BMI (Body Mass Index) is calculated the same way for children as for adults: body weight [kg] / height² [m²]. However, the interpretation is entirely different. For adults, fixed thresholds apply (18.5–24.9 = normal), but for children the normal range shifts with age and sex — which is why results must be compared against percentile charts.
What Is a Percentile Chart?
A percentile chart is a graph showing the distribution of BMI (or height or weight) among children of the same age and sex. The percentile indicates what proportion of peers have a lower value than the child being assessed.
- Below the 5th percentile — underweight (paediatric assessment needed).
- 5th–85th percentile — healthy weight.
- 85th–95th percentile — overweight.
- Above the 95th percentile — obesity.
In Poland, paediatricians use growth charts developed by the Institute of Mother and Child (IMiD), last updated in 2012 based on a national survey.
Worked Example: BMI Calculation for a Child
Consider an 8-year-old girl, height 128 cm, weight 28 kg:
- BMI = 28 / (1.28)² = 28 / 1.6384 ≈ 17.1
The raw value 17.1 kg/m² tells us very little. Only after plotting it on the IMiD chart for 8-year-old girls does it become apparent that BMI = 17.1 corresponds to approximately the 60th percentile — a healthy result.
By comparison, an 8-year-old boy with the same BMI of 17.1 may fall on a different percentile because the charts for boys and girls differ.
When Should a Child's BMI Be a Concern?
It is worth consulting a paediatrician or dietitian when:
- A child's BMI is above the 95th percentile (obesity) or below the 5th percentile (underweight).
- The BMI percentile rises or falls sharply within a single year.
- The child shows other symptoms: fatigue, joint pain, difficulty concentrating.
Important: BMI is only one indicator. Assessing a child's health should also take into account waist circumference, physical activity levels, diet, and family history.
Growth vs BMI — Which Matters More?
During periods of rapid growth (the pubertal growth spurt), a child's BMI may temporarily drop even though they are healthy — the body lengthens faster than it gains weight. Conversely, in children with delayed growth, BMI may be temporarily elevated. Always assess BMI alongside height and weight charts individually.
BMI Calculator for Children
Our BMI calculator covers both adults and children. Enter the child's age, sex, height, and weight and it automatically compares the result against IMiD percentile charts, indicating which percentile band the result falls in. Remember that the calculator is for informational purposes only — always consult a doctor with any concerns about your child's weight.