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The formula of calculating the BMI is the same as for adult people. But the meaning is different.
A team of scientists (K. Kromeyer-Hauschild, M. Wabitsch, D. Kunze et al.) has taken 2001 the values of a big amount of boys and girls. These shows the spreading of the Body Mass Index with age as reference.
The charts have the age from left to right and the BMI from bottom to top.
You see Percentil-Lines in the chart. P50 means, that 50% of the kids have a higher and 50% have a lower BMI. P3 means, that 3% of the kids have a lower BMI.
Overweight is from P90 and higher. This means, 10% of the kids are overweight. Underweight is lower tahn P3. In the charts above the areas are colored in red. Normal weight is with green color. This borders are more to fit for scientists needs and less for the child.
More info about BMI:BMI Script, BMI Origin, BMI Formula, BMI Meaning, BMI Statistic, BMI Kids, BMI Crisitism, BMI-Pedia BMI-Factsheet