# Calculation of the BMI of Children and Adolescents

BMI for Children ﹣ Calculator

BMI (Body Mass Index) is a measure of weight in relation to height. The general formula is: BMI = weight / (height in m)2. Converted into Anglo-American units, the formula for BMI is: 703 × weight in lbs / (height in inches)². However, the interpretation of this BMI value is different for children compared to adults. The BMI calculator for children takes into account the assessment of BMI according to the S2 guideline, as described in more detail below.

## Is my child overweight?

Many concerned parents wonder if their child is overweight or if their development is normal. The assessment of weight for children is different from the BMI calculation for adults. For children and adolescents, reference curves are used to determine if and how much their BMI deviates from the normal range. Accordingly, the weight of the children is assessed. Weight classification is carried out according to thev S2 guideline of the Working Group on Childhood and Adolescent Obesity using gender-specific age percentiles (reference curves) as noted by Kromeyer-Hauschild et. al..

These reference curves specifically include values of age, gender, height and weight, which come from earlier surveys when relatively few children were overweight. If one were to use more recent surveys as a comparative value or reference, one would be comparing a child with an 'overweight reference' to assess BMI because of the increase number of obese children. This would lead to the conclusion that the child's BMI is average, i.e. within the normal range, although the child is obese.

## What is body mass index?

The body mass index (BMI) is used to estimate the body fat percentage in children and adolescents. The body weight is put in relation to the child’s height. It is calculated as follows: BMI = weight / (height in m)² or to take into account the Anglo-American units BMI = 703 × weight in lbs / (height in inches)². The BMI value indicates whether or not children and adolescents are overweight. Sporty children and adolescents can have an increased BMI value without being overweight. An elevated body mass index neither tells you anything about the cause of the overweight condition nor does it indicate which therapy is most suitable. In this case, please consult a general practitioner.

## What age is the BMI formula for children suitable for?

In theory, BMI can be calculated from the first year of life. However, this formula should not be used for babies. At this age, children and their weight change so dynamically that a BMI calculation makes almost no sense. Your paediatrician should therefore check your child's weight and progress as part of the U examinations. You are welcome to use the BMI calculator for toddlers and adolescents. Based on the age of majority, the Body Mass Index Formula for Adults applies.

## What BMI is normal for children?

Depending on the age, very different BMI values are considered normal. If your child is developing faster or slower than their peers, their BMI may be different from the average. Use the BMI calculator for children to compare your child's body weight to their age and height. Consider the results of our calculator as indications. Never put your child on a diet on your own and consult a paediatrician.

## What BMI is not normal for children? (Example)

Since the interpretation or assessment of the BMI of children is gender- and age-dependent, we show below by way of example, how the BMI of a 15-year-old girl with a height of 5 feet and 2.5 inch is to be evaluated. (The BMI calculator for children can of course be used to make concrete BMI calculations for children and adolescents of all other ages.)

### Severely underweight at 88 lbs

According to the Federal Centre for Health Education, a 15-year-old girl with a height of 5 feet and 2.5 inch is extremely light and severely underweight. The BMI of 15.84 is below the 3rd percentile. This means that less than 3 percent of all girls of the same age have such a low weight. She should seek advice from a doctor along with her parents.

### Slightly underweight at 94 lbs

The adolescent is slightly underweight in relation to her height. The BMI of 16.92 is below the 10th percentile. This means that less than 10 percent of all girls of the same age have such a weight. Her parents should be consulted about this. If there is frequent tiredness, listlessness or illness, they should visit a doctor together.

### Normal weight in the lower normal range at 101 lbs

According to the Federal Centre for Health Education and the S2 guideline of the Working Group on Obesity in Childhood and Adolescence, the weight of 15-year-olds in relation to their height of 5 feet 2.5 inch is in the lower normal range. The BMI of 18.18 lies between the 10th and the 25th percentile. As long as she is healthy and feels strong and fit, everything is fine.

### Normal weight at 116 lbs

According to the Federal Centre for Health Education and the S2 guideline, the weight of adolescents in relation to their height is within the normal range. The BMI of 20.88 lies between the 25th and 75th percentiles.

### Normal weight in the upper normal range at 130 lbs

The weight of the 15-year-old girl, at 130 lbs, is in the upper normal range in relation to her height of 5 feet 2.5 inch. The BMI of 23.40 is between the 75th and 90th percentiles. This is fine, however, a bit closer attention should be paid to what and how much she eats. Perhaps a little more exercise should also be done.

### Overweight at 145 lbs

According to the S2 guideline, the adolescent is overweight in relation to her height. The BMI of 26.10 is between the 90th and 97th percentiles. She should talk to her parents or consult with her doctor.

### Obesity at 160 lbs

According to the S2 guideline of the Working Group on Obesity in Childhood and Adolescence, the girl has obesity in relation to her height. The BMI of 28.80 lies between the 97th and 99.5th percentiles. This means that less than 3 percent of all girls of the same age have a weight this high. Together with her parents, she should see a doctor in order to possibly set up a suitable weight loss programme with other people who are affected.

### Extreme obesity at 180 lbs

At 180 lbs and a height of 5 feet 2.5 inch, a 15-year-old girl has extreme obesity according to the S2 guideline. The BMI of 32.40 is above the 99.5 percentile. This means that less than 0.5 percent of all girls of the same age have a weight this high. Together with her parents, she should see a doctor in order to find a suitable weight-loss programme with other people who are affected.

## Input 'help' on the BMI Calculator for Children

The child BMI or more precisely the assessment of the BMI for children depends on the following inputs or parameters:

### Units of measurement

Please select the appropriate units of measurement.

#### US/UK

By selecting 'US/UK', Anglo-American units of measurement are taken into account. Pounds (lbs) are used for weight and feet (ft) or inches (in) are used for height.

#### Metric

After selecting 'Metric', metric units of measurement are taken into account. Kilograms (kg) are used for weight and centimetres (cm) are used for height.

### Weight

Please state the child's body weight in kg or in lbs, as the BMI (Body Mass Index) is a measure for assessing weight in relation to the child's height.

### Height

Please indicate the child's height in cm or in feet and inches. The BMI is also a measure for assessing weight in relation to height in children.

### Date of birth

Please indicate the child's date of birth. Age-specific BMI reference values are used to assess the weight of children and adolescents.

### Gender

Please indicate the gender of the child. For the assessment of the weight of children and adolescents, gender­specific reference values of BMI are used in addition to age­specific reference values.

## Source information

As source for the information in the 'BMI for Children' category, we have used in particular:

## Last update

This page of the 'BMI for Children' category was last edited or reviewed by Stefan Banse on February 20, 2023. It corresponds to the current status.

### Changes in this category "BMI for Children"

• Expand the page to BMI calculator for children
• Adaptation of the BMI calculator for children and the texts to the new S2 guidelines from 2020