If using the metric system, BMI is calculated by dividing the weight (in kilograms) by the height (in metres) squared. As height is typically recorded in centimetres, height in centimetres will need to be divided by 100 to obtain the height in metres.
BMI = Weight (kg) / Height (m)2
If using pounds and inches, the formula needs to be altered slightly. The weight in pounds is multiplied by 703 and then divided by the height in inches, squared.
The Imperial BMI Formula is: BMI = (Weight (lbs.) * 703) / Height (inches)2
BMI categories and cut-offs are commonly used to guide patient management (Table 1).
Table 1 The cut-off points for defining malnutrition in adults by WHO.
|Classification||Body mass index (kg/m2|
|Obese class III||40 +|
|Obese class II||35.0 - 39.9|
|Obese class I||30.0 - 34.9|
|Overweight||25.0 - 29.9|
|Normal||18.5 - 24.9|
|Mild underweight||17.0 - 18.4|
|Moderate underweight||16.0 - 16.9|
|Severe underweight||< 16.0|
Population specific cut offs are also available for assessing overweight and obesity in Asians as these populations show different associations between BMI, percentage body fat and health outcomes than their white counterparts. The proportion of Asians with a higher risk of diabetes and cardiovascular disease is significantly higher at BMIs lower than the recommended WHO cut offs for overweight. Thresholds of 23 kg/m2 or and 27.5 kg/m2 have been identified as increased risk and high risk respectively. The following categories have therefore been suggested:
However, one clear BMI cut off point is not applicable to all Asian populations for overweight and obesity as the observed risk varies from 22 kg/m2 to 25 kg/m2 and for high risk from 26 kg/m2 to 31 kg/m2.
Increasing risk is a continuum with increasing BMI, and the cut off values are merely a convenience for public health and clinical use.
Considerations relating to the use of body mass index in specific populations are described in Table 2.
Table 2 Application of body mass index in different populations.
|Pregnancy||Suitable. Women will also have a higher BMI during pregnancy because of increased weight associated with pregnancy, but not necessarily due to increased fat. Therefore, BMI will overestimate body fat in this population. Pre-pregnancy and post pregnancy BMI as well as weight gain during pregnancy are typically used to assess a pregnant woman’s weight status.|
|Infancy and lactation||Refer to BMI for age and growth indices as the relationship between body mass index and fatness in children varies substantially with age, height and sexual maturation. Age, sex, height and weight are taken into account when calculating these indices.|
|Toddlers and young children||Refer to BMI for age and growth indices as the relationship between body mass index and fatness in children varies substantially with age, height and sexual maturation. Age, sex, height and weight are taken into account when calculating these indices.|
|Adolescents||Refer to BMI for age and growth indices as the relationship between body mass index and fatness in children varies substantially with age, height and sexual maturation. Age, sex, height and weight are taken into account when calculating these indices.|
|Older Adults||Suitable, however, BMI cut-offs may not be appropriate in over 70 years due to body composition changes such as: 1) loss of muscle and bone and gain in body fat; 2) for any given BMI, loss of muscle may mask higher fat; 3) with aging, fat accumulates around the waist area (central obesity); 4) BMI is a poor indicator of fat distribution, unable to detect sarcopenic obesity prevalent in this population.|
Refer to section: practical considerations for objective anthropometry