Child BMI Calculator (Ages 2–20)
Calculate BMI for children and teenagers with age and sex-specific percentiles. Child BMI is interpreted differently from adult BMI — this calculator uses the correct growth reference charts.
Child BMI Guide
Why Child BMI Is Different
Adult BMI uses fixed thresholds (18.5, 25, 30) that apply to all adults. Child BMI must account for normal changes in body composition during growth — healthy body fat percentage varies significantly with age and sex. A BMI of 20 is very different for a 6-year-old versus a 16-year-old. Child BMI is therefore expressed as a centile position relative to the reference population rather than an absolute value. The UK 1990 (UK90) growth reference is used by the NHS and RCPCH for children in the UK.
UK Growth Centile Thresholds
UK NHS classification for children: below 2nd centile — underweight. 2nd to 85th centile — healthy weight. 85th to 95th centile — overweight. Above 95th centile — obese. A child at the 50th centile has a BMI equal to the median for their age and sex. A child at the 85th centile has a higher BMI than 85% of children of the same age and sex. The NHS does not use BMI alone for clinical assessment in children — growth trajectories over time are more informative than a single measurement.
Limitations of Child BMI
BMI in children has the same limitations as in adults — it cannot distinguish muscle from fat. A very athletic child may have a high BMI centile due to muscle mass. BMI is also affected by early or late puberty timing — puberty typically increases BMI centile in girls, particularly for body fat. Ethnicity: South Asian and Chinese children tend to have higher body fat at the same BMI compared to White European children. For clinical decisions about child weight, a paediatrician or GP uses BMI cen
Supporting a Healthy Weight in Children
For children identified as overweight or obese, the recommended approach differs from adults. Calorie restriction is generally not appropriate in growing children. Instead: focus on reducing highly processed foods, sugar-sweetened drinks, and excessive screen time. Increase physical activity through enjoyable activities — team sports, swimming, cycling — rather than exercise programmes. Maintaining weight while growing taller naturally reduces BMI centile without restriction. The MEND programme
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