Definitions of underweight, desirable weight, overweight and obesity among both adults and children are defined on the basis of the body mass index (BMI) which is a measure of the weight to height ratio. It is calculated by taking the weight (in kilograms) and dividing it by the square of height (in metres).
Among adults, underweight is defined as having a BMI of either less than 18.5 or less than 20, overweight is defined as having a BMI of 25 or more, obese is defined as having a BMI of 30 or more, and morbidly obese is defined as having a BMI of 40 or more.
For instance, if an adult weighed 10 stone (63.5 kilograms) and was five feet five inches (1.65m) tall then their body mass index would be 63.5 divided by 1.65 divided by 1.65 which is 23.3 and they would be in the healthy weight range.
Among children, the cut-off values to define overweight and obesity differ depending on the age and gender of the child. The cut-off values are based on 1990 reference curves and defined on the basis of centiles. Underweight was defined as a BMI less than or equal to the second centile, healthy weight as a BMI above the second centile but less than the 85th centile, overweight as a BMI the 85th centile or above but less than the 95th centile, and obese as a BMI the 95th centile or higher. Thus in 1990, 2% of all children were defined as underweight, 15% as overweight and 5% as obese.
Cole, T.J., J.V. Freeman, and M.A. Preece, Body mass index reference curves for the UK 1990. Archives of Diseases in Childhood, 1995. 73: p. 25-29.
NHS Digital. National Child Measurement Programme. https://digital.nhs.uk/services/national-child-measurement-programme
National Obesity Observatory. A simple guide to classifying body mass index in children, June 2011.
Also see National Child Measurement Programme.