The main source for this part is the National Child Measurement Programme for England (NCMP) which includes nearly all children in reception year (aged 4-5) and year 6 (aged 10-11). 95% of eligible children were measured in 2018/19.
Health Survey for England (HSE) also collects data on childhood obesity; covering all children aged 2-15, although as a sample it has much lower coverage than NCMP and therefore the estimates are less precise.
NCMP and HSE collect height and weight measurements to calculate BMI for each child. BMI (adjusted for age and sex) is recommended as a practical estimate of overweight and obesity in children as it takes into account different growth patterns in boys and girls at different ages.
BMI = Person’s weight (kg) / Person’s height (in metres)2
Each age and sex group needs its own level of classification and this section uses the British 1990 growth reference (UK90) to describe childhood overweight and obesity.