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Obesity Classification

BMI (kg/m)Classification
fromup to
18.5underweight
18.525.0normal weight
25.030.0overweight
30.035.0class I obesity
35.040.0class II obesity
40.0class III obesity  

Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health. It is defined by body mass index (BMI) and further evaluated in terms of fat distribution via the waist–hip ratio and total cardiovascular risk factors. BMI is closely related to both percentage body fat and total body fat. In children, a healthy weight varies with age and sex. Obesity in children and adolescents is defined not as an absolute number but in relation to a historical normal group, such that obesity is a BMI greater than the 95th percentile. The reference data on which these percentiles were based date from 1963 to 1994, and thus have not been affected by the recent increases in weight. BMI is defined as the subject’s weight divided by the square of their height and is calculated as follows.

where m and h are the subject’s weight and height respectively.

BMI is usually expressed in kilograms per square metre, resulting when weight is measured in kilograms and height in metres. To convert from pounds per square inch multiply by 703 (kg/m)/(lb/sq in).

The most commonly used definitions, established by the World Health Organization (WHO) in 1997 and published in 2000, provide the values listed in the table.

Some modifications to the WHO definitions have been made by particular organizations. The surgical literature breaks down class II and III obesity into further categories whose exact values are still disputed.

  • Any BMI ≥ 35 or 40 kg/m is severe obesity.
  • A BMI of ≥ 35 kg/m and experiencing obesity-related health conditions or ≥40–44.9 kg/m is morbid obesity.
  • A BMI of ≥ 45 or 50 kg/m is super obesity.

As Asian populations develop negative health consequences at a lower BMI than Caucasians, some nations have redefined obesity; Japan have defined obesity as any BMI greater than 25 kg/m while China uses a BMI of greater than 28 kg/m.

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