Child BMI is assessed differently from adults — using age- and sex-specific growth percentiles. Enter your child's details for an instant result.
Unlike adult BMI, children's BMI is compared to growth charts by age and sex.
When most people think of BMI, they think of the standard adult categories: underweight below 18.5, normal up to 24.9, overweight up to 29.9, and obese at 30 and above. These cutoffs work reasonably well for adults whose bodies have finished developing. But for children and teenagers, a fixed cutoff system would be misleading.
A 6-year-old and a 16-year-old are growing at completely different rates and naturally carry different amounts of body fat at different stages of development. Girls typically gain body fat in early puberty; boys gain more muscle mass later. Using the same BMI number to classify both would produce wildly inaccurate results.
To solve this, the CDC developed BMI-for-age growth charts — separate charts for boys and girls, covering ages 2 to 20 — based on national health surveys of thousands of children. Your child's BMI is plotted against these charts to produce a percentile ranking.
A percentile tells you how your child's BMI compares to other children of the same age and sex. If your child is at the 60th percentile, their BMI is higher than 60% of children their age and sex, and lower than 40%. This is squarely in the healthy range.
💡 Important: A higher percentile is not automatically bad, and a lower percentile is not automatically good. What matters is whether the percentile falls within the healthy range (5th–85th) and whether it is stable over time. A sudden jump in percentile — even within the healthy range — may be worth discussing with your doctor.
Underweight children may not be getting enough calories or nutrients to support healthy growth. This can sometimes reflect a medical condition, a very fast growth phase, or difficulty eating. Do not try to address this with supplements or high-calorie foods on your own — speak with your paediatrician first.
The approach for children is very different from adults. The goal is rarely weight loss — it is usually to slow the rate of weight gain while the child continues to grow in height, which naturally brings BMI down over time. Practical steps include:
⚠️ Never comment negatively on a child's weight or body shape. Research consistently shows that weight stigma in childhood increases the risk of disordered eating, anxiety, and long-term poor health outcomes. Focus on healthy habits for the whole family, not on the child's weight specifically.
BMI is a screening tool, not a diagnosis. It cannot distinguish between muscle mass and body fat. A very active, muscular child may have a high BMI percentile despite having healthy body composition. Conversely, a child with low muscle mass may have a normal BMI percentile but still have relatively high body fat. For a thorough assessment, your child's paediatrician will consider BMI alongside growth trends, physical examination, and family history.
Next step
Calculate daily calories, water intake, and ideal weight for adults.
Child BMI — percentile categories explained