.5 as underweight and possibly indicative of malnutrition, an eating disorder, or other health problems, while a BMI of 25 or more is considered overweight and 30 or more is considered obese. 

BMI Category
16 and under Underweight (Severe thinness)
16 –18,5 Underweight
18,5 – 25 Normal range
25 – 30 Overweight (Pre-obese)
30 – 35 Obese (Class I)
35 – 40 Obese (Class II)
40 or more Obese (Class III)

A common use of the BMI is to assess how far an individual's body weight departs from what is normal for a person's height. The weight excess or deficiency may, in part, be accounted for by body fat (adipose tissue) although other factors such as muscularity also affect BMI significantly (see discussion below and overweight).