The most widely-used method of measuring and identifying obesity is Body Mass Index (BMI).
BMI = weight in kg/height m2
In adults Overweight, or pre-obesity, is defined as a BMI of 25-29.9 kg/m², while a BMI ≥ 30 kg/m² defines obesity. These BMI thresholds were proposed by WHO expert report and reflect the increasing risk of excess weight as BMI increases above an optimal range of 21-23 kg/m², the recommended median goal for adult Caucasian populations (WHO/NUT/NCD, 2000).
While BMI is a simple measure that is very useful for populations, it should be considered a rough guide for predicting risk in individuals. Athletes are commonly misclassified as are those who are particularly short or tall. In brief BMI is most useful on a population not individual basis.
Other methods include waist circumference, waist to hip ratio and biometric impedance.
Waist circumference (wc) is a cheap and easy method of measurement. Waist circumference is considered a reasonable indicator of intra-abdominal or visceral fat. This fat is closely associated with increased risk of comorbidity. NICE cut off points suggest Males with wc >94cm or Females with wc => 85cm are considered to be at increased risk. The World Health Organisation classify have identified levels of risk combining both BMI and WC these are shown below
WHO classification for risk of obesity-related ill health