There are many different ways in which to measure obesity, which are outlined below:

Measures used at home or at a GP clinic

Body Mass Index (weight [kg]/height [m]2)

• Is the most widely used approach
• Simple to calculate (although more difficult to classify in children – variety of definitions – IOTF, UK1990 and WHO 2007)
• Varies with age and sex in children & adolescents
• Not a direct measure of body fat mass or distribution
• Skewed by high muscle mass
• Correlation to health varies with ethnicity.
• This measure is used in the National Child Measurement Programme and Health Survey for England.

Skin Fold Thickness – biceps, triceps, subscapula, suprailiac

• Requires callipers and some basic training.

Waist circumference and waist to hip ratio

• Examines fat distribution
• Waist hip ratio used less frequently
• Often subject to measurement error
• Adult waist circumference cut points are:
  Increased risk ♂ ≥ 94cm ♀ ≥ 80cm Greatly increased risk ♂ ≥ 102cm ♀ ≥ 88cm
  – Child cut points vary with sex and age and have been published by McCarthy, 2001.

Bio-impedance

• Given lean mass is made up of 73% water and fat has no water content, the method estimates lean tissue mass (which acts as a conductor) and fat mass (which acts as an insulator) through changes in voltage.
• Home machines can be inaccurate (as they often estimate from the legs only)

The most common method of measuring obesity is the Body Mass Index (BMI). BMI is calculated by dividing body weight (kilograms) by height (metres) squared. A BMI between 25 and 29.9 is classified as overweight and any BMI of 30 or over is obese (Table 1).

Table 1: World Health Organisation BMI classification system for adults

An online ADULT BMI calculator is available at: http://www.bdaweightwise.com/lose/lose_bmi.aspx/

An online CHILD BMI calculator is available at: http://tools.erpho.org.uk/bmi.aspx/