Question 1

How do I calculate BMI for children?

Accepted Answer

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.

Question 2

What does the BMI percentile mean?

Accepted Answer

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.

Question 3

At which percentile is a child considered overweight?

Accepted Answer

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.

Question 4

Which reference values does the calculator use?

Accepted Answer

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.

Question 5

What is the SDS value?

Accepted Answer

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.

Question 6

Why does BMI fall in toddlers at first?

Accepted Answer

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.

Question 7

Is BMI meaningful for babies and toddlers?

Accepted Answer

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.

Question 8

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

Accepted Answer

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.

Question 9

How precisely do I need to measure?

Accepted Answer

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.

Guide: Child BMI Calculator

BMI in Children: Understanding and Interpreting Percentiles

Reading Percentile Charts: How to Interpret Growth Curves

BMI in Teenagers: What Changes During Puberty

Child BMI vs. Adult BMI: Why 18.5 and 25 Don't Apply to Children

Underweight in Children: When Parents Should Take a Closer Look

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