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BMI Calculator for Children and Teenagers

In short: For children, what counts is not the BMI value alone but the age- and sex-specific percentile. According to the AGA cut-offs, normal weight lies between the 10th and 90th percentile, overweight starts above the 90th, obesity above the 97th.

Calculate your child's BMI with the classification that pediatricians use as well: as a percentile based on the Kromeyer-Hauschild reference data, specific to age and sex, for ages 2 to 18. Free and without sign-up. All calculations run directly in your browser.

Sex

This calculator offers orientation for healthy children and teenagers and does not replace a medical assessment. The reliable classification, especially over time, belongs in pediatric care, for example during regular well-child checkups. Reference: Kromeyer-Hauschild et al. 2001 (updated 2015), cut-offs according to the AGA.

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How the classification works

The BMI formula for children is the same as for adults: weight in kilograms divided by height in meters squared. But because fat and muscle mass shift constantly during growth, the raw number says little on its own. That is why the value is compared with reference curves built from measurements of over 34,000 children (Kromeyer-Hauschild et al.), separated by age and sex. The result is a percentile: your child's position compared to peers. Our calculator uses the LMS method for this, the same procedure used by pediatric practice software.

Classification according to the AGA

The German Working Group on Obesity in Childhood and Adolescence (AGA) recommends this classification:

BMI percentileClassification
below the 3rd percentileSevere underweight
3rd to below the 10th percentileUnderweight
10th to 90th percentileNormal weight
above the 90th percentileOverweight
above the 97th percentileObesity
above the 99.5th percentileExtreme obesity

BMI percentiles as a reference table

For orientation, here are the most important percentile values based on Kromeyer-Hauschild, calculated with the same reference data as the calculator above. P50 is the median BMI of peers, overweight starts above P90, obesity above P97:

Boys (BMI in kg/m²)

AgeP3P50P90P97
2 years14.016.118.019.1
4 years13.415.517.518.8
6 years13.215.417.919.4
8 years13.416.019.021.1
10 years13.816.920.623.4
12 years14.518.022.325.5
14 years15.519.323.726.9
16 years16.620.524.928.0
18 years17.621.626.129.1

Girls (BMI in kg/m²)

AgeP3P50P90P97
2 years13.715.917.919.0
4 years13.115.317.518.9
6 years12.915.418.019.7
8 years13.216.019.321.5
10 years13.616.920.823.5
12 years14.418.222.525.5
14 years15.619.624.127.0
16 years16.720.825.128.0
18 years17.521.726.429.7

What the result means (and what it does not)

  • Snapshot: A single value decides nothing. Pediatricians assess the development over years, which is what regular checkups are for.
  • Growth spurts: Before and during puberty, weight temporarily does not match height. That is part of normal development.
  • Build and sports: As with adults, BMI does not distinguish between muscle and fat mass.
  • No reason for diets: Children do not need diets. Balanced eating and physical activity matter more. Weight goals belong in medical or therapeutic care.

Good next steps for families

If the result is on your mind, talk to your pediatrician first. For everyday life, shared meals, water instead of sugary drinks and physical activity that is fun all help. You can find clear background knowledge in our nutrition basics and family-friendly ideas in our healthy recipes. And if you would like support: through the coach directory you can find qualified nutrition professionals, many of them experienced in family counseling. For yourself, there is the BMI calculator for adults.

Frequently asked questions about BMI in children

How is BMI calculated for children?

The formula is the same as for adults: weight in kilograms divided by height in meters squared. The difference lies in the assessment: for children, BMI is compared with age- and sex-specific reference curves (percentiles), because body composition changes constantly during growth.

What does the percentile mean?

The percentile tells you how your child's BMI compares to peers of the same age and sex. Percentile 75, for example, means that 75 out of 100 children have a lower BMI and 25 a higher one. Values between the 10th and 90th percentile are considered normal weight.

When is a child considered overweight?

According to the guidelines of the German Working Group on Obesity in Childhood and Adolescence (AGA), a child above the 90th percentile is considered overweight and above the 97th percentile obese. Below the 10th percentile is considered underweight, below the 3rd severely underweight.

Which reference values does this calculator use?

The Kromeyer-Hauschild reference values (2001, updated 2015), which are the clinical standard in Germany and are used by the AGA and pediatricians. The calculator classifies the value using the LMS method, the same procedure used in pediatric practice.

My child is above the 90th percentile. What now?

A single value is not a diagnosis. Growth spurts naturally shift the relationship between height and weight for a while. Mention the result at your child's next pediatric appointment. Your pediatrician looks at the development over time, and that says more than a single value.

Does the calculator work for babies?

The reference values start at birth, but in the first year of life BMI is only of limited use and the assessment belongs entirely in pediatric well-child checkups. Our calculator is designed for children and teenagers from 2 to 18 years.

Why does the calculator show different values than an adult table?

Adult cut-offs like BMI 25 do not apply to children. An 8-year-old boy with a BMI of 21.5, for example, is already above the 97th percentile, while a BMI of 21.5 is completely normal for an 18-year-old. That is why age-specific percentile curves are needed.

Sources

  • Kromeyer-Hauschild K et al.: Percentiles for the body mass index in childhood and adolescence based on various German samples. Monatsschrift Kinderheilkunde 2001; 149: 807-818 (reference values incl. 2015 update, provided by the AGA).
  • German Working Group on Obesity in Childhood and Adolescence (AGA): guidelines and percentile cut-offs, adipositas-gesellschaft.de.
  • Cole TJ: The LMS method for constructing normalized growth standards. European Journal of Clinical Nutrition 1990; 44(1): 45-60.