BMI for Kids & Children
Children's BMI cannot be interpreted using adult thresholds. A BMI of 22 means something completely different in a 9-year-old vs a 35-year-old. Here's how the paediatric system works — and what the numbers actually mean.
Why Children Need a Different System
Children's bodies change rapidly. As they grow, their ratio of muscle, fat, bone, and height shifts constantly — so a fixed BMI threshold like "18.5–24.9 = healthy" is meaningless for a growing child.
Instead, paediatric BMI uses BMI-for-age percentiles. A child's BMI is calculated the same way (weight ÷ height²), then compared to a reference population of children the same age and sex. The result is a percentile, not a category.
CDC BMI-for-Age Percentile Categories
| Percentile range | Category | Action |
|---|---|---|
| Below 5th | Underweight | Speak with a doctor — may indicate nutritional issues |
| 5th to 84th | Healthy weight | Normal — continue healthy habits |
| 85th to 94th | Overweight | Monitor — discuss with paediatrician |
| 95th and above | Obese | Clinical assessment recommended |
Source: CDC (Centers for Disease Control and Prevention). These categories apply to children aged 2–19. The WHO uses slightly different reference charts but the same percentile-based approach.
BMI Changes Through Childhood
Children's BMI follows a characteristic pattern:
- Birth to age 1: BMI rises rapidly as infants gain weight faster than height.
- Ages 1–6 ("adiposity rebound"): BMI typically falls as children grow taller and leaner.
- Around age 5–7: BMI starts rising again — this is the "adiposity rebound." Children who experience this rebound earlier (before age 5) have higher adult obesity risk.
- Adolescence: BMI rises with puberty. Girls typically see a larger increase in body fat; boys gain more lean mass.
Average BMI Reference by Age
| Age | Boys median BMI (50th %ile) | Girls median BMI (50th %ile) |
|---|---|---|
| 2 years | 16.5 | 16.4 |
| 4 years | 15.8 | 15.5 |
| 6 years | 15.5 | 15.3 |
| 8 years | 16.3 | 16.2 |
| 10 years | 17.5 | 17.5 |
| 12 years | 18.9 | 19.2 |
| 14 years | 20.5 | 21.0 |
| 16 years | 22.0 | 21.9 |
| 18 years | 23.2 | 22.4 |
Source: CDC growth charts (2000). Median values — half of healthy children fall above, half below.
Talking to Children About Weight
Research consistently shows that how parents and healthcare providers discuss weight with children matters significantly for long-term outcomes:
- Avoid using the words "fat," "obese," or "diet" with children — they correlate with higher risk of disordered eating and lower self-esteem.
- Frame discussions around health behaviours (sleep, activity, food variety) rather than weight or appearance.
- BMI screening in schools without proper context or counselling has been shown to cause harm — several countries have discontinued it.
- If a paediatrician raises concerns about a child's BMI, ask for a full assessment including growth trajectory, not just a single measurement.
Adult BMI dashboard
Our calculator is designed for adults (18+). For children's BMI, use the CDC's BMI-for-age calculator.
Open Adult Dashboard →