Body mass index (BMI) is a measurement for under and overweight. The BMI index of persons aged 20 years and older is defined as body weight (in kilos) divided by reported height (in metres) squared.

According to the World Health Organisation, a BMI index under 18.5 for grown-ups indicates underweight. A BMI index ranging from 18.5 to 25 indicates normal weight and an index exceeding 25 indicates overweight. Obesity is defined as a BMI of 30 and above.

The term overweight refers to all people with a BMI index of 25 and above and therefore includes obese persons. Obesity exclusively refers to people with a BMI of 30 and over.