The World Health Organization (WHO) states that the Body Mass Index (BMI) provides the most useful measure of overweight and obesity as it is the same for both sexes and for all ages of adults. But it is only a rough guide because it may not correspond to the same degree of fatness in different individuals.WHO defines overweight and obesity as follows.For AdultsOverweight is a Body Mass Index BMI greater than or equal to 25.Obesity is a BMI greater than or equal to 30.For ChildrenAge is considered when defining overweight and obesity. For children under 5 years of age, overweight is weight-for-height greater than 2 standard deviations above WHO Child Growth Standards median. And obesity is weight-for-height greater than 3 standard deviations above the WHO Child Growth Standards median.