How to understand BMI-for-age
Children’s BMI uses age-specific references
A child’s BMI needs to be interpreted using their exact age and sex. The same BMI can mean different things at different ages, so this tool uses the corresponding BMI-for-age table.
How does WHO classify BMI-for-age?
For ages 5–19, WHO uses BMI-for-age z-scores: below -2 SD indicates thinness, above +1 SD indicates overweight, and above +2 SD indicates obesity. Up to 60 months, the upper thresholds differ: above +1 SD indicates risk of overweight, above +2 SD indicates overweight, and above +3 SD indicates obesity.
Example: 150 cm and 40 kg
Parents often look for a “free WHO-based BMI calculator for children” to understand a child’s weight. At 150 cm and 40 kg, BMI is approximately 17.8. Its meaning still depends on the child’s age and sex.
The BMI is approximately 17.8 kg/m². A weight category cannot be assigned without age and sex. For example, at age 10 this is below +1 SD for both boys and girls in the embedded WHO tables; the interpretation can differ at younger ages.
Why enter both height and weight?
BMI is weight divided by height squared. Weight alone gives an incomplete picture: children who are 150 cm and 125 cm tall should not be assessed in the same way simply because both weigh 40 kg.
What does Transform Kid focus on?
Transform Kid focuses on age-appropriate strength, endurance, coordination, agility, balance, flexibility and movement habits. Goals reflect each child’s abilities; the same weight-change target is not applied to every child.