Health⏱ 5 min read
How to Calculate BMI for Children (and Why It Works Differently)
Children's BMI uses age- and sex-adjusted percentiles, not fixed cut-offs. A BMI of 18 means something completely different for a 10-year-old than for an adult. Here is how paediatric BMI works.
Adult BMI uses fixed thresholds (18.5, 25, 30). Children's BMI uses centile charts because a healthy weight varies significantly with age and sex during growth. The raw number alone is almost meaningless without the chart.
Calculating BMI (Same Formula as Adults)
BMI = weight (kg) / height (m)^2
Example: 10-year-old girl, 35kg, 140cm (1.40m)
BMI = 35 / (1.40 x 1.40) = 35 / 1.96 = 17.9
Same calculation as adult BMI.
The difference is in interpretation -- you need a centile chart.
UK Centile Thresholds (Ages 2-18)
BMI CentileClassificationMeaning
Below 2ndUnderweightBelow 98% of children this age/sex
2nd to 91stHealthy weightNormal range
91st to 98thOverweightAbove 91% of same age/sex
98th and aboveVery overweight (obese)Above 98% of same age/sex
The Same BMI Means Different Things at Different Ages
BMI of 17 interpretation by age and sex (UK charts):
5-year-old boy: 97th centile -- very overweight
10-year-old boy: 75th centile -- healthy weight
15-year-old boy: 25th centile -- healthy weight (lean)
Adult (any age): 17.0 -- underweight (below 18.5 threshold)
This is why comparing a child's BMI to adult thresholds is meaningless.
The same number spans the full range from underweight to obese
depending on the child's age and sex.
The BMI Rebound (Age 5-7)
Children's BMI naturally rises until age 1, falls to age 5-6,
then rises again through adolescence. This dip and rise is normal.
"Adiposity rebound" (the rise after age 5) is normal development.
Research shows earlier rebound (before age 5) correlates with
higher adult BMI -- but this is epidemiological, not diagnostic.
A child whose BMI rises from age 5 onwards:
Normal if following their centile channel (same centile +/- 1-2 bands)
Concerning if crossing centiles upward (e.g. 50th to 91st over 1-2 years)
What to Do With the Number
Paediatric BMI is a screening tool, not a diagnosis. A child above the 91st centile should be assessed by a GP or health visitor who will consider family history, growth trajectory, activity levels, and diet before any action is taken. For children at the 91st centile or above, the NHS offers the National Child Measurement Programme and GP referral to the Healthy Child Programme.
BMI does not measure body composition. A muscular or early-developing child may have a higher BMI centile without excess body fat. Clinical assessment always takes precedence over the number alone.