About this tool
The Child BMI Percentile Calculator is a free, in-browser tool that turns a child's height, weight, age and sex into a BMI percentile and z-score, then classifies the result into the standard WHO categories (thinness, normal, overweight, obesity). Adult BMI cut-offs like 25 or 30 do not apply to children, because healthy BMI changes a lot with age and differs between boys and girls — percentiles compare a child with others of the same age and sex instead.
It uses the WHO 2007 growth reference for ages 5–19 (BMI-for-age LMS tables) embedded directly in the page, so nothing is looked up online and everything is computed on your device. The z-score is calculated with the standard LMS formula and converted to a percentile with the normal distribution.
Frequently asked questions
Which reference data does this calculator use?
The WHO 2007 growth reference for 5–19 years (BMI-for-age). The published LMS values are embedded at 6-month intervals and linearly interpolated for ages in between, which stays within about 0.01 z of the full monthly table.
What do the WHO categories mean?
Following WHO cut-offs: below −3 SD is severe thinness, below −2 SD thinness, −2 to +1 SD normal, above +1 SD overweight (roughly BMI 25 at age 19) and above +2 SD obesity (roughly BMI 30 at age 19).
Why can't I enter an age under 5 or over 19?
The WHO 5–19 y reference only covers 60 to 228 months. Younger children are assessed with the WHO Child Growth Standards (0–5 y), which use different tables, and adults use fixed BMI cut-offs.
Is my child's data uploaded anywhere?
No. The reference table ships inside the page and the percentile is computed entirely in your browser. This is an educational estimate, not a medical assessment — growth concerns belong with a pediatrician.
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