How do I calculate BMI for children?

The formula is the same as for adults: weight in kilograms divided by height in metres squared. A child of 130 cm and 27 kg therefore has a BMI of 27 ÷ (1.30 × 1.30) = 16.0. Only the assessment differs: the value is compared with children of the same age and sex and expressed as a percentile. That is exactly what the calculator above does.

What does the BMI percentile mean?

The percentile shows where your child's BMI lies within a reference group of girls or boys of the same age. Percentile 60 means that about 60% of the children in this group have a lower BMI and about 40% a higher one. The percentile is not a grade and not a body-fat percentage — it only describes the position in comparison.

At which percentile is a child considered overweight?

According to the guideline of the German Working Group on Obesity in Childhood and Adolescence (AGA), a BMI above the 90th percentile is classed as overweight, above the 97th as obesity and above the 99.5th as severe obesity. Below the 10th percentile is underweight, below the 3rd severe underweight. These are statistical thresholds — a medical assessment also considers the trend, build and overall health.

Which reference values does the calculator use?

The calculator uses the BMI percentiles by Kromeyer-Hauschild et al. that are standard in Germany (Monatsschrift Kinderheilkunde 2001; adjusted for ages 15 to 18 in 2015). They are based on measurements of around 34,000 children and adolescents from 17 studies and are recommended by the AGA guideline. Other countries and the WHO use their own references, so calculators from abroad may show slightly different percentiles.

What is the SDS value?

SDS stands for standard deviation score. It indicates by how many standard deviations the BMI differs from the median of children of the same age: 0 corresponds to the 50th percentile, +1 to roughly the 84th and +2 to roughly the 97.7th percentile. The SDS is particularly useful for very high or very low values, where changes barely show in the percentile but are clearly visible in the SDS.

Why does BMI fall in toddlers at first?

In the first year of life BMI rises steeply, then falls until around the age of 4 to 5 because children mainly grow in length during this time. Afterwards it rises again — this turning point is called the adiposity rebound. That is why the same BMI can be unremarkable at age 1 and raised at age 5. The percentile curves reflect this pattern.

Is BMI meaningful for babies and toddlers?

Only to a limited extent. Although the reference values start at birth, in the first two years paediatricians mainly look at length, weight and head circumference over time, as recorded in the child health booklet. At this age the lying length is measured. For children under 2, treat the result as a rough guide at most.

My child is above the 90th percentile — what now?

First of all: a single value is a snapshot. Growth spurts, muscle and build influence BMI. Raise the result at the next check-up or make an appointment. Your paediatric practice will assess the trend and discuss with you whether support makes sense. Please do not put your child on a diet yourself, and talk about bodies and food respectfully in front of them — children should never have to feel ashamed of their weight.

How precisely do I need to measure?

Ideally in the morning, without shoes and in light clothing. Measure height against a wall with a straight back and a book or set square on the head. Even a 2 cm error in height noticeably shifts a school child's BMI. For the most accurate result, use the values from the last check-up and enter the age on the day of measurement via the date of birth.

BMI in Teenagers: What Changes During Puberty | Child BMI Calculator

Puberty: a time of big changes

Hardly any phase of life brings such marked physical change as puberty. Teenagers grow many centimetres in a short time, muscles and bones develop, and the distribution of body fat changes. All of this affects BMI. If you want to understand a teenager's BMI, it helps to know about these changes — and why adult cut-offs do not yet apply.

Why BMI rises in adolescence

BMI rises steadily from school age to the end of puberty. According to the German reference values, the median for boys is 18.0 at age 12 and 21.0 at age 17. For girls it rises from 18.2 to 21.3 over the same period. A BMI of 20, which is already in the upper healthy range for a 12-year-old boy, is below average for a 17-year-old.

Girls and boys differ markedly. On average, girls enter puberty earlier. Between about 11 and 16 they have a slightly higher median BMI than boys. After that the rise flattens in girls, while boys continue to gain through muscle growth.

Early and late developers

The reference curves are based on calendar age, not on stage of development. Yet two 13-year-olds can be years apart: one is already in the middle of the growth spurt, the other still has it ahead. Early developers often have a temporarily higher BMI, late developers a lower one. These differences usually even out later.

For parents this means that a notable value during puberty should always be seen in context. Your paediatrician can judge which stage of development your child is in and whether the BMI fits.

Muscle or fat? The limits of BMI

BMI does not distinguish between muscle and fat. Athletic teenagers with a lot of muscle can be above the 90th percentile without carrying too much body fat. Conversely, an unremarkable BMI can hide a high fat percentage if there is little muscle. Doctors therefore take further information into account, such as waist circumference, physical activity and family history.

The transition to adult BMI

At 18 the reference for children and adolescents ends, and the fixed adult cut-offs apply from then on. The German reference values for 15- to 18-year-olds were adjusted in 2015 so that the transition is seamless. At 18, a BMI of 25 corresponds to roughly the 83rd to 85th percentile and a BMI of 30 to about the 97th to 98th percentile — close to the thresholds used during adolescence. From the 18th birthday, use the adult BMI calculator.

How to support teenagers well

Teenagers are often particularly sensitive about their bodies during puberty. Comparisons with others, comments on appearance or pressure to lose weight can harm their self-image. It is more helpful to talk about well-being, energy and health rather than numbers. If your child has questions about their weight, the paediatric or adolescent practice is a good place to go — including for a one-to-one conversation.

Also watch for warning signs in the other direction: marked weight loss, unusual eating behaviour or excessive exercise can indicate an eating disorder. Early medical help is particularly important here.

In summary

Teenagers' BMI rises with age, develops differently in girls and boys and fluctuates with the pace of development. The percentile in the BMI calculator for children and teens shows where your child stands compared with peers. For the assessment, the trend across several measurements remains decisive.

BMI in Teenagers: What Changes During Puberty

Puberty: a time of big changes

Why BMI rises in adolescence

Early and late developers

Muscle or fat? The limits of BMI

The transition to adult BMI

How to support teenagers well

In summary

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