Children's BMI is interpreted differently than adults. It is calculated by height and weight, and then compared to children of the same age and sex.
While the formula for BMI is the same for children and adults (weight / height²), its meaning varies. Children's body fat levels change as they grow, and the relationship between body fat and height differs between boys and girls. This is why children's BMI is always expressed relative to other children of the same age and gender.
A BMI-for-age percentile shows how a child's BMI compares to a reference group. For example, if a child is in the 75th percentile, it means their BMI is higher than 75% of children of the same sex and age. The 50th percentile is the exact middle (average).
| Category | Percentile Range |
|---|---|
| Underweight | Less than 5th percentile |
| Healthy weight | 5th to less than 85th percentile |
| Overweight | 85th to less than 95th percentile |
| Obese | 95th percentile or higher |