Why Children's BMI Works Differently from Adults
Adult BMI uses fixed thresholds: below 18.5 = underweight, 18.5–24.9 = normal, 25–29.9 = overweight, 30+ = obese. These fixed numbers cannot be applied to children because children's bodies change dramatically as they grow — the amount of body fat naturally rises and falls at different stages of development, and differs significantly between boys and girls.
Instead, children's BMI uses BMI-for-age percentile charts developed by the CDC (Centers for Disease Control) and WHO. Your child's BMI number is compared to thousands of other children of the same age and sex, and expressed as a percentile ranking.
BMI-for-age percentile charts apply to children and teenagers aged 2 to 19 years. For children under 2, different weight-for-height charts are used. Source: CDC Growth Charts
How to Calculate BMI for a Child — Step by Step
The BMI formula is exactly the same as for adults. The difference is only in how you interpret the result:
Metric formula (same for children and adults):
BMI = weight (kg) ÷ height² (m²)
Example — boy aged 10, weight 35 kg, height 140 cm (1.40 m):
BMI = 35 ÷ (1.40²) = 35 ÷ 1.96 = 17.9 kg/m²
This 17.9 result then needs to be looked up on the CDC BMI-for-age percentile chart for 10-year-old boys to determine the category — you cannot use adult thresholds.
Children's BMI Percentile Categories (CDC)
| BMI Percentile | Category | What it means |
|---|---|---|
| Below 5th percentile | Underweight | Child weighs less than 95% of peers — doctor review advised |
| 5th – 84th percentile | Healthy Weight ✓ | Normal, healthy weight range for their age and sex |
| 85th – 94th percentile | Overweight | Child weighs more than 85% of peers — lifestyle review advised |
| 95th percentile and above | Obese | Child weighs more than 95% of peers — medical consultation advised |
BMI Reference Chart for Boys by Age
The table below shows the BMI values at key percentile thresholds for boys aged 5–18. These are approximate reference values from CDC data:
| Age | 5th %ile (Underweight) | 50th %ile (Median) | 85th %ile (Overweight) | 95th %ile (Obese) |
|---|---|---|---|---|
| 5 years | 13.8 | 15.3 | 16.8 | 17.9 |
| 6 years | 13.5 | 15.3 | 17.0 | 18.4 |
| 8 years | 13.6 | 15.7 | 18.0 | 19.8 |
| 10 years | 14.0 | 16.6 | 19.6 | 21.9 |
| 12 years | 14.7 | 17.8 | 21.5 | 24.3 |
| 14 years | 15.7 | 19.2 | 23.2 | 26.4 |
| 16 years | 16.9 | 20.6 | 24.6 | 28.0 |
| 18 years | 17.9 | 21.7 | 25.8 | 29.3 |
BMI Reference Chart for Girls by Age
Girls have different BMI thresholds at each age because their body fat distribution and development patterns differ from boys:
| Age | 5th %ile (Underweight) | 50th %ile (Median) | 85th %ile (Overweight) | 95th %ile (Obese) |
|---|---|---|---|---|
| 5 years | 13.5 | 15.1 | 16.8 | 18.1 |
| 6 years | 13.2 | 15.1 | 17.1 | 18.6 |
| 8 years | 13.3 | 15.7 | 18.4 | 20.6 |
| 10 years | 13.8 | 16.9 | 20.5 | 23.3 |
| 12 years | 14.6 | 18.4 | 22.6 | 25.9 |
| 14 years | 15.8 | 19.9 | 24.3 | 28.1 |
| 16 years | 16.8 | 20.9 | 25.4 | 29.4 |
| 18 years | 17.5 | 21.4 | 26.0 | 30.4 |
How to Interpret Your Child's BMI Result
- Do not panic if the result is borderline. BMI is a screening tool, not a diagnosis. A child at the 86th percentile is not necessarily unhealthy — build, bone structure, and growth spurts all affect BMI temporarily.
- Track trends, not single numbers. A BMI that is consistently in the 90th percentile matters more than a single measurement.
- Always use age and sex-specific charts. The adult thresholds (25 = overweight, 30 = obese) do not apply to children under 18.
- Consult your doctor if your child is consistently below the 5th or above the 95th percentile. Never put a child on a calorie-restriction diet without professional guidance.
BMI for children should only be interpreted by a healthcare professional in the context of the child's full growth history, diet, activity level, and development. This article is for informational purposes only.
Calculate Your Child's BMI
Our free BMI Calculator uses the standard formula. Enter your child's height and weight — then use the percentile charts above to interpret the result for their age and sex.
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