Measurement of Obesity
There are various ways in which to measure different aspects of obesity. They include Body Mass Index (BMI), skin fold thickness, waist circumference, waist to hip ratio and bio-impedance.
Body Mass Index (BMI)
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. An adult BMI of between 25 and 29.9 is classified as overweight and a BMI of 30 or over is classified as obese (Table 1).
Table 1: World Health Organization BMI classification system for adults
BMI is the most widely used approach in the UK, but it is important to note that it is not a direct measure of body fat mass or distribution, and BMI measures may be skewed by very high muscle mass. The relationship between BMI and health also varies with ethnicity.
In children and adolescents BMI varies with age and sex, for this reason a growth reference must be used. In England, the British 1990 growth reference charts are used to classify the weight status of children according to their age and sex for the National Child Measurement Programme and Health Survey for England. To check out your own or your child’s BMI, click here
For more information about using BMI for children please see our page on measuring and interpreting BMI in children.
NOO has produced a number of papers on BMI measurement:
- Body Mass Index as a measure of obesity published June 2009
- Variation in children’s BMI by month of measurement published December 2010
- A simple guide to classifying body mass index in children published June 2011
Skin fold thickness
Skin fold thickness refers to the measurement of subcutaneous fat located directly beneath the skin by grasping a fold of skin and subcutaneous fat and measuring it using calipers. It is used mainly to determine relative fatness and the percentage of body fat. Measurement requires callipers and some basic training.
Waist circumference and waist to hip ratio
The circumference of the waist is sometimes used as a simple measure of body fatness, though it can be subject to measurement error. Adult waist circumference cut points are:
- Increased risk of health problems: Men≥ 94cm Women ≥ 80cm
- Greatly increased risk of health problems: Men ≥ 102cm Women ≥ 88cm
Waist to hip ratio examines fat distribution and in practice is used less frequently, given the established links between waist circumference alone and health risk.
Click here for more information on central adiposity as an indicator for obesity: Measures of central adiposity as an indicator of obesity published August 2009
This measures the impedance or opposition to the flow of a very small electric current as it passes through the body. As lean mass is made up of 73% water and fat has no water content, this method estimates lean tissue mass (which acts as a conductor) and fat mass (which acts as an insulator), through changes in voltage. Home machines are available for bio-impedance measurement though these can be inaccurate as they often estimate from the legs only.