Child body mass index classification

Assessing the body mass index (BMI) of children is more complicated than for adults because a child’s BMI changes as they mature. Growth patterns differ between boys and girls, so both the age and sex of a child needs to be taken into account when estimating whether BMI is too high or too low. Instead of using fixed BMI thresholds to classify individuals (as used for adults) children’s BMI is categorised using variable thresholds that take into account the child’s age and sex. These thresholds are usually derived from a reference population, known as a child growth reference, in England the National Institute for Health and Care Excellence (NICE) recommend using the British 1990 (UK90) growth reference.

For population monitoring purposes, a child’s BMI is classed as overweight or obese where it is on or above the 85th centile or 95th centile respectively, based on the UK90 growth reference data. The population monitoring cut offs for overweight and obesity are lower than the clinical cut offs (91st and 98th centiles for overweight and obesity) used to assess individual children; this is to capture children in the population in the clinical overweight or obesity BMI categories and those who are at high risk of moving into the clinical overweight or clinical obesity categories. This helps ensure that adequate services are planned and delivered for the whole population.


Table showing UK90 population monitoring cut offs used in the Obesity Profile

BMI centile range

BMI Category

Less than or equal to 2nd centile


Greater than 2nd and less than 85th centile

Healthy weight

Greater than or equal to 85th and less than 95th centile


Greater than or equal to 95th centile


Greater than or equal to 99.6th centile

Severe obesity