Paediatric BMI percentile classification
Why children use percentiles rather than fixed thresholds
Body composition changes dramatically throughout childhood and adolescence. As children grow, the healthy ratio of weight to height varies by age and sex in ways that fixed thresholds cannot capture.
For example, it is normal for boys' BMI to decrease between ages 1 and 6 as they grow taller and leaner, then increase through puberty as muscle mass develops. Girls' BMI increases through puberty as fat mass develops naturally. A BMI of 18 is healthy for a 16-year-old girl but may indicate underweight for a 12-year-old boy of the same height.
Percentiles solve this problem by comparing a child's BMI to the distribution of BMI values in a reference population of children of the same age and sex. A child at the 75th percentile has a higher BMI than 75% of children of the same age and sex in the reference population. This comparison accounts for age-related and sex-related differences in healthy body composition.
BMI formula — same as adults
Worked examples
BMI = 35 / (1,42)^2 = 35 / 2,0164 = 17,4. For a 10-year-old boy, this BMI falls near the 50th to 60th percentile on the WHO growth chart, meaning this child's BMI is typical for his age and sex. The adult classification would suggest this is underweight (below 18,5), which would be completely wrong for a 10-year-old.
BMI = 58 / (1,60)^2 = 58 / 2,56 = 22,7. For a 14-year-old girl, BMI 22,7 falls near the 75th percentile — higher than 75% of girls the same age, but still within the healthy range. The adult threshold of 25 for overweight cannot be applied here: a 14-year-old girl with BMI 24,9 may be at the 90th percentile and classified as overweight by paediatric standards.
Calculate BMI
Enter height and weight to calculate BMI. For children, always interpret the result using age and sex-specific percentile charts rather than adult thresholds.
Paediatric BMI percentile classification — WHO and CDC
| Percentile Range | Classification | Clinical meaning |
|---|---|---|
| Below 3rd | Severely underweight | Warrants immediate clinical evaluation |
| 3rd to 5th | Underweight | Monitor — may indicate nutritional concern |
| 5th to 84th | Healthy weight | Normal range for age and sex |
| 85th to 94th | Overweight | Above healthy range — lifestyle review advised |
| 95th and above | Obese | Significantly above healthy range — clinical referral |
WHO vs CDC growth charts
Two main reference datasets are used for paediatric BMI percentiles: the WHO Child Growth Standards and the CDC Growth Charts.
The WHO growth charts (2006) are based on children raised in optimal conditions across six countries and represent how children should grow. They are recommended by most international health organisations for children from birth to age 5.
The CDC growth charts (2000) are based on a representative sample of US children and describe how children did grow in the reference population. They are widely used in clinical practice for children aged 2 to 20 in the United States and are also used internationally.
For children under 2 years, the WHO weight-for-length charts are more appropriate than BMI. BMI is generally considered appropriate from age 2 onwards.
The choice between WHO and CDC charts does not typically change the clinical classification substantially, but the two charts produce slightly different percentile values for the same BMI at a given age and sex.
What to do with results
A paediatric BMI result outside the healthy range (below the 5th or above the 85th percentile) should be discussed with a paediatrician or general practitioner. BMI percentile is a screening tool, not a diagnosis.
For results in the overweight or obese range, a clinical assessment will consider: the child's growth trajectory over time (is the BMI rising relative to peers or stable?), family history, dietary patterns, physical activity, and other health markers including blood pressure and blood glucose.
For results in the underweight range, the assessment will consider: growth rate, pubertal stage, dietary intake, and any symptoms of underlying conditions.
Importantly, discussing a child's weight and BMI requires care. Research shows that how weight is communicated to children and adolescents affects body image and can influence eating behaviour. Many health organisations recommend focusing on health behaviours (eating, activity, sleep) rather than weight or BMI numbers when talking with children.
Common mistakes
Methodology
Paediatric BMI uses the same formula as adult BMI: weight (kg) / height (m) squared. Percentile classifications follow WHO Child Growth Standards (2006) for children under 5 and CDC Growth Charts (2000) for ages 2 to 20 in clinical practice. The cut-off points (5th, 85th, 95th percentiles) follow international consensus guidelines.
BMI percentile is a screening tool requiring clinical interpretation. No conclusion about a child's health should be drawn from BMI percentile alone without professional assessment.
Calculate BMI now
Enter height and weight to calculate BMI. Remember to use age and sex-specific percentile charts for anyone under 18.
Frequently asked questions
Formula based on standard mathematical and financial methods. Results are for informational purposes. Last reviewed May 2026. Version 1.