Child BMI (approximate Z-score)

Pediatric BMI: percentile by age and sex

For kids and teens, the math behind BMI is identical to the adult version, BMI = weight(kg) / height(m)², but the reading is different. Instead of fixed adult cut-offs, you check the result against age- and sex-specific growth curves and report a percentile. The reference here is the WHO Child Growth Standards (0–5 years) and the WHO Growth Reference (5–19 years). On those curves, P<5 is underweight, P5–P85 healthy, P85–P95 overweight and ≥P95 obesity. Take a worked Example: a 10-year-old boy at 35 kg and 1.40 m comes out at BMI = 17.9 kg/m², roughly the 75th percentile and well inside the healthy band. Hand that same 17.9 kg/m² to an adult chart and it reads as underweight, which is exactly why pediatric interpretation has to go through the curves.

Clinical context

In Brazil, the Sociedade Brasileira de Pediatria (SBP) follows the WHO curves, and the Caderneta da Criança from the Ministry of Health is the official record for tracking growth from birth through age 9. You'll see the BMI percentile show up in prenatal and pediatric care, in school health programs, and as one of the metrics behind the Bolsa Família conditionalities, which monitor the health of children in low-income families. The ECA (Estatuto da Criança e do Adolescente) treats the right to growth monitoring as a public-health obligation.

FAQ

Can I use the adult cut-offs (18.5 / 25 / 30) on a child? No. Applied to a child, they'd put most kids in the wrong category. Stick to age- and sex-specific percentiles.

Does this calculator replace pediatric assessment? No, treat it as an approximation. A pediatrician will also weigh Tanner stage, growth velocity, family history and lab results before calling a child under- or overweight.

What's the difference between WHO and CDC curves? The WHO curves describe how children should grow when conditions are good. The CDC curves describe how US children actually did grow between the 1960s and the 1990s. WHO is the international standard, and it's the one Brazil uses.