Obesity: measurement

There are a number of measurements which can be used to identify obese (and overweight) adults and children including body mass index (BMI), waist circumference and waist hip ratio (WHR). A brief definition of these measures is provided below. Further information on the measurement of obesity has been provided by Public Health England in Measurement of Obesity (129kb).

Adults

BMI: A calculation based on weight in kilograms divided by height in metres squared. For epidemiological purposes, obesity is defined as BMI greater than or equal to 30kg/m² which is the level at which the risk of developing many diseases increases. BMI does not directly measure percentage of body fat, but it is a more accurate indicator of obesity than relying on weight alone. BMI is the most commonly used measure of obesity.

Waist circumference: Considered to be a better assessor of metabolic risk than BMI because it is more directly proportional to total body fat and the amount of metabolically active visceral fat. A measurement of >102 cm in men or > 88 cm in women identifies those considered obese.

WHR: Defined as waist circumference divided by hip circumference. Obesity is measured by a raised WHR, taken to be 0.95 or more in men and 0.85 or more in women.

Children

In adults, BMI can be directly classified into different categories since age in adulthood does not greatly affect weight in relation to height. However, these adult BMI category cut-offs are not appropriate for children since BMI changes markedly as a child ages. Instead, for children, BMI can be converted into centiles using 1990 UK reference data based on sex and age (Cole, Freeman and Preece 1995). Children whose BMI is within the top 5% of the 1990 UK reference range for their age and sex are considered obese.