Same formula, different assessment
Children's BMI is calculated exactly like adults': weight in kilograms divided by height in metres squared. Even so, it would be a mistake to apply the familiar cut-offs of 18.5 for underweight and 25 for overweight to children. This article explains why and how the assessment works instead.
Why fixed cut-offs do not work for children
In adults, body build is largely stable. In children, the relationship between weight and height changes constantly. Babies gain a lot of weight in the first year, toddlers mainly grow in length, and teenagers build muscle and bone mass during puberty. The typical BMI therefore differs considerably depending on age.
A few figures from the German reference values make this clear. The median BMI of a five-year-old boy is 15.4, that of a 17-year-old young woman 21.3. By adult standards almost all primary-school children would be underweight — which is obviously not the case.
An example: BMI 17
How strongly the assessment depends on age is shown by a BMI of 17:
1. For a one-year-old boy, BMI 17 is on roughly the 56th percentile — right in the middle.
2. For a five-year-old boy, the same value is on roughly the 83rd percentile — in the upper healthy range.
3. For an eight-year-old girl, BMI 17 corresponds to roughly the 67th percentile.
4. For a 15-year-old girl, BMI 17 is on roughly the 8th percentile — underweight by the AGA definition.
By adult cut-offs all four children would be underweight. The percentiles, by contrast, show a completely different and realistic picture.
A second example: BMI 24
Adult values mislead in the other direction too. A BMI of 24 counts as a healthy weight in adults. For a ten-year-old boy, however, this value is on roughly the 98th percentile, which by the AGA definition is in the obesity range. Anyone using the adult yardstick here would overlook a value that should be looked at by a doctor.
The solution: percentiles
Instead of fixed cut-offs, children are compared with peers of the same age and sex. The German Working Group on Obesity in Childhood and Adolescence (AGA) recommends the German reference values by Kromeyer-Hauschild for this. Above the 90th percentile is classed as overweight, above the 97th as obesity, below the 10th as underweight. The child BMI calculator does this classification for you.
The transition at 18
The reference values for children run until the 18th birthday. They were designed to merge almost seamlessly into the adult cut-offs. At 18, a BMI of 25 corresponds to roughly the 83rd to 85th percentile, a BMI of 30 to about the 97th to 98th percentile, and a BMI of 18.5 to about the 8th to 9th percentile. From 18, the adult BMI calculator is the right tool.
What they share — and their limits
Child and adult BMI have one thing in common: neither distinguishes between muscle and fat, and neither says anything about fat distribution. Both are indicators, not diagnoses. In children, values also change with growth. A single value is therefore only a snapshot — it becomes meaningful over time and in conversation with your paediatrician.