The adult chart isn't helpful when it comes to children's BMI: A 10-year-old with a BMI of 18 may be of completely normal weight, but for an adult, that same value would indicate being underweight. What matters is a comparison with peers—and that's exactly what our percentile calculator does, based on the WHO reference.
Why Children Need Their Own Measure
Children do not grow at a steady rate. In preschool age, the BMI initially decreases, but starting around age six, it rises again—experts refer to this as the “obesity rebound.” In addition, there are significant differences between boys and girls, especially during puberty.
That is why a child’s BMI is always considered in relation to age and gender. The result is a percentile: it indicates what percentage of children of the same age have a lower BMI. A percentile of 60 therefore means that 60 out of 100 children of the same age are lighter—relative to their height.
The WHO Reference for Ages 5 to 19
The calculator uses the 2007 WHO Growth Reference. It describes the distribution of BMI in standard deviations, abbreviated as SD or Z-score. This yields the following ranges:
| Range | Z-Score | Percentile |
| Severely underweight | below −3 SD | below 0.1 |
| Underweight | −3 to −2 SD | 0.1 to 2.3 |
| Normal weight | −2 to +1 SD | 2.3 to 84.1 |
| Overweight | +1 to +2 SD | 84.1 to 97.7 |
| Obesity | Over +2 SD | over 97.7 |
An example: A 10-year-old boy who is 140 centimeters tall and weighs 34 kilograms has a BMI of 17.3. The median for his age is 16.4—so he is slightly above that, with a Z-score of about +0.5, placing him around the 68th percentile. This is clearly within the normal range.
WHO or German percentile curves?
In Germany, pediatric practices typically use the Kromeyer-Hauschild reference values, which are recommended by the Working Group on Childhood and Adolescent Obesity. According to these guidelines, the 90th percentile marks the threshold for being overweight, and the 97th percentile marks the threshold for obesity.
The WHO reference is somewhat stricter, as it classifies individuals as overweight starting at the 85th percentile. Both systems are scientifically recognized—but the values are not directly comparable. You’ll find the growth chart your pediatrician’s office uses in your child’s yellow health record booklet.
What a high value means—and what it doesn’t
A single measurement says very little. The trend over the years is much more meaningful: If a child stays on their percentile curve, this is usually nothing to worry about, even if that curve is at the 90th percentile. If, on the other hand, it jumps from the 50th to the 90th percentile within one to two years, it’s worth taking a closer look.
It’s also important to note that BMI does not measure body composition. An athletic child with a lot of muscle mass may fall above the reference range on paper without this being a cause for concern.
What parents should not do: Start diets on their own. For children, the goal is usually to maintain their weight and grow into their height, rather than to lose weight. Anything else should be done under the supervision of a pediatrician.
Physical activity is the more effective approach
The WHO recommends an average of 60 minutes of moderate to vigorous physical activity per day for children and adolescents, plus activities three days a week that strengthen muscles and bones—such as climbing, running around, ball sports, and gymnastics.
For gyms and sports clubs, this is a growing market: children’s classes, vacation programs, and parent-child activities appeal to families who don’t typically participate in traditional fitness programs. Our training heart rate calculator shows how to assess exercise intensity—for adults; with children, the focus is on fun and play rather than heart rate zones.
What the calculator can’t do
It uses WHO values for full years of age and calculates linearly between them. This is accurate enough for home use, but not for tracking progress over time—that’s what the growth charts in the child’s health record are for.
It does not account for children under five years of age, premature babies, chronic illnesses, or growth disorders. If your child is gaining an unusually large or small amount of weight, is barely growing, or if you’re unsure, your pediatrician’s office is the right place to go—not an online calculator. You can find the BMI for adults in our BMI calculator.
Frequently Asked Questions / FAQ
At what age does the pediatric BMI become useful?
The calculator uses the WHO reference for ages 5 to 19. For younger children, there are separate growth standards that are used during well-child checkups.
What is the difference between a percentile and a Z-score?
Both describe the same position, just in different terms. The Z-score indicates the deviation from the median in standard deviations, while the percentile translates that into “out of 100 children.” A Z-score of 0 corresponds to the 50th percentile.
My child is very athletic—is the measurement still accurate?
To a limited extent. Muscle mass weighs more than fat, so athletic children often score higher without there being any underlying cause. The trend and the overall picture matter more than the single number.
How often should I check these measurements?
Once or twice a year is plenty—ideally around the time of their well-child checkups. Measuring and weighing them more frequently makes weight an unnecessary issue for children.
When should I see the pediatrician?
If the value is above the 97th percentile or below the 3rd percentile, if it fluctuates significantly within a short period of time, or if your child complains of symptoms. Even if you’re unsure, making an appointment is always the better choice than waiting to see what happens.
Editorial team at fitnessmarkt.de
Image source: #17539155 freepik / freepik.com
Published on: 21 October 2026