Measuring Obesity

A crude population measure of obesity is the body mass index (BMI) which is a simple index of weight-for-height that is commonly used in classifying overweight and obesity in adult populations and individuals - a person's weight in kilograms is divided by the square of the height in meters (kg/m2). BMI provides the most useful population-level measure of overweight and obesity as it is the same for both sexes and for all ages of adults but it is merely a rough guide because it may not correspond to the same degree of fatness in different individuals.
The WHO defines an adult who has a BMI between 25 and 29.9 as overweight - an adult who has a BMI of 30 or higher is considered obese - a BMI below 18.5 is considered underweight, and between 18.5 to 24.9 a healthy weight .

BMI provides a benchmark for individual assessment, but experts suspect that the risk of chronic disease in populations increases progressively from a BMI of 21 upwards.
Measuring overweight and obesity in children aged 5 to 14 years is challenging - the WHO Child Growth Standards includes BMI charts for infants and young children up to age 5 - childhood obesity is associated with a higher chance of premature death and disability in adulthood.
BMI ranges for children and teens are defined so that they take into account normal differences in body fat between boys and girls and differences in body fat at various ages. However although BMI correlates with the amount of body fat, BMI does not directly measure body fat and some people, such as athletes, may have a BMI that identifies them as overweight even though they do not have excess body fat.

Other methods of estimating body fat and body fat distribution include measurements of skinfold thickness and waist circumference, calculation of waist-to-hip circumference ratios, and techniques such as ultrasound, computed tomography, and magnetic resonance imaging (MRI).