BMI Calculator for Children

BMI-for-age in children is calculated using the same formula as adults, weight in kilograms divided by height in metres squared, but the result is then plotted against age-and-sex-specific growth charts to give a centile. Unlike adult BMI, where fixed figures such as 18.5 and 25 mark the boundaries, no single number defines overweight or obese in childhood, because what counts as healthy changes as children grow.

The NHS uses the UK-WHO growth reference, which follows the WHO growth standard up to age 4 and the UK90 reference from 4 to 18. A BMI on or above the 91st centile is classified as overweight and on or above the 98th centile as obese, while on or below the 2nd centile indicates underweight. Anything between the 2nd and 91st centiles falls in the healthy band. The calculator works out the exact centile for your child's age in months and sex from that reference and returns the classification that goes with it.

Children's body composition changes significantly during puberty, so tracking the centile over time is more informative than a single reading. A child who has sat on much the same centile for years is growing along their own line, which is exactly what the charts were designed to show. If you are concerned about your child's weight, the NHS recommends speaking to your GP or health visitor, who can plot measurements on official growth charts and advise on next steps.

England measures children twice through the National Child Measurement Programme, in Reception at age 4 or 5 and again in Year 6 at age 10 or 11. The 2024/25 figures, published in November 2025, put 10.5% of Reception children in the obese category and 25% overweight or obese. By Year 6 that rises to 22.2% obese and 38% overweight or obese. The programme uses population cut-offs at the 85th and 95th centiles, lower than the clinical 91st and 98th, so its labels can differ from a GP's. Deprivation shows through plainly, with Year 6 obesity at 29% in the most deprived areas against 14% in the least, and rates run higher among Black African children in particular. Parental obesity correlates strongly, and Reception obesity rose in 2024/25 to one of the highest rates since the programme began, pandemic years aside.

Strict diets are not appropriate for anyone under 18 without medical supervision, so the guidance concentrates on habits rather than calorie counting. The Chief Medical Officer recommends 60 minutes of moderate to vigorous activity a day, five or more portions of fruit and vegetables, and keeping sugar below 5% of daily calories. Less screen time, eating together as a family, swapping crisps and sweets for fruit or yoghurt, and cutting back on ultra-processed food all pull in the same direction. Sleep belongs on the list too, at 9 to 12 hours for ages 6 to 12 and 8 to 10 hours for teenagers.

Local Tier 2 services include HENRY for under-fives, MEND for ages 6 to 13 and Beezee Bodies, with NHS Tier 3 paediatric weight management clinics handling more severe cases. Schools add cooking classes, the daily mile and healthier lunchtimes, and the Soft Drinks Industry Levy has cut sugar in soft drinks by 30% since 2018. Book an appointment if your child sits consistently above the 91st or 98th centile, since childhood obesity raises lifetime type 2 diabetes risk by five to seven times and heart disease by two to three times, and intervention before the age of 12 works far better than later. Sudden weight loss, restrictive eating, vomiting after meals or excessive exercise need a GP referral just as much, and the eating disorders charity Beat runs a UK helpline at beateatingdisorders.org.uk.