How is obesity defined?

BODY MASS INDEX

Overweight and obesity are measured and determined using the body mass index, or BMI. This is a simple, easy to apply, index of weight-for-height that is commonly used to classify underweight, overweight and obesity in adults. It is defined as the weight in kilograms divided by the square of the height in metres (kg/m2).

BMI = weight (kg) / height (m) x height (m)

The classification of overweight and obesity, according to BMI, is shown in the table below. This applies to men and women over the age of 18 years.

Classification              BMI
Underweight              Below 18.5
Normal range             18.5 - 24.9
Overweight               25.0 - 29.9
Obese                      30 or above

Therefore, a person is considered overweight when his or her BMI is between 25.0 and 29.9, and obese when his or her BMI is greater than or equal to 30.

NOTE:

• As BMI is calculated using height and weight only, it therefore does not measure body fat, and cannot differentiate between weight that represents muscle and weight that represents fat. This means that people who have the same BMI can have a different percent body fat.
• The limits defined above cannot be used to classify underweight, overweight and obesity in children; different BMI cut-offs are required. As BMI changes substantially with age in children, BMI needs to be assessed using BMI-for-age charts, where the child's BMI is plotted against their age. 
  

WAIST CIRCUMFERENCE

Waist circumference is another widely used measurement to determine overweight and obesity, especially in the abdominal or waist area.

                   Males           Females (aged >18 y)
Overweight    > 94 cm       > 80 cm
Obese           > 102 cm     > 88 cm

Waist circumference is a good indicator of increased risk of developing health- and obesity-related problems and diseases, particularly type 2 diabetes and heart disease. Therefore, waist circumference is not only a measure of overweight and obesity, but also an indicator of potential risk of other health problems.