BMI z-score calculator for children and adolescents (5 to 18 years)
Enter sex, weight, height and the dates of birth and assessment. You get the BMI-for-age z-score, its classification and every step of the formula.
For: Children and adolescents aged 5 to 18 years (60 to 228 months). Free, no sign-up. Everything is worked out in your browser and nothing is stored.
How the BMI z-score is worked out
First the body mass index is calculated: weight in kilograms divided by height in metres squared. Then the L (skewness), M (median) and S (coefficient of variation) values for that sex and age in months are read from the table, and the LMS formula is applied:
Z = [(BMI / M)L − 1] / (L × S)
How to read the result
The classification compares the BMI with the cut-off points of the same table for that age and sex:
| BMI for age | Classification |
|---|---|
| +3 SD or above | Severe obesity |
| From +2 SD and below +3 SD | Obesity |
| From +1 SD and below +2 SD | Overweight |
| From −1 SD and below +1 SD | Normal |
| From −2 SD and below −1 SD | Underweight |
| −2 SD or below | Undernutrition |
SD: standard deviations. A nutritional diagnosis also takes in height for age, the child's history and the clinical examination.
Frequently asked questions
What is the BMI z-score?
It is how many standard deviations a child's or adolescent's BMI sits from the median for their age and sex. A z-score of 0 is the median; +1 and −1 mark the limits of the normal range.
What ages does this calculator cover?
Children and adolescents from 5 to 18 years: between 60 and 228 months, which is the range covered by the MINSAL 2018 BMI-for-age tables.
Which tables does it use?
The growth standards for the nutritional assessment of children and adolescents published by the Chilean Ministry of Health (MINSAL, 2018), which use the WHO L, M and S values.
How is age in months worked out?
Completed years and months between the date of birth and the date of assessment, adding one month when more than 15 days are left over.
Is the data I enter stored?
No. Everything is worked out in your browser; no data is sent or stored.
Source
Patrones de crecimiento para la evaluación nutricional de niños, niñas y adolescentes, desde el nacimiento hasta los 19 años de edad (Growth standards for the nutritional assessment of children and adolescents from birth to 19 years). Ministerio de Salud, Chile, 2018.
A support tool for health professionals. It does not replace clinical assessment or the technical standard in force.